Network Working Group J. Arkko
Internet-Draft Ericsson
Expires: August 25, 2003 J. Kempf
DoCoMo Communications Labs USA
B. Sommerfeld
SUN Microsystems
B. Zill
Microsoft
February 24, 2003
SEcure Neighbor Discovery (SEND)
draft-ietf-send-ipsec-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on August 25, 2003.
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 an extension to IPsec for
securing ND. Contrary to the original ND specifications that also
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called for use of IPsec, this extension does not require the creation
of a large number of manually configured security associations.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Neighbor and Router Discovery Overview . . . . . . . . . . . 6
4. Secure Neighbor Discovery Overview . . . . . . . . . . . . . 9
5. Cryptographically Generated Addresses . . . . . . . . . . . 11
5.1 Address Format . . . . . . . . . . . . . . . . . . . . 12
5.2 Basic Interface Identifier Generation . . . . . . . . 12
5.3 Address Generation . . . . . . . . . . . . . . . . . . 13
5.4 Duplicate Address Detection . . . . . . . . . . . . . 14
6. Authorization Delegation Discovery . . . . . . . . . . . . . 15
6.1 Delegation Chain Solicitation Message Format . . . . . 15
6.2 Delegation Chain Advertisement Message Format . . . . 17
6.3 Trusted Root Option . . . . . . . . . . . . . . . . . 19
6.4 Certificate Option . . . . . . . . . . . . . . . . . . 20
6.5 Processing Rules for Routers . . . . . . . . . . . . . 21
6.6 Processing Rules for Hosts . . . . . . . . . . . . . . 22
7. IPsec Extensions . . . . . . . . . . . . . . . . . . . . . . 25
7.1 The AH_RSA_Sig Transform . . . . . . . . . . . . . . . 25
7.1.1 Reserved SPI Number . . . . . . . . . . . . . . 25
7.1.2 Authentication Data Format . . . . . . . . . . . 25
7.1.3 AH_RSA_Sig Security Associations . . . . . . . . 27
7.1.4 Replay Protection . . . . . . . . . . . . . . . 28
7.1.5 Processing Rules for Senders . . . . . . . . . . 28
7.1.6 Processing Rules for Receivers . . . . . . . . . 29
7.2 Other IPsec Extensions . . . . . . . . . . . . . . . . 30
7.2.1 Destination Agnostic Security Associations . . . 30
7.2.2 ICMP Type Specific Selectors . . . . . . . . . . 31
8. Securing Neighbor Discovery with SEND . . . . . . . . . . . 32
8.1 Using IPsec to Secure Neighbor Advertisement Messages 32
8.2 Security Policy and SA Database Configuration . . . . 32
9. Securing Router Discovery with SEND . . . . . . . . . . . . 34
9.1 Using IPsec to Secure Router Advertisement Messages . 34
9.2 Using IPsec to Secure Redirect Messages . . . . . . . 34
9.3 Security Policy and SA Database Configuration . . . . 35
10. Operational Considerations . . . . . . . . . . . . . . . . . 37
11. Performance Considerations . . . . . . . . . . . . . . . . . 39
12. Security Considerations . . . . . . . . . . . . . . . . . . 40
12.1 Achieved Security Properties . . . . . . . . . . . . . 40
12.2 Attacks against SEND Itself . . . . . . . . . . . . . 40
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 42
14. Conclusions and Remaining Work . . . . . . . . . . . . . . . 43
Normative References . . . . . . . . . . . . . . . . . . . . 44
Informative References . . . . . . . . . . . . . . . . . . . 45
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 46
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 47
B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 48
C. IPR Considerations . . . . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . 50
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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 [17] in using IKE [15] before ND is
operational. Furthermore, the number of security associations needed
for protecting ND is impractically large [18]. 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 IPsec AH [3] to secure all
advertisements relating to Neighbor and Router Discovery. A new
transform for AH allows public keys to be used. Routers are
certified by a trusted root, and a zero-configuration mechanism for
showing address ownership.
Section 5 describes the mechanism for showing address ownership,
based on the use of Cryptographically Generated Addresses (CGAs).
Section 6 describes the mechanism for distributing certificate chains
to establish authorization delegation chain to a common trusted root.
Section 7 describes the necessary modifications to IPsec. Finally,
Section 8 show how to apply these components to securing Neighbor and
Router Discovery.
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2. Terms
Cryptographically Generated Addresses (CGAs) A technique [22] 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.
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].
Security Association (SA) A Security Association (SA) is a simplex
"connection" that affords security services to the traffic carried
by it. Security services are afforded to an SA by the use of AH,
or ESP, but not both. A SA is uniquely identified by a triple
consisting of a Security Parameter Index (SPI), an IP Destination
Address, and a security protocol (AH or ESP) identifier [2].
Security Association Database (SAD) A nominal database containing
parameters that are associated with each (active) security
association. For inbound and outbound IPsec processing, these
databases are separate.
Security Parameters Index (SPI) An arbitrary 32-bit value. Together
with the destination IP address and security protocol (ESP or AH)
identifier, the SPI uniquely identifies the Security Association.
Values from 1 to 255 are reserved.
Special SPI A Security Parameters Index from the reserved range 1 to
255.
Security Policy The Security Policy determines the security services
afforded to an IPsec protected packet and the treatment of the
packet in the network.
Security Policy Database (SPD) A nominal database containing a list
of policy entries. Each policy entry is keyed by one or more
selectors that define the set of IP traffic encompassed by this
policy entry. Separate entries for inbound and outbound traffic
is required [2].
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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 Neighbour 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]. No higher level traffic can
proceed if this procedure flushes out neighbour cache entries
after (perhaps incorrectly) determining that the peer is not
reachable.
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.
o Address Resolution is similar to IPv4 ARP [14]. 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 another 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 [19],
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 [13].
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 the 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't 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 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 Neighbour 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
IPsec AH is used in to protect Neighbor and Router Discovery
messages. This specification introduces the use of a new transform
for IPsec AH, extensions to the current IPsec selectors, an
authorization delegation discovery process, and an address ownership
proof mechanism.
The components of the solution specified in this document are as
follows:
o IPsec AH is used to protect all advertisement messages relating to
Neighbor and Router discovery. Solicitation messages are not
protected, as they do not carry any information.
o IPsec security policy database and security association database
are configured to require the protection as indicated above. Note
that such configuration may take place manually or the operating
system may perform it automatically upon enabling Secure Neighbor
Discovery.
This specification introduces extensions to the required selectors
used in security policy database entries. This is necessary in
order to enable the protection of specific ICMP message types,
while leaving other messages unprotected.
o A new transform for IPsec AH allows public keys to be used on a
security association directly without the involvement of a key
management protocol. Symmetric session keys are not used, public
key signatures are used instead. 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.
The new transform uses also a fixed, standardized SPI (Security
Parameters Index) number. This necessary again in order to avoid
the involvement of a key management protocol.
Given that Neighbor and Router Discovery messages are in some
cases sent to multicast addresses, the new transform uses a
timestamp mechanism as a replay mechanism instead of sequence
numbers.
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 accept adopt the router as its default router.
Delegation Chain Solicitation and Advertisement messages are used
to discover a certificate chain to the trusted root without
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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.
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5. Cryptographically Generated Addresses
Cryptographically Generated Addresses (CGAs) [22][23][20][25] are a
technique whereby a node's IPv6 address can be unalterably tied to
the node's public key. Conceptually, CGAs allow a recipient of a
message to determine whether the sender is authorized to use the
public key and address claimed to be associated with the packet.
Typically, this requires the sender to use the hash of the node's
public key as the interface identifier in the bottom 64 bits of the
IPv6 address.
Authorization through CGAs and certificates are related, but separate
mechanisms. It is separate in that other techniques of authorization
(i.e. digital certificates) can be used instead of CGAs to achieve
the same effect. However, certificates require a means to create and
distribute them, thereby imposing more overhead than CGA. It is
related in that a digital signature is required in addition to the
CGA address and the signature must cover the address, in order that
the recipient can have the confidence that the address was not
altered in transit. Furthermore, to properly authorize the address
use, the issuer of the certificate must be considered as a valid
source of authority for certifying address usage, and must be capable
of making statements about an individual's use of IP addresses.
Theoretically, proper use of certificates provides more assurance
about address usage authorization than CGA. However, it is often
practically difficult to arrange the certificate authorities so that
they can control which IP addresses can be used by which parties.
The authorization provide by CGA is computational in nature, deriving
its strength from the computational difficulty of creating duplicate
CGA addresses. It does not require any configuration tasks, and it
does not impose any requirements on the infrastructure.
Respectively, certificate based authorization is administrative in
nature, and does not impose restrictions to the structure of the
addresses.
CGAs are particularly useful for Neighbor Discovery because they
provide a low overhead way for the sender of a Neighbor Advertisement
to indicate their authorization for claiming the address. The
recipient of a Neighbor Advertisement with a CGA Address, a public
key, and a digital signature in the header can have confidence that:
o The packet was not modified in transit (due to the signature),
o The sender of the packet has a right to claim possession of the
address (due to the authenticated CGA address).
In this section, we describe how a sender generates CGA addresses and
digital signatures for the AH header in Neighbor Advertisement
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packets, and how the receiver of such a packet verifies it. The
description of CGA use for IPv6 Neighbor Discovery follows closely
that described in [24].
5.1 Address Format
The basic idea behind CGA addresses is to use some function of the
host's public key as input to a hash function to generate the
interface identifier (bottom 64 bits) in the IPv6 address.
Variations on this basic theme provide additional security against
denial of service attacks and futureproofing against increases in
attacker processing power due to Moore's Law. For purposes of secure
Neighbor Discovery, CGA addresses are modified EUI-64 addresses [1]
in which the "universal/local" bit (bit 6) is set to 1 (indicating
global scope) and the "individual/group" bit (bit 7) is set to 1
(indicating the CGA group). Correct handling of these bits
effectively reduces the size of the interface identifier to 62 bits.
5.2 Basic Interface Identifier Generation
The basic hash algorithm for CGA addresses generates a 160 bit hash
by concatenating the node's public key, a nonce, and routing prefix
for the address in question. This result is then hashed to obtain
the actual interface identifier. The input hash is generated as
follows:
Equation (1).
H(N) = Hash-160(public_key |
nonce |
routing_prefix)
H(i) = Hash-160(H(i+1))
where Hash-160 is the 160 bits obtained from applying the SHA-1
secure hashing algorithm [12], public_key is the node's public key in
the format defined in Section 7.1.2, and nonce is a random octet
string of 8 or more bytes. The selection N is a local matter, but it
MUST be at least 3. N is used as a defense mechanism against
denial-of-service attacks.
The routing_prefix is the routing prefix for the address in question.
Note that this value is used regardless of whether the scope of the
address to be generated is link-local, site-local, or global. If the
scope is not global, it is possible that different networks will be
using the same routing prefix, such as the FE80::/10 prefix for
link-local addresses. This is allowed, as the addresses are not used
in the same network. In any case, other components in Equation (1)
typically provide sufficient randomness to avoid collisions and
Duplicate Address Detection would avoid possibly remaining address
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collisions.
The host generates a series of these hash values. The actual
interface identifier is then generated by performing taking the
rightmost 60 bits of the SHA-1 hash applied to the input value:
Equation (2).
interface_id = Hash-60(H(i))
where Hash-60 is the rightmost 60 bits obtained from the application
of the SHA-1 algorithm, i starts at 0 and increases depending on
whether additional rounds of duplicate address detection must be
negotiated (see Section 5.4).
The routing_prefix term is included in the above in order to
introduce a strong binding between the prefixes and interface
identifiers, and to add some randomness in order to defeat brute
force and birthday attacks. If it is not included, an attacker can
generate a lookup table of key pairs for each of the possible 2**60
values of the interface identifier and use them to disrupt duplicate
address detection. Note, however, that including these in the
address requires the host to perform duplicate address detection for
each address configured on the interface, not just for the link local
address, as is allowed by RFC 2462.
5.3 Address Generation
Equation (2) in Section 4.1.1 only takes 60 bits of the SHA-1 hash,
although 62 bits are theoretically available after the "u" and "g"
bits are omitted. This is because the right most 2 bits of the
interface identifier are reserved for a security parameter. The
security parameter can have a value of 0 through 3, and is a way of
future-proofing the CGA address against increases in processing power
in attackers due to Moore's Law, since 62 bits is on the borderline
of what is today computationally difficult to attack. Conceptually,
the security parameter is a way to increase the computational effort
of both generating and attacking an address. While this has the side
effect of increasing the effort for the client, the client presumably
only has to generate the address once, while an attacker may have to
generate the address multiple times.
The algorithm for generating the actual address is as follows, given
the security parameter has value Sec:
1. Generate a key pair.
2. Generate a nonce value.
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3. Generate a table of hash values according to Equation (1).
4. Generate a target identifier according to Equation (2), but
taking 20 x Sec + 60 bits instead of just 60 bits.
5. Compare the leftmost 20 x Sec to zero. If not zero, go back to
Step 2.
6. Set the universal and group bits to 1 and the rightmost two bits
to Sec.
7. Use the result of Step 6 as the interface identifier for the
address.
If the security parameter is zero, this algorithm is the basic CGA
algorithm.
If the security parameter is greater than zero, the algorithm is not
guaranteed to terminate after a certain number of iterations (though
it will ultimately terminate). For security parameter values 1, 2,
and 3, the average number of iterations required to produce a
matching hash output are 2**19, 2**39, and 2**59, i.e. 2**(20 x Sec
-1) [24]. The additional amount of computational effort involved in
increasing the security parameter allows the SEND algorithm to scale
as Moore's law increases processing power.
5.4 Duplicate Address Detection
During Duplicate Address Detection, a node may encounter a clash with
another node on the link. One possible denial of service attack
occurs when the attacker deliberately provokes an address clash, in
order to prevent the victim from claiming the address. RFC 2462 [7]
inadvertently facilitates this attack, by requiring nodes to
terminate Duplicate Address Detection when a clash is detected.
For Secure Neighbor Discovery, a node performs Duplicate Address
Detection a maximum of 3 times. If an address clash is detected, the
node restarts interface identifier generation at Step 2 of the
algorithm described in Section 4.1.2, by selecting a different hash
input for target identifier generation. If clashes are detected
after three tries, the node is probably under attack, so it should
shut down and report the situation to an administrator.
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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 to
communicate off-link.
Since the newly-connected node likely can't communicate off-link, it
can't 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 can be 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. Due to the size of certificates and
potentially long certificate chains, the advertisement message may be
large. The messages have been made separate from the rest of
Neighbor Discovery in order to reduce their effect on the size of
other messages. Long certificate chains may also be broken to
multiple 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
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, 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 |M| Component |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
MUST be the link-local address assigned to the interface from
which this message is sent.
Destination Address
Typically the Source Address of a host invoking Delegation
Chain Solicitation 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.
M
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 'M' flag,
when set, indicates that there are more components coming in
this advertisement.
Component
This is a 15 bit unsigned integer field. The first message in
a multi-component advertisement has the Component field set to
0, the second set to 1, and so on.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
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Valid Options:
Certificate
Zero or one certificates are provided in Certificate options,
to establish a 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.
Name Type
The type of the name included in the Name field. This
specification defines only one legal value for this field:
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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:
1 X.509 Certificate
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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 [10].
6.5 Processing Rules for Routers
Routers SHOULD possess a keypair 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
with Router Solicitation messages MUST be ignored and the packet
processed as normal. 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
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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 MAY choose to unicast the
response directly to the soliciting host's address (if the
solicitation's source address is not the unspecified address), but
the usual case is to multicast the response to the all-nodes group.
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].
6.6 Processing Rules for Hosts
Hosts SHOULD possess the certificate of at least one certificate
authority, and MAY possess their own keypair and certificate from
this authority.
A host MUST silently discard any received Router Advertisement
messages that do not satisfy all of the following validity checks:
o IP Source Address is a link-local address. Routers must use their
link-local address as the source for Router Advertisement and
Redirect messages so that hosts can uniquely identify routers.
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.
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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 Router Advertisement messages MUST be ignored and the packet
processed as normal. 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 all certificates retrieved in Delegation Chain
Advertisements for use in subsequent verification of Router (and
optionally Neighbor) Advertisements. Note that it may be useful to
cache this information and implied verification results for use over
multiple attachments to the network.
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 may 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
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authorization delegation chain to the host's trusted root.
Delegation Chain Solicitations MUST NOT be sent if a valid
certificate chain exists in the host's cache from the desired router
(or host) to the host's trusted root.
A host MUST send Delegation Chain Solicitations either to the
All-Routers multicast address, if it hasn't 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, these MUST be unicast to the hosts's address.
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 make Denial-of-Service attacks against the
mechanism harder (see Section 12.2).
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7. IPsec Extensions
In order to use IPsec in securing Neighbor and Router Discovery some
extensions have been specified in this document. These include a new
transform suitable for the use of public keys and/or CGAs, a
timestamp mechanism suitable for replay protection in a multicast
environment, and some extensions to security association and security
policy databases.
7.1 The AH_RSA_Sig Transform
The AH_RSA_Sig transform specifies how AH can be used without a
symmetric key. This transform introduces the use of a new reserved
SPI number and a new format for the Authentication Data field in AH.
AH_RSA_Sig MUST NOT be negotiated in IKE. For consistency it has an
IPsec DOI [4] Transform ID TBD <To Be Assigned by IANA>, however.
7.1.1 Reserved SPI Number
The AH_RSA_Sig MUST be only be used with the reserved SPI number TBD
<To Be Assigned by IANA>.
7.1.2 Authentication Data Format
The format of the Authentication Data field in AH depends on the
chosen transform. For the AH_RSA_Sig transform, the format is as
follows:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PK_Len | Nonce_Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Public key .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Nonce (optional) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Digital Signature (remaining bytes) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The meaning of the fields is described below:
PK_Len
This 16 bit unsigned integer field contains the length of the
Public Key field in bytes.
Nonce_Len
This 16 bit unsigned integer field contains the length of the
Nonce field in bytes. The length is set to zero if that field is
not present.
Timestamp
This 64 bit unsigned integer field contains a timestamp used for
replay protection (the Sequence Number field in AH is not used for
AH_RSA_Sig). The use of this field is discussed in Section 7.1.4.
Public key
This variable length field contains the public key of the sender
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in X.509 format [10].
Nonce
This variable length field, if present, contains the nonce used in
the construction of the CGA address.
Digital Signatures
This variable length field, if present, 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 [11].
7.1.3 AH_RSA_Sig Security Associations
Security associations that specify the use of AH_RSA_Sig transform
MUST record the following additional configuration information:
o A flag that indicates whether or not authorization delegation to a
trusted root is used.
o A flag that indicates whether or not CGA addresses are used.
Incoming security associations MUST also record the following
additional information:
o The public key of the trusted root, if authorization delegation is
in use.
o The minimum acceptable key length for peer public keys (and any
intermediaries between the trusted root and the peer). The
default SHOULD be 768 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.
o The minimum acceptable Sec value, if CGA verification is required.
Outgoing security associations MUST also record the following
additional information:
o A public-private keypair. If authorization delegation is in use,
there must exist a delegation chain from a trusted root to this
keypair.
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o Optionally any information required to construct CGA signatures,
including the used Sec value and nonce, and the resulting CGA
address.
7.1.4 Replay Protection
For AH_RSA_Sig, the Sequence Number field in AH MUST be set to zero
by the sender and ignored by receivers.
If anti-replay has been enabled in the security association, senders
MUST set the Timestamp field to the current time. The format is 64
bits, and the contents are the number of milliseconds since January
1, 1970 00:00 UTC.
If anti-replay has been enabled, receivers MUST be configured with an
allowed Delta value and maintain a cache of messages received within
this time period from each specific source address. Receivers MUST
then check the Timestamp field as follows:
o A packet with a Timestamp field value beyond the current time plus
or minus the allowed Delta value MUST be silently discarded.
o A packet accepted according to the above rule MUST be checked for
uniqueness within the cache of received messages from the given
source address. A packet that has already been seen from the same
source with the same Timestamp field value MUST be silently
discard.
o A packet that passes both of the above tests 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.
7.1.5 Processing Rules for Senders
A node sending a packet using the AH_RSA_Sig transform MUST construct
the packet as follows:
o The Next Header, Payload Len, and Reserved fields are set as
described in RFC 2402.
o The Security Parameters Index is set to the value specified in
Section 7.1.1.
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o The Sequence Number field is set to 0.
o The PK_Len field in Authentication Data is set to the length of
the public key used for signing this packet. This public key is
stored in the security association. The key itself is put to the
Public key field.
o If the security association has specified the use of the CGA
method, the Nonce_len field is set to the length of the nonce used
in the construction of the CGA address. In this case the nonce is
copied to the Nonce field. Otherwise, the Nonce_Len field is set
to zero and the Nonce field is omitted.
o The Timestamp field is set as described in Section 7.1.4.
o The packet, in the form defined for AH's coverage, is signed using
the private key in the security association, and the resulting
PCKS #1 signature is put to the Digital Signature field.
o Additionally, if the use of CGA has been specified for the
security association we require that the source address of the
packet has been constructed as specified in Section 5. A sending
node uses as inputs the sender's public key, nonce, the subnet
prefix from the Target Address, and the Target Link Layer Address.
The Target Address (including the subnet prefix) is put to the
Source Address field in the IPv6 header, and the public key and
the nonce are put to the Authentication Data field in the AH
header.
7.1.6 Processing Rules for Receivers
A packet received on a security association employing AH_RSA_Sig
transform MUST be checked as follows:
o Next Header and Payload Len fields are valid as specified in RFC
2402.
o The SPI field is equal to the value defined in Section 7.1.1.
o The sum of the PK_Len, Nonce_Len, and LLA_Len fields does not
exceed the length of the Authentication Data field.
o The Nonce_Len field is non-zero if the use of CGA has been
specified in the security association.
o The Timestamp field is verified as described in Section 7.1.4.
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o If the use of CGA has been specified in the security association,
we additionally require that A node receiving an Neighbor or
Router Advertisement message with CGA protection first checks the
CGA address in the Target Address field by generating the address
using the algorithm described in Section 5.3. The inputs for the
algorithm are the sender's public key and nonce, included in the
AH packet as described in Section 7.1.2, the subnet prefix from
the Target Address, the Target Link Layer Address, which MUST be
included in a Target Link Layer Address option, and the security
parameter from the rightmost two bits of the Target Address. If
the interface identifier checks, the recipient proceeds with the
cryptographically more time consuming check of the AH signature.
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.
o The Public key and Digital Signature fields can be correctly
decoded, and the the Digital Signature verifies as specified in
the previous section.
o If the use of a trusted root has been configured for the security
association, 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.
Packets that do not pass all the above tests MUST be silently
discarded.
7.2 Other IPsec Extensions
7.2.1 Destination Agnostic Security Associations
In order to allow the same security association to be used when the
the node sends packets to different peers using the same addresses, a
change must be provided to the RFC 2401 rules on how security
associations are identified. This change is particularly important,
for instance, when routers use the same keys and security association
to send Router Advertisements for up to number of prefixes x 2^64
hosts on an interface.
The change is mandatory for all nodes that support the AH_RSA_Sig
transform. Security associations that use the SPI value specified in
Section 7.1.1 MUST be identified solely by the SPI and protocol
numbers, not by the destination IP address.
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7.2.2 ICMP Type Specific Selectors
In order to allow finer granularity of protection for various ICMPv6
messages, it is necessary to extend the security policy database and
security association selectors with the capability to distinguish
between different messages.
All nodes that support the AH_RSA_Sig transform MUST be capable of
using ICMP and ICMPv6 Type field as a selector.
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8. Securing Neighbor Discovery with SEND
This section describes how to use IPsec and the mechanisms from
Section 5, Section 6, Section 7 in order to provide security for
Neighbor Discovery.
8.1 Using IPsec to Secure Neighbor Advertisement Messages
All Neighbor Solicitation messages SHOULD be sent without protection.
All Neighbor Advertisement messages MUST be protected with IPsec,
using the AH_RSA_Sig transform. The protection can be based on CGA
addresses, node certificates and trusted roots, or both as specified
in the security association.
All nodes MUST have the necessary key pairs, and as applicable,
certificates and CGA parameters associated with their relationship to
trusted root or to an address.
Hosts that use stateless address autoconfiguration MUST generate new
CGA addresses as specified in Section 5 for each new
autoconfiguration run.
It is outside the scope of this specification to describe trusted
roots and address autoconfiguration (stateful or stateless) with
dynamically changing addresses works. It is also outside the scope
of this specification to describe how stateful address
autoconfiguration works with the CGA method.
Hosts MAY use Authorization Delegation Discovery to learn the
certificate chain of their default router or peer host.
8.2 Security Policy and SA Database Configuration
This section gives a description for the security policy and security
associations database entries, under which the outbound and inbound
Neighbor Advertisement messages can be protected.
The following table summarizes the inbound security policy data base
along with the inbound security associations:
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Policy entries:
*------------------------------------------------------------------*
| Proto: Type | Source | Destination | Treatment |
*------------------------------------------------------------------*
| ICMPv6: NS | * | * | pass |
*------------------------------------------------------------------*
| ICMPv6: NA | * | own | SA = NA_In |
*------------------------------------------------------------------*
| ICMPv6: NA | * | all-nodes MC | SA = NA_In |
*------------------------------------------------------------------*
Security associations:
+------------------------------------------------------------------+
| Name | Direction | SPI | Proto | Transform |
+------------------------------------------------------------------+
| NA_In | Inbound | TBD (fixed) | AH | AH_RSA_Sig |
| | | | | CGA = yes/no |
| | | | | root = ... (opt)|
+------------------------------------------------------------------+
The following table summarizes outbound security policy database:
Policy entries:
*------------------------------------------------------------------*
| Proto: Type | Source | Destination | Treatment |
*------------------------------------------------------------------*
| ICMPv6: NS | * | * | pass |
*------------------------------------------------------------------*
| ICMPv6: NA | own | * | SA = NA_Out |
*------------------------------------------------------------------*
Security associations:
+------------------------------------------------------------------+
| Name | Direction | SPI | Proto | Transform |
+------------------------------------------------------------------+
| NA_Out | Outbound | TBD (fixed) | AH | AH_RSA_Sig |
| | | | | key pair = ... |
| | | | | CGA = yes/no |
| | | | | CGA params = ...|
| | | | | root = ... (opt)|
+------------------------------------------------------------------+
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9. Securing Router Discovery with SEND
This section describes how to use IPsec and the mechanisms from
Section 5, Section 6, Section 7 in order to provide security for
Router Discovery.
9.1 Using IPsec to Secure Router Advertisement Messages
All Router Solicitation messages SHOULD be sent without protection.
All Router Advertisement messages MUST be protected with IPsec, using
the AH_RSA_Sig transform. The protection can be based on CGA
addresses, node certificates and trusted roots, or both as specified
in the security association.
All routers MUST have the necessary key pairs, and as applicable,
certificates and CGA parameters associated with their relationship to
trusted root or to an address. 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.
9.2 Using IPsec to Secure Redirect Messages
All Redirect messages MUST be protected with IPsec, using the
AH_RSA_Sig transform. The protection can be based on CGA addresses,
node certificates and trusted roots, or both as specified in the
security association.
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.
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9.3 Security Policy and SA Database Configuration
This section gives a description for the security policy and security
associations database entries, under which the outbound and inbound
Router Advertisement and Redirect messages can be protected.
The following table summarizes the inbound security policy data base
along with the inbound security associations:
Policy entries:
*------------------------------------------------------------------*
| Proto: Type | Source | Destination | Treatment |
*------------------------------------------------------------------*
| ICMPv6: RS | * | * | pass |
*------------------------------------------------------------------*
| ICMPv6: RA | * | own | SA = RA_In |
*------------------------------------------------------------------*
| ICMPv6: RA | * | all-nodes MC | SA = RA_In |
*------------------------------------------------------------------*
| ICMPv6: REDIRECT | * | own | SA = RE_In |
*------------------------------------------------------------------*
Security associations:
+------------------------------------------------------------------+
| Name | Direction | SPI | Proto | Transform |
+------------------------------------------------------------------+
| RA_In | Inbound | TBD (fixed) | AH | AH_RSA_Sig |
| | | | | CGA = yes/no |
| | | | | root = ... (opt)|
+------------------------------------------------------------------+
| RE_In | Inbound | TBD (fixed) | AH | AH_RSA_Sig |
| | | | | CGA = yes/no |
| | | | | root = ... (opt)|
+------------------------------------------------------------------+
The following table summarizes outbound security policy database.
The Router Advertisement and Redirect entries are only present in
routers.
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Policy entries:
*------------------------------------------------------------------*
| Proto: Type | Source | Destination | Treatment |
*------------------------------------------------------------------*
| ICMPv6: RS | * | * | pass |
*------------------------------------------------------------------*
| ICMPv6: RA | own | * | SA = RA_Out |
*------------------------------------------------------------------*
| ICMPv6: REDIRECT | own | * | SA = RE_Out |
*------------------------------------------------------------------*
Security associations:
+------------------------------------------------------------------+
| Name | Direction | SPI | Proto | Transform |
+------------------------------------------------------------------+
| RA_Out | Outbound | TBD (fixed) | AH | AH_RSA_Sig |
| | | | | key pair = ... |
| | | | | CGA = yes/no |
| | | | | CGA params = ...|
| | | | | root = ... (opt)|
+------------------------------------------------------------------+
| RE_Out | Outbound | TBD (fixed) | AH | AH_RSA_Sig |
| | | | | key pair = ... |
| | | | | CGA = yes/no |
| | | | | CGA params = ...|
| | | | | root = ... (opt)|
+------------------------------------------------------------------+
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10. Operational Considerations
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. In such a case, the link is required to
operate as two separate logical links, and packets between a secure
and insecure node always go through the router.
Routers configured for SEND advertise two sets of globally routable
prefixes: one set for SEND nodes and one set for nodes that implement
insecure Neighbor Discovery. The insecure nodes will ignore the
advertisements sent using SEND, as the original Neighbor Discovery
specifications require silently discarding packets if they contain an
AH header that they can not verify.
The following considerations apply to hosts:
o Hosts configured for SEND MUST use SEND for all of their
addresses, including link local addresses.
o Hosts configured for SEND MUST validate all Router Advertisements
with the protocol described in Section 8. Note that this includes
discarding advertisements received without a valid IPsec AH header
and CGA address, thus making insecure prefixes invisible to them.
o Hosts configured for SEND MUST secure and validate all Neighbor
Advertisements with the protocol described in Section 8. Note
that this includes discarding advertisements received without a
valid IPsec AH header and CGA address.
The following considerations apply to routers:
o Routers MUST send two sets of Router Advertisements. The
advertisements containing the secure prefixes MUST be secured with
the protocol described in Section 9. The advertisements
containing the insecure prefixes MUST be sent without security.
o Routers MUST assign different addresses for their secure and
insecure communications, including their link-local addresses.
Secure Router and Neighbor Advertisements MUST use a source
address that satisfies the security properties outlined in Section
9. Unless this address is link-local, it MUST belong to one of
the advertised secure prefixes. Similarly, source addresses for
insecure advertisements MUST belong to one of the advertised
insecure prefixes, unless the address is link-local.
o Routers MUST refrain from sending Redirects to a SEND-secured node
with the Destination Address field set to an address for an
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insecure node. Similarly, routers MUST refrain from sending
Redirects to a insecure node with the Destination Address field
set to an address for a SEND-secured node
The above rules require secure nodes to ignore all insecure Neighbor
and Router Discovery messages, and all insecure nodes to ignore all
SEND-secured messages. This implies that the secure and insecure
nodes will not be able to discover each other, or even realize that
the other prefixes are on-link. Thus, these hosts will request the
router to route packets destined to the a host in the other group.
The rules regarding Redirect messages above have been provided to
ensure that the router performs its routing task and does not
instruct the hosts to communicate directly.
One effect of this is that secure hosts can not communicate with
insecure hosts using link-local addresses, and vice versa.
The security policy or security association database entries are
needed for insecure nodes as far as Neighbor Discovery is concerned.
SEND-secured nodes have the usual entries required by SEND.
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11. Performance Considerations
The computations related to AH_RSA_Sig transform are substantially
more expensive than those with traditional symmetric transforms.
While computational power is increasing, it appears still impractical
to use asymmetric transforms for a significant amount packets.
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 to communicate with.
Routers are required to perform a larger amount of operations,
particularly when the frequency of router advertisements is high due
to mobility requirements. Still, the number of operations on a
router is in 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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12. Security Considerations
12.1 Achieved Security Properties
The CGA method assures that the received messages are coming from the
owner of the address. However, this method does not eliminate all
security vulnerabilities related to the ND functions. CGA prevents
spoofed answers to DAD queries. An attacker may still be able to
prevent valid responses or requests from reaching the intended
recipient. As a result both participants are forced to believe that
no address collision exists, when there in fact is.
Within Address Resolution and NUD functions CGA can be used to
prevent spoofed responses. However, it is still possible to prevent
the Address Resolution and NUD from completing for a given address.
For the NUD, this means that a node is claimed to be unreachable,
when it really is not.
Hosts can use CGA to show that the Redirect messages come from their
current router. Still, we cannot say anything about the other router
mentioned in the Redirect message. When trusted roots are used to
certify routers, this is, however, not an issue.
Within the Router Discovery functionality the CGA method ensures that
we are communicating with the same router all the time, and prevents
spoofing of the link-layer address of the router. But it does not
help to verify that the router is connected to the Internet or that
it is authorized to advertise a specific route prefix. A proper
verification of these properties will not be possible without
involving a trusted root.
Protection of Router (or Neighbor) Discovery with trusted roots
ensures that the given router (or neighbor) belongs to the set of
trusted entities. It does not provide assurance that the given
router is not spoofing another legitimate router (but see Section
14).
12.2 Attacks against SEND Itself
The CGA addresses have a 60-bit hash. This length is in within the
range of an feasible attack in the future. The following mechanisms
have been built in this draft to counteract such attacks:
The inclusion of the routing prefix prevents precomputation
attacks.
The Sec parameter helps the SEND algorithm to scale as Moore's law
increases processing power. Additional amount of computational
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effort is involved in for both attackers and owners of an address;
verifiers of a message still need to spend the same amount of
effort.
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 the SEND in-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.
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 Section 7.1.6. 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 in SEND, the defenses
are not quite as effective. Implementations SHOULD track how much
resources are being devoted to the processing of packets received
with the AH_RSA_Sig transform, and start selectively dropping packets
if too much resources are spent. Implementations MAY also start
first dropping packets that which 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 over
multicast advertisements.
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13. 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.
This document defines a new reserved SPI number in the Reserved SPI
range 1-255 [3].
This document defines a new IPSEC AH Transform Identifier for the
IPsec DOI [4]. This identifier represents the AH_RSA_Sig transform
from Section 7.1.
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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14. Conclusions and Remaining Work
This draft documents ongoing work. The following areas are still
being studied:
o Protection of solicitations. There are no provisions yet for the
protection of Address Resolution which takes place as a
side-effect of Neighbor Solicitations. Similarly, the effects of
Duplicate Address Detection probes on other nodes currently doing
DAD have not been covered, as they too are carried by
solicitations.
o CGA detailed format and calculation formulas: The CGA formulas
used in this document are from an early approach to the control of
the security level in an environment with a constrained number of
output bits. An advanced version of this approach will be
published soon and appears interesting [21].
o Transition issues: Security policy and security association
database entry examples are needed before the correctness of the
approach outlined in Section 10 can be estimated. Also, the
ability of hosts to simultaneously use SEND and insecure ND
without a router. The ability of a non-SEND router to participate
on a link with SEND-capable hosts and other routers.
o The security considerations, achieved security properties, and the
treatment of Denial-of-Service attacks on the SEND mechanisms
themselves need further work.
o The formats used to carry trusted root references, certificates,
and public keys may change.
o It is unclear at this time how, and if, router and neighbor
protection based on trusted roots relates to addresses and
prefixes. Is a router only certified to use a particular IP
address, or to provide a particular prefix to the link?
o It is unclear whether MLD [16] protection is needed or not.
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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] International Organization for Standardization, "The Directory
- Authentication Framework", ISO Standard X.509, 2000.
[11] RSA Laboratories, "RSA Encryption Standard, Version 1.5", PKCS
1, November 1993.
[12] 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
[13] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[14] 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.
[15] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[16] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[17] Arkko, J., "Effects of ICMPv6 on IKE and IPsec Policies",
draft-arkko-icmpv6-ike-effects-01 (work in progress), June
2002.
[18] Arkko, J., "Manual SA Configuration for IPv6 Link Local
Messages", draft-arkko-manual-icmpv6-sas-01 (work in progress),
June 2002.
[19] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
November 2002.
[20] Montenegro, G. and C. Castelluccia, "SUCV Identifiers and
Addresses", draft-montenegro-sucv-03 (work in progress), July
2002.
[21] Aura, T., "Cryptographically Generated Addresses (CGA)",
draft-aura-cga-00.txt (work in progress), February 2003.
[22] O'Shea, G. and M. Roe, "Child-proof Authentication for MIPv6",
Computer Communications Review, April 2001.
[23] Nikander, P., "Denial-of-Service, Address Ownership, and Early
Authentication in the IPv6 World", Proceedings of the Cambridge
Security Protocols Workshop, April 2001.
[24] Arkko, J., Aura, T., Kempf, J., Mantyla, V., Nikander, P. and
M. Roe, "Securing IPv6 Neighbor Discovery", Wireless Security
Workshop, September 2002.
[25] Montenegro, G. and C. Castelluccia, "Statistically Unique and
Cryptographically Verifiable (SUCV) Identifiers and Addresses",
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NDSS, February 2002.
Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@ericsson.com
James Kempf
DoCoMo Communications Labs USA
181 Metro Drive
San Jose, CA 94043
USA
EMail: kempf@docomolabs-usa.com
Bill Sommerfeld
SUN Microsystems
USA
EMail: sommerfeld@east.sun.com
Brian Zill
Microsoft
USA
EMail: bzill@microsoft.com
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Appendix A. Contributors
Steven Bellovin was the first to suggest the use of IPsec in this
manner for the protection of Neighbor Discovery. Pekka Nikander and
Vesa-Matti Mantyla were co-authors of an unpublished draft from which
many of the details of this document have been inherited. The
theoretical foundations of protecting Neighbor Discovery were laid
out in a paper [24] where Tuomas Aura, Vesa-Matti Mantyla, Pekka
Nikander, and Mike Roe were co-authors.
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Appendix B. Acknowledgements
The authors would like to thank Erik Nordmark and Gabriel Montenegro
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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Arkko, et al. Expires August 25, 2003 [Page 51]
