IPv6 Operations WG R. Graveman
Internet-Draft RFG Security, LLC
Expires: December 10, 2004 M. Parthasarathy
Nokia
P. Savola
CSC/FUNET
H. Tschofenig
Siemens
June 11, 2004
Using IPsec to Secure IPv6-over-IPv4 Tunnels
draft-tschofenig-v6ops-secure-tunnels-00.txt
Status of this Memo
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patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
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This Internet-Draft will expire on December 10, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document gives guidance on securing IPv6-in-IPv4 tunnels using
IPsec. No additional protocol extensions are described beyond those
available with the revised IPsec framework. IKEv2 is extensively
used as an authentication and key exchange protocol to cover address
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configuration procedures, and the usage of the Extensible
Authentication Procotol and NAT traversal capabilities is also
described.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Threats and the Use of IPsec . . . . . . . . . . . . . . . . . 3
2.1 IPsec in Transport Mode . . . . . . . . . . . . . . . . . 4
2.2 IPsec in Tunnel Mode . . . . . . . . . . . . . . . . . . . 4
3. Scenarios and Overview . . . . . . . . . . . . . . . . . . . . 5
3.1 Router-to-Router Tunnels . . . . . . . . . . . . . . . . . 5
3.2 Site-to-Router/Router-to-Site Tunnels . . . . . . . . . . 5
3.3 Host-to-Host Tunnels . . . . . . . . . . . . . . . . . . . 7
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. IPsec Configuration Details . . . . . . . . . . . . . . . . . 9
5.1 IPsec Transport mode . . . . . . . . . . . . . . . . . . . 9
5.2 IPsec Tunnel mode . . . . . . . . . . . . . . . . . . . . 10
5.2.1 SPD for Host-to-Host Scenario . . . . . . . . . . . . 10
5.2.2 SPD for Host-to-Router scenario . . . . . . . . . . . 10
6. Dynamic Address Configuration . . . . . . . . . . . . . . . . 12
7. Extensible Authentication Support . . . . . . . . . . . . . . 13
8. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Tunnel Endpoint Discovery . . . . . . . . . . . . . . . . . . 16
10. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . 17
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 18
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 19
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
15.1 Normative References . . . . . . . . . . . . . . . . . . . . 19
15.2 Informative References . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . 23
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1. Introduction
The IPv6 operations (v6ops) working group has selected IPv6-in-IPv4
tunneling [I-D.ietf-v6ops-mech-v2] as one of the IPv6 transition
mechanisms for IPv6 deployment. A number of threats have been
identified with possible solutions to mitigate them
[I-D.ietf-v6ops-mech-v2]. One of the solutions is the use of IPsec
protected tunnels, but there is little detail on how IPsec would
actually be used in an interoperable manner. This memo describes the
use of IPsec in detail.
First this document analyses the threats that can be addressed by
IPsec. Next, this document discusses some of the assumptions made by
this document for successful IPsec SA establishment. Then, it gives
the details of IKE/IPsec exchange with packet formats and SPD
entries. Finally, it discusses the usage of IPsec NAT-traversal
mechanism that can be used with configured tunnels in some scenarios.
2. Threats and the Use of IPsec
Following threats have been identified in [I-D.ietf-v6ops-mech-v2]:
1. IPv4 address of the encapsulating ("outer") packet can be
spoofed.
2. IPv6 address of the encapsualted ("inner") packet can be spoofed.
The reason for threat (1) is due to the lack of widespread deployment
of IPv4 ingress filtering in the network. The reason for threat (2)
is that the IPv6 packet is encapsulated in IPv4 and hence escapes
IPv6 ingress filtering. [I-D.ietf-v6ops-mech-v2] specifies following
strict address checks as mitigating measures.
To mitigate threat (1), the decapsulator verifies that the IPv4
source address of the packet is the same as the address of the
configured tunnel endpoint. The decapsulator may also implement IPv4
ingress filtering, i.e., checks whether the packet is received on the
expected interface.
To mitigate threat (2), the decapsulator verifies whether the inner
IPv6 address is a valid IPv6 address and also applies IPv6 ingress
filtering before accepting the IPv6 packet.
This memo proposes to use IPsec for providing stronger security in
preventing these threats. IPsec can be used in two ways, in
transport and tunnel mode.
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2.1 IPsec in Transport Mode
In transport mode, the IPsec security association (SA) is established
to protect the traffic defined by (IPv4-source, IPv4-dest, protocol =
41). On receiving such an IPsec packet, the receiver first applies
the IPsec transform (ESP) and then matches the packet against the
inbound selectors associated with the SA to verify that the packet is
appropriate for the SA via which it was received. The successful
verification implies that the packet came from the right IPv4
endpoint as the SA is bound to the IPv4 source address.
This prevents threat (1) but not the threat (2). IPsec in transport
mode does not verify the contents of the payload itself where the
IPv6 addresses are carried, that is, two nodes that are using IPsec
transport mode to secure the tunnel can spoof the inner payload. The
packet will be decapsulated successfully and accepted.
The shortcoming can be mitigated by IPv6 ingress filtering i.e.,
check that the packet is arriving from the interface in the direction
of the route towards the tunnel end-point, similar to a Strict
Reverse Path Forwarding (RPF) check [RFC3704].
For performing ingress filtering, it is assumed that the tunnel is
modelled as an interface and the traffic of the tunnel is protected
using IPsec transport mode SA.
2.2 IPsec in Tunnel Mode
In tunnel mode, the IPsec SA is established to protect the traffic
defined by (IPv6-source, IPv6-destination). On receiving such an
IPsec packet, the receiver first applies the IPsec transform (ESP)
and then matches the packet against the inbound selectors associated
with the SA to verify that the packet is appropriate for the SA via
which it was received. The successful verification implies that the
packet came from the right IPv6 endpoint as the SA is bound to the
IPv6 source address.
The IPv4 addresses may be spoofed and IPsec cannot detect it in this
mode, that is, two nodes that are using IPsec tunnel mode to secure
the tunnel with a common tunnel endpoint can spoof each other's IPv4
address. But, the packet will not be accepted by IPsec as the IPv6
address bound to the SA will not match the address in the spoofed
packet. Thus, the outer address spoofing is irrelevant as long as
the inner IPv6 packet can be verified to come from the right IPv6
endpoint.
It may not be possible to always verify the IPv6 address -- for
example with link-local addresses. The additional issues with
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address verification are discussed in each of the scenarios section
appropriately.
3. Scenarios and Overview
There are roughly three kinds of scenarios: (generic)
router-to-router tunnels, site-to-router/router-to-site tunnels (a
generalization of host-to-router/router-to-host scenarios,
respectively), and host-to-host tunnels.
3.1 Router-to-Router Tunnels
IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
IPv4 routing topology by encapsulating them within IPv4 packets.
Tunneling can be used in a variety of ways.
.--------. _----_ .--------.
|v6-in-v4| _( IPv4 )_ |v6-in-v4|
| Router | <======( Internet )=====> | Router |
| A | (_ _) | B |
'--------' '----' '--------'
^ IPsec tunnel between ^
| Router A and Router B |
V V
.--------. .-------.
| End | | End |
| Host | | Host |
'--------' '--------'
Figure 1: Router-to-Router Scenario
IPv6/IPv4 routers interconnected by an IPv4 infrastructure can tunnel
IPv6 packets between themselves. In this case, the tunnel spans one
segment of the end-to-end path that the IPv6 packet takes.
The source and destination addresses of the IPv6 packets traversing
the tunnel could come from a wide range of IPv6 prefixes. It is not
scalable to establish IPsec tunnel mode SAs for all such packets.
Hence, IPsec transport mode SA is recommended for this scenario.
IPv6 ingress filtering should be performed to mitigate the IPv6
address spoofing threat.
A specific case of router-to-router tunnels, when one router resides
at an end site, is described in the next section.
3.2 Site-to-Router/Router-to-Site Tunnels
This is a generalization of site-to-router and router-to-site
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tunneling, because the issues when connecting a whole site (using a
router), and connecting a single host are roughly equal.
_----_ .---------. IPsec _----_ IPsec .-------.
_( IPv6 )_ |v6-in-v4 | Tunnel _( IPv4 )_ Tunnel | V4/V6 |
( Internet )<--->| Router |<=======( Internet )=======>| Site B |
(_ _) | A | (_ _) '--------'
'----' '---------' '----'
^
|
V
.--------.
| Native |
| IPv6 |
| node |
'--------'
Figure 2: Router-to-Site Scenario
IPv6/IPv4 routers can tunnel IPv6 packets to their final destination
IPv6/IPv4 host. This tunnel spans only the last segment of the
end-to-end path.
This is the same as the Site-to-Router case.
+---------------------+
| IPv6 Network |
| |
.--------. _----_ | .--------. |
| V6/V4 | _( IPv4 )_ | |v6-in-v4| |
| Site B |<====( Internet )==========>| Router | |
'--------' (_ _) | | A | |
'----' | '--------' |
IPsec tunnel between | ^ |
V6 Site and Router A | | |
| V |
| .-------. |
| | V6 | |
| | Host | |
| '--------' |
+---------------------+
Figure 3: Site-to-Router Scenario
IPv6/IPv4 hosts can tunnel IPv6 packets to an intermediary IPv6/IPv4
router that is reachable via an IPv4 infrastructure. This type of
tunnel spans the first segment of the packet's end-to-end path.
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In this case, the host(s) originate the packet with source address
coming from a well known prefix whereas the destination address could
be any node on the internet. In this case, the IPsec tunnel mode SA
can be bound to the prefix that was allocated to the router at Site B
and router A can verify that the source address of the packet matches
the prefix. Site B will not be able to do a similar verification for
the packets it receives. This may be quite reasonable for most of
the deployment cases, for example, the ISP allocating a /48 to a
customer. The CPE (where the tunnel is terminated) "trusts" (in a
weak sense) the ISP's router and the ISP's router can verify that the
Site B is the only one that can originate packets within the /48.
IPsec tunnel mode SA is recommended for this case, though similar
amount of protection can be obtained with transport mode SA with
strict ingress filtering as well.
3.3 Host-to-Host Tunnels
.--------. _----_ .--------.
| V6/V4 | _( IPv4 )_ | V6/V4 |
| Host | <======( Internet )=====> | Host |
| A | (_ _) | B |
'--------' '----' '--------'
IPsec tunnel between
Host A and Host B
Figure 4: Host-to-Host Scenario
IPv6/IPv4 hosts that are interconnected by an IPv4 infrastructure can
tunnel IPv6 packets between themselves. In this case, the tunnel
spans the entire end-to-end path that the packet takes.
In this case, the source and the destination IPv6 address are known a
priori. A tunnel mode SA can be bound to the specific address. The
address verification prevents IPv6 address spoofing completely.
4. Assumptions
Throughout this document we make a few assumptions which are briefly
listed here. The following documents are used as a basis:
o Revised 'Security Architecture for the Internet Protocol' as
described in [I-D.ietf-ipsec-rfc2401bis].
o IKEv2 as described in [I-D.ietf-ipsec-ikev2] and the accompagning
documents for cipher suites and cryptgraphic algorithms (see
[I-D.ietf-ipsec-ikev2-algorithms], [I-D.ietf-ipsec-ikev2-iana] and
[I-D.ietf-ipsec-ui-suites]).
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o 'IP Encapsulating Security Payload (ESP)' as described in
[I-D.ietf-ipsec-esp-v3]
o 'UDP Encapsulation of IPsec Packets' as described in
[I-D.ietf-ipsec-udp-encaps] for the purpose of NAT traversal.
Please note that we do not consider the usage of the IP
Authentication Header (AH) [I-D.ietf-ipsec-rfc2402bis] since Section
3.2 of [I-D.ietf-ipsec-rfc2401bis] specifies that IPsec
implementations MUST implement ESP and MAY implement AH with the
reasoning that ESP provides security services (such as integrity
protection without confidentiality protection using 'NULL'
encryption) which are comparable with AH.
Furthermore, we focus on IKEv2 since [I-D.ietf-ipsec-rfc2401bis]
assumes use of IKEv2 as a key and security association management
system and not IKEv1 with its extensions.
The decision to focus on IKEv2 and newer IPsec documents is based on
the premise that doing so allows using "mixed-mode" transforms as
described below. This is useful for Transport mode SAs. Some
implementations might, however, support these SAs already, at least
using manual configuration.
XXX TBD: the issue may be revisited, and/or "legacy" considerations
added in an Appendix, etc.
In supporting IPv4/IPv6 transition the following functionality
described in section 5.1.2 of [I-D.ietf-ipsec-rfc2401bis] is
particularly useful:
"
o IPsec allows the IP version of the encapsulating header to be
different from that of the inner header.
"
The IPsec framework does not allow IKEv1/IKEv2 to be used to create
tunnels which do not experience cryptographic protection although
this functionality might be useful in some environments. IKEv2 would
then migrate into a secure signaling protocol for tunnel
establishment (without implementing data traffic protection) in a
fashion similar to the 'IPv6 Tunnel Broker with the Tunnel Setup
Protocol (TSP)' [I-D.blanchet-v6ops-tunnelbroker-tsp] protocol
proposal. Section 4.2 of [I-D.ietf-ipsec-rfc2401bis], however,
prohibits this functionality by stating:
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"
A compliant implementation MUST NOT allow instantiation of an ESP SA
that employs both NULL encryption and no integrity algorithm.
"
Regarding the usage of the Explicit Congestion Notification (ECN), it
appears that the ECN bits in the IPv4 and IPv6 headers have exactly
the same semantics, so the bits just need to be copied from the outer
IPv4 header to the inner IPv6 header on tunnel exit.
5. IPsec Configuration Details
This section describes details about the IPsec tunnel establishment
for protection of IPv4/IPv6 data traffic.
5.1 IPsec Transport mode
This is typically used in Router-to-Router scenario.
The following SPD entries assume that there are two routers Router1
and Router2, whose tunnel endpoint's IPv4 address is denoted by
IPV4-TEP1 and IPV4-TEP2 respectively.
Router1's SPD OUT :
IF SRC = IPV4-TEP1 && DST = IPV4-TEP2 && protocol = 41
THEN USE ESP TRANSPORT MODE SA
Router1's SPD IN:
IF SRC = IPV4-TEP2 && DST = IPV4-TEP1 && protocol = 41
THEN USE ESP TRANSPORT MODE SA
Router2's SPD OUT:
IF SRC = IPV4-TEP2 && DST = IPV4-TEP1 && protocol = 41
THEN USE ESP TRANSPORT MODE SA
Router2's SPD IN:
IF SRC = IPV4-TEP1 && DST = IPV4-TEP2 && protocol = 41
THEN USE ESP TRANSPORT MODE SA
The IDci and IDcr payloads of IKEv1 carry the IPv4-TEP1, IPV4-TEP2
and protocol value 41 as phase 2 identities. With IKEv2, the traffic
selectors are used to carry the same information.
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5.2 IPsec Tunnel mode
5.2.1 SPD for Host-to-Host Scenario
The following SPD entries assume that there are two hosts Host1 and
Host2, whose IPv6 addresses are denoted by IPV6-EP1 and IPV6-EP2 and
IPV4 addresses of the tunnel endpoints are denoted by IPV4-TEP1 and
IPV4-TEP2 respectively.
Host1's SPD OUT :
IF SRC = IPV6-EP1 && DST = IPV6-EP2
THEN USE ESP TUNNEL MODE SA:
outer source = IPv4-TEP1
outer dest = IPV4-TEP2
Host1's SPD IN:
IF SRC = IPV4-EP2 && DST = IPV6-EP1
THEN USE ESP TUNNEL MODE SA
outer source = IPV4-TEP2
outer dest = IPV4-TEP1
Host2's SPD OUT:
IF SRC = IPV4-EP2 && DST = IPV6-EP1
THEN USE ESP TUNNEL MODE SA
outer source = IPV4-TEP2
outer dest = IPV4-TEP1
Host2's SPD IN:
IF SRC = IPV6-EP1 && DST = IPV6-EP2
THEN USE ESP TUNNEL MODE SA:
outer source = IPv4-TEP1
outer dest = IPV4-TEP2
The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and IPV6-TEP2
as phase 2 identities. With IKEv2, the traffic selectors are used to
carry the same information.
5.2.2 SPD for Host-to-Router scenario
The following SPD entries assume that the host has the IPv6 address
IPV6-EP1 and the tunnel end points of the host and router are
IPV4-TEP1 and IPV4-TEP2 respectively. If the tunnel is between a
router and a CPE device where the router has allocated a IPV6-PREF/48
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to the CPE device, the corresponding SPD entries can be derived by
substituting IPV6-EP1 by IPV6-PREF/48.
Host's SPD OUT:
IF SRC = IPV6-EP1 && DST = any
THEN use ESP TUNNEL MODE SA
outer source = IPV4-TEP1
outer dest = IPV4-TEP2
Host's SPD IN:
IF SRC = any && DST = IPV6-EP1
THEN use ESP TUNNEL MODE SA
outer source = IPV4-TEP1
outer dest = IPV4-TEP2
Router's SPD OUT:
IF SRC = any && DST = IPV6-EP1
THEN use ESP TUNNEL MODE SA
outer source = IPV4-TEP1
outer dest = IPV4-TEP2
Router's SPD IN:
IF SRC = IPV6-EP1 && DST = any
THEN use ESP TUNNEL MODE SA
outer source = IPV4-TEP1
outer dest = IPV4-TEP2
The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and
ID_IPV6_ADDR_RANGE or ID_IPV6_ADDR_SUBNET as its phase 2 identity.
The starting address is zero IP address and the end address is all
ones for ID_IPV6_ADDR_RANGE. The starting address is zero IP address
and the end address is all zeroes for ID_IPV6_ADDR_SUBNET. With
IKEv2, the traffic selectors are used to carry the same information.
To describe the packet format the following acronyms are used
throughout this document:
o IPV4-TEP1 and IPV4-TEP2 denote the IPv4 address of the tunnel
endpoints.
o IPV6-EP1 and IPV6-EP2 denote the IPv6 address of the two IPv6
endpoints of the communication.
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The packet format is the same for both transport mode and tunnel mode
as shown in Figure 10.
IPv4 header (source = IPV4-TEP1,
destination = IPV4-TEP2)
ESP header
IPv6 header (source = IPV6-EP1,
destination = IPV6-EP2)
Figure 10: Packet Format for transport and tunnel mode
This type of layering may not be valid with [RFC2401] since, with a
strict definition, IP does not meet the definition of a "higher layer
protocol" being the next protocol after an IP header.
With [I-D.ietf-ipsec-rfc2401bis] the definition about the "next layer
protocol" was explicitly expanded and hence this type of layering is
valid.
6. Dynamic Address Configuration
With the exchange of protected configuration payloads, IKEv2 is able
to provide the IKEv2 peer with DHCP-like information payloads. These
configuration payloads are exchanged between the IKEv2 initiator and
the responder with the help of the CFG_REQUEST/CFG_REPLY and CFG_SET/
CFG_ACK payloads. The former is used to request information and the
latter allows to push configuration data. Configuration information
(e.g., a temporary address) can be carried in any request to create a
CHILD_SA by including a CP payload.
These configuration payloads are primiarly used for bootstrapping the
IKEv2 peer. Although these payloads are extensible they are not used
as a generic purpose management.
The following example exchange illustrates the usage of configuration
payloads:
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Initiator Responder
------------- --------------
HDR, SAi1, KEi, Ni -->
<-- HDR(A,0), N(COOKIE)
HDR(A,0), N(COOKIE), SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr,[CERTREQ]
HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, CP(CFG_REQUEST), SAi2, TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
CP(CFG_REPLY), SAr2, TSi, TSr}
Figure 11: IKEv2 Configuration payload payload exchange
The example message exchange shown in Figure 11 shows IKEv2 with a
denial of service protection enabled exchange and a CFG_REQUEST in
message 5 and the corresponding response in message 6. The content
of these payloads, for example, contains (as given in Section 2.19 of
[I-D.ietf-ipsec-ikev2]):
CP(CFG_REQUEST)=
INTERNAL_ADDRESS(0.0.0.0)
INTERNAL_NETMASK(0.0.0.0)
INTERNAL_DNS(0.0.0.0)
TSi = (0, 0-65536,0.0.0.0-255.255.255.255)
TSr = (0, 0-65536,0.0.0.0-255.255.255.255)
CP(CFG_REPLY)=
INTERNAL_ADDRESS(10.168.219.202)
INTERNAL_NETMASK(255.255.255.0)
INTERNAL_SUBNET(10.168.219.0/255.255.255.0)
TSi = (0, 0-65536,10.168.219.202-10.168.219.202)
TSr = (0, 0-65536,10.168.219.0-10.168.219.255)
7. Extensible Authentication Support
In addition to the authentication mechanisms provided in IKEv2 the
Extensible Authentication Protocol (EAP) [I-D.ietf-eap-rfc2284bis] is
included which provides some flexibility for authentication
mechanisms. Figure 13 shows an example IKEv2 exchange with EAP
support. The usage of EAP offers two interesting features:
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o User authentication is terminated at a different entity other than
the IKEv2 responder. This allows users' security credentials to
be kept in a central place (e.g., AAA server) and to terminate the
EAP method at this entity instead at the IKEv2 responder.
Authorization can also be executed at the same entity.
o A number of authentication and key exchange protocols are
supported via EAP method (such as EAP-AKA, EAP-SIM, SRP, etc.).
Each EAP methods provides its own properties and usage
environment. This provides a certain degree of flexibility.
Note that IKEv2 with EAP authentication still requires public key
based authentication of the IKEv2 responder outside the EAP
authentication. In most deployments this requires a server-side
public-key based authentication to protect the EAP exchange with a
uni-lateral authenticated tunnel. With the extensions proposed in
[I-D.eronen-ipsec-ikev2-eap-auth] only EAP authentication is used by
omitting the IKEv2 responder authentication.
Please note that Section 3.16 of [I-D.ietf-ipsec-ikev2] indicates
that the EAP Identity-Request/Identity-Response payload SHOULD NOT be
used. The IDr payload (message 3 in Figure 13) carries this identity
instead. As a consequence active user identity confidentiality for
the IKEv2 initiator is not provided. Special purpose EAP methods
must be used instead if this features is desired.
Figure 13 shows an example IKEv2 message exchange with EAP-AKA as an
EAP method. Note that the interaction between the IKEv2 responder
and the AAA infrastructure is not shown.
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Initiator Responder
------------- --------------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK {IDi, [CERTREQ,] [IDr,]
SAi2, TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
EAP-Request/AKA-Challenge (AT_RAND, AT_AUTN, AT_MAC) }
HDR, SK {EAP-Response/AKA-Challenge(AT_RES, AT_MAC)} -->
<-- HDR, SK {EAP-Success, AUTH}
HDR, SK { AUTH } -->
<-- HDR, SK { SAr2, TSi, TSr }
Figure 13: EAP usage in IKEv2 with EAP-AKA
EAP will typically be used with a backend AAA server which raises
some security concerns. See [I-D.ietf-eap-keying] for a more
complete discussion of these security issues.
When a backend server is used, there are actually two authentication
exchanges: the EAP method between the client and the AAA server, and
another authentication between the AAA server and IKEv2 gateway. The
AAA server authenticates the client using the selected EAP method,
and they establish a session key. The AAA server then sends this key
to the IKEv2 gateway over a connection authenticated using e.g.
IPsec or TLS. The protocol used between the IKEv2 responder and the
AAA server could be, for instance, Diameter or RADIUS [RFC3579].
RADIUS and Diameter are able to carry EAP payloads as described in
[RFC3579] and in [I-D.ietf-aaa-eap], respectively.
8. NAT Traversal
Network address (and port) translation devices are commonly found in
today's networks. A detailed description of the problem of IPsec
protected data traffic traversing a NAT including requirements are
discussed in [RFC3715].
IKEv2 can detect the presence of a NAT automatically by sending an
Informational exchange with NAT_DETECTION_SOURCE_IP and
NAT_DETECTION_DESTINATION_IP payloads before establishing an IPsec
SA. These payloads are processed the same way as in the initial
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IKE_SA_INIT exchange. Once a NAT is detected and both end points
support IPsec NAT traversal extensions UDP encapsulation can be
enabled.
More details about UDP encapsulation of IPsec protected IP packets
can be found in [I-D.ietf-ipsec-udp-encaps].
For IPv6-over-IPv4 tunneling, NAT traversal is interesting for two
reasons:
1. One of the tunnel endpoints is often behind a NAT, and configured
tunneling, using protocol 41, is not guaranteed to traverse the
NAT. Hence, using IPsec tunnels would enable one to both set-up
a secure tunnel, and set-up a tunnel where it might not always be
possible without other tunneling mechanisms.
2. Using NAT traversal allows the outer address to change without
having to renegotiate the SAs. This could be very beneficial for
a crude form of mobility, and in scenarios the NAT changes the IP
addresses frequently. However, as the outer address may change,
this might introduce new security issues, and using tunnel mode
would be most appropriate.
XXX: can you force the use of NAT traversal so that you don't need to
renegotiate SAs even if you aren't behind a NAT -- could be useful
for MOBIKE applications.
9. Tunnel Endpoint Discovery
The IKEv2 initiator needs to know the address of the IKEv2 responder
for IKEv2 signaling. A number of ways can be used to provide the
initiator with this information, for example:
o Using off-band mechanisms, e.g., from the ISP's web page.
o Using DNS to look up a service name by appending it to the DNS
search path provided by DHCPv4 (e.g.
"tunnel-service.example.com").
o Using a DHCP option.
o Using a pre-configured or pre-determined IPv4 anycast address.
o Using other, unspecified or proprietary methods such as TED (see
[I-D.fluhrer-ted]).
For the purpose of this document it is assumed that this address can
be obtained somehow. Once the address has been learned, it is
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configured as the tunnel end-point for the configured IPv6-over-IPv4
tunnel.
10. Example
TBD: Full-fledged example
11. Security Considerations
When you run IPv6-in-IPv4 tunnels (unsecured) over the Internet, it
is possible to "inject" packets in the tunnel by spoofing the source
address (data plane security), or if the tunnel is signalled somehow
(e.g., some messages where you authenticate to the server, so that
you would get a static v6 prefix), someone might be able to spoof the
signalling (control plane security).
To add security to both, the protocol for tunnel setup and to the
data traffic, the IPsec framework plays an important role.
IKEv2 is a signaling protocol with optional Denial of Service
protection which authenticates both end points (with different
identifities) and establishes two types of security associations
(CHILD-SAs and IKE-SA). The authentication mechanisms are very
flexible due to the built-in support for symmetric and asymmetric
cryptography (or even a combination of both) and the Extensible
Authentication Protocol support (as desribed in Figure 13). The
IKE-SA is used to secure most of the IKEv2 message exchange. In
particular the CHILD-SA exchange, Informational exchanges (such as
the dead-peer detection mechanisms used for liveness checks) and the
exchange of configuration messages are secured. The CHILD-SA
exchange leads to the establishment of a IPsec tunnel and the
creation of SAD and SPD entries.
As a summary, IKEv2 provides a secure signaling protocol for
establishing, maintaining and deleting an IPsec tunnel.
IPsec, with the Encapsulating Security Payload (ESP), offers
integrity and data origin authentication, confidentiality, with
optional (at the discretion of the receiver) anti-replay features.
The usage of confidentity-only is discouraged. ESP furthermore
provides limited traffic flow confidentality.
IPsec provides access control mechanisms through the distribution of
keys and also through the usage of policies dictated by the Security
Policy Database (SPD). Furthermore, through the usage of EAP and the
backend AAA infrastructure it is possible to enforce additional
authorization mechanisms (at the user level) at entities other than
the tunnel end points.
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The NAT traversal mechanism provided by IKEv2 introduces some
weaknesses into IKEv2 and IPsec. These issues are discussed in more
detail in [I-D.ietf-ipsec-ikev2].
Please note that the usage of IPsec for the scenarios described in
Figure 3, Figure 2 and Figure 1 does not aim to protect the
end-to-end communication. It protects just the tunnel part. It is
still possible for an IPv6 endpoint that is not attached to the IPsec
tunnel to spoof packets.
12. Open Issues
This section lists some open issues that will be resolved in future
versions of this document.
o More considerations of IKEv1 might be useful.
o Discussion of 'Use of IPsec Transport Mode for Dynamic Routing'
[I-D.touch-ipsec-vpn] would be appropriate.
o A more detailed description of the address configuration mechanism
would be helpful. The configuration example with CFG_REQUEST/
CFG_REPLY payloads should contain IPv6 addresses.
o The full-fledged example of Section 10 is still missing.
o Some notes on the implications of mobility interworking are still
missing.
o Discuss the use of link-local etc. messages with Tunnel mode SAs
-- i.e., how many SAs will be needed (and how they are negotiated)
if link-local messages will be present as well?
o The "Site-to-Router" scenarios separation is a bit weak -- any
better ideas how to categorize these would be appreciated.
o Better discussion of when transport/tunnel mode SAs make more
sense would probably be useful.
13. Contributors
Please note that the authors are listed in alphabetical order.
Suresh Satapati also participated in the discussions.
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14. Acknowledgments
The authors would like to thank Stephen Kent and Michael Richardson
for their comments.
We would like to thank Pasi Eronen for his text contributions.
15. References
15.1 Normative References
[I-D.ietf-eap-rfc2284bis]
Blunk, L., Vollbrecht, J., Aboba, B., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
draft-ietf-eap-rfc2284bis-07 (work in progress), December
2003.
[I-D.ietf-ipsec-esp-v3]
Kent, S., "IP Encapsulating Security Payload (ESP)",
draft-ietf-ipsec-esp-v3-08 (work in progress), March 2004,
<reference.I-D.ietf-ipsec-esp-v3.xml>.
[I-D.ietf-ipsec-ikev2]
Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-13 (work in progress), March 2004,
<reference.I-D.ietf-ipsec-ikev2.xml>.
[I-D.ietf-ipsec-ikev2-algorithms]
Schiller, J., "Cryptographic Algorithms for use in the
Internet Key Exchange Version 2",
draft-ietf-ipsec-ikev2-algorithms-04 (work in progress),
September 2003,
<reference.I-D.ietf-ipsec-ikev2-algorithms.xml>.
[I-D.ietf-ipsec-rfc2401bis]
Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", draft-ietf-ipsec-rfc2401bis-01 (work
in progress), January 2004,
<reference.I-D.ietf-ipsec-rfc2401bis.xml>.
[I-D.ietf-ipsec-udp-encaps]
Huttunen, A., "UDP Encapsulation of IPsec Packets",
draft-ietf-ipsec-udp-encaps-08 (work in progress),
February 2004, <reference.I-D.ietf-ipsec-udp-encaps.xml>.
[I-D.ietf-ipsec-ui-suites]
Hoffman, P., "Cryptographic Suites for IPsec",
draft-ietf-ipsec-ui-suites-04 (work in progress), August
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2003, <reference.I-D.ietf-ipsec-ui-suites.xml>.
[I-D.ietf-v6ops-mech-v2]
Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-02
(work in progress), February 2004,
<reference.I-D.ietf-v6ops-mech-v2.xml>.
15.2 Informative References
[I-D.bellovin-useipsec]
Bellovin, S., "Guidelines for Mandating the Use of IPsec",
draft-bellovin-useipsec-03 (work in progress), March 2004,
<reference.I-D.bellovin-useipsec.xml>.
[I-D.blanchet-v6ops-tunnelbroker-tsp]
Blanchet, M., "IPv6 Tunnel Broker with the Tunnel Setup
Protocol(TSP)", draft-blanchet-v6ops-tunnelbroker-tsp-00
(work in progress), February 2004,
<reference.I-D.blanchet-v6ops-tunnelbroker-tsp.xml>.
[I-D.dupont-ikev2-addrmgmt]
Dupont, F., "Address Management for IKE version 2",
draft-dupont-ikev2-addrmgmt-04 (work in progress),
February 2004, <reference.I-D.dupont-ikev2-addrmgmt.xml>.
[I-D.eronen-ipsec-ikev2-eap-auth]
Eronen, P., "Extension for EAP Authentication in IKEv2",
draft-eronen-ipsec-ikev2-eap-auth-00 (work in progress),
February 2004,
<reference.I-D.eronen-ipsec-ikev2-eap-auth.xml>.
[I-D.fluhrer-ted]
Fluhrer, S., "Tunnel Endpoint Discovery",
draft-fluhrer-ted-00 (work in progress), November 2001,
<reference.I-D.fluhrer-ted.xml>.
[I-D.ietf-aaa-eap]
Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application",
draft-ietf-aaa-eap-03 (work in progress), October 2003.
[I-D.ietf-eap-keying]
Aboba, B., Simon, D., Arkko, J. and H. Levkowetz, "EAP Key
Management Framework", draft-ietf-eap-keying-01 (work in
progress), October 2003.
[I-D.ietf-ipsec-ikev2-iana]
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Richardson, M., "Initial IANA registry contents",
draft-ietf-ipsec-ikev2-iana-01 (work in progress), January
2004, <reference.I-D.ietf-ipsec-ikev2-iana.xml>.
[I-D.ietf-ipsec-rfc2402bis]
Kent, S., "IP Authentication Header",
draft-ietf-ipsec-rfc2402bis-07 (work in progress), March
2004, <reference.I-D.ietf-ipsec-rfc2402bis.xml>.
[I-D.ietf-pana-ipsec]
Parthasarathy, M., "PANA enabling IPsec based Access
Control", draft-ietf-pana-ipsec-02 (work in progress),
February 2004, <reference.I-D.ietf-pana-ipsec.xml>.
[I-D.kivinen-mobike-design]
Kivinen, T., "Design of the MOBIKE protocol",
draft-kivinen-mobike-design-00 (work in progress), March
2004, <reference.I-D.kivinen-mobike-design.xml>.
[I-D.touch-ipsec-vpn]
Touch, J., Eggert, L. and Y. Wang, "Use of IPsec Transport
Mode for Dynamic Routing", draft-touch-ipsec-vpn-07 (work
in progress), March 2004,
<reference.I-D.touch-ipsec-vpn.xml>.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998,
<reference.RFC.2401.xml>.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999, <reference.RFC.2663.xml>.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004,
<reference.RFC.3704.xml>.
[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004,
<reference.RFC.3715.xml>.
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Authors' Addresses
Richard Graveman
RFG Security, LLC
15 Park Avenue
Morristown, New Jersey 07960
USA
EMail: rfg@acm.org
Mohan Parthasarathy
Nokia
313 Fairchild Drive
Mountain View CA-94043
USA
EMail: mohanp@sbcglobal.net
Pekka Savola
CSC/FUNET
Espoo
Finnland
EMail: psavola@funet.fi
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
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