Network Working Group P. Nikander
Internet-Draft Ericsson Research Nomadic Lab
Expires: April 8, 2004 October 9, 2003
A Bound End-to-End Tunnel (BEET) mode for ESP
draft-nikander-esp-beet-mode-00
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies a new mode, called Bound End-to-End Tunnel
(BEET) mode, for IPsec ESP. The new mode augments the existing ESP
tunnel and transport modes. For end-to-end tunnels, the new mode
provides limited tunnel mode semantics without the regular tunnel
mode overhead. The mode is intended to support new uses of ESP,
including mobility and multi-address multi-homing.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . 4
2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Related work . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Use scenarios . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 NAT traversal . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Mobile IP . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.1 Mobile IPv4 . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.2 Mobile IPv6 . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3 End-node multi-address multi-homing . . . . . . . . . . . . 9
4.4 Host Identity Protocol . . . . . . . . . . . . . . . . . . . 10
5. Protocol definition . . . . . . . . . . . . . . . . . . . . 12
5.1 Changes to Security Association data structures . . . . . . 12
5.2 Packet format . . . . . . . . . . . . . . . . . . . . . . . 12
5.3 Cryptographic processing . . . . . . . . . . . . . . . . . . 13
5.4 IP header processing . . . . . . . . . . . . . . . . . . . . 14
5.5 Handling of outgoing packets . . . . . . . . . . . . . . . . 14
5.6 Handling of incoming packets . . . . . . . . . . . . . . . . 15
6. Policy considerations . . . . . . . . . . . . . . . . . . . 16
7. PF_KEY extensions . . . . . . . . . . . . . . . . . . . . . 17
8. New requirements on Key Management protocols . . . . . . . . 18
9. Implementing the functionality with other means . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 22
Normative references . . . . . . . . . . . . . . . . . . . . 23
Informative references . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . 24
A. Implementation experiences . . . . . . . . . . . . . . . . . 25
B. Garden beets . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . 27
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1. Introduction
The current IPsec ESP specification [4] defines two modes of
operation: tunnel mode and transport mode. The tunnel mode is mainly
intended for non-end-to-end use where one or both of the ends of the
ESP Security Associations (SAs) are located in security gateways,
separate from the actual end-nodes. The transport mode is intended
for end-to-end use, where both ends of the security association are
terminated at the end-nodes themselves.
This document defines a new mode for ESP, called Bound End-to-End
Tunnel (BEET) mode. The purpose of the mode is to provide limited
tunnel mode semantics without the overhead associated with the
regular tunnel mode. As the name states, the BEET mode is intended
solely for end-to-end use. It provides tunnel mode semantics in the
sense that the IP addresses seen by the applications and the IP
addresses used on the wire are distinct from each other, providing
the illusion that the application level IP addresses are tunneled
over the network level IP addresses. However, the mode does not
support full tunnel semantics. More specifically, the IP addresses
as seen by the application are strictly bound, and only one pair of
bound addresses can be used on any given BEET mode Security
Association. This is in contrast to the regular tunnel mode, where
the inner IP addresses can be any addresses from a defined range.
An BEET mode Security Associations records two pairs of IP addresses,
called inner addresses and outer addresses. The inner addresses are
what the applications see. The outer addresses are what appear on
the wire. Since the inner addresses are fixed for the life time of
the Security Association, they need not to be sent in individual
packets. Instead, they are set up as the Security Associations are
created, they are verified when packets are sent, and they are
restored as packets are received.
This all gives the BEET mode the efficienty of transport mode with a
limited set of end-to-end tunnel semantics. The semantics are
limited in the sense that only one fixed pair of inner addresses are
allowed. The outer addresses may change over the life time of the
SA, but the inner addresses cannot. If a new pair of inner addresses
is needed, a new pair of BEET mode Security Associations must be
established, or the regular tunnel mode must be used. However, in
the cases considered, a single pair of security associations is
usually sufficient between any single pair of nodes.
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2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [1].
This document contains both normative and informative sections. The
normative sections define the BEET mode. The informative sections
provide background information that aim to motivate the need for the
new mode. Whenever it may not be clear from the context whether a
given major section is normative or informative, it is defined in the
beginning of the section.
2.1 Terminology
In this section we define the terms specific to this document. This
section is normative.
Inner IP address An IP address as seen by the applications, stored in
TCB or other upper layer data structures, and processed by the IP
stack prior to ESP processing in the output side and after ESP
processing in the input side.
Outer IP address An IP address as seen in the wire and processed by
the IP stack after ESP processing in the output side and before
ESP processing in the input side.
Inner IP header An IP header that contains inner IP addresses. In
some cases an inner IP header may be represented as an internal
data structure containing the data equivalent to an IP header.
Outer IP header An IP header that contains outer IP addresses. In
some cases an outer IP header may be represented as an internal
data structure containing the data equivalent to an IP header.
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3. Background
For a number of years people have been talking about using IPsec for
other purposes than VPN. In fact, the current specifications do
provide support for end-to-end protection of data. However, that
mode is rarely used, for a number of reasons [5], [6]. One of the
reasons, though, seems to be address agility. That is, due to NAT,
mobility, multi-address multi-homing, etc., the addresses that are
used actually on the wire do not necesarily match with the addresses
that the applications expect to see. In the NAT case the addresses
are changed on the fly, thereby invalidating any transport mode
checksums (unless, of course, a tunnel is used). Mobile nodes change
their addresses periodically, and the existing applications rarely
survive the address changes without some help, e.g., Mobile IP.
Multi-addressing based multi-homed nodes would prefer to keep their
connections active even when the primary (or currently used) IP
address becomes unusable in the face of an network outage.
Based on the reasons above, there is clearly a need for a mode of
communication where the addresses that the applications see are
distinct from the addresses that are actually used in the wire. The
current IPsec tunnel mode provides the required functionality, but at
the cost of additional overhead in terms of larger packets and more
complicated processing.
3.1 Related work
The basic idea captured by this draft has been floating around for a
long time. Steven Bellovin's HostNAT talk [7] at the Los Angeles
IETF is an early example. After that, basically the same idea has
surfaced several times. Perhaps the most concrete current proposal
is the Host Identity Protocol (HIP) [9], where BEET mode ESP
processing is an integral part of the overall protocol.
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4. Use scenarios
In this section we describe a number of possible use scenarios. None
of these use scenarios are meant to be complete specifications on how
exactly to support the functionality. Separate specifications are
needed for that. Instead, the purpose of this section is to discuss
the overall benefits of the BEET mode, and to lay out a road map for
possible future documents. This section is informative.
4.1 NAT traversal
NAT traversal is currently a major problem in IPsec. It is not
sufficient to encapsulate the packets into UDP; additionally, tunnel
mode must be used. Tunnel mode is required since the outer IP
addresses at the ends of the protected connection differ. If
transport mode was used, the differing IP addresses would lead to
failing upper layer TCP/UDP checksums.
The BEET mode provides sufficient tunnel mode semantics without the
packet overhead of the tunnel mode. A pair of BEET mode SAs can be
effectively used to "un-NAT" packets that have been NATed during
their travel through the network. Figure Figure 1 illustrates the
process.
Packet contents on a client -> server packet
+--------+
| Client | src = 131.160.175.2 dst = 129.15.6.1 clear text
+--------+ ^
| 10.0.0.1 |
| | src = 10.0.0.1 dst = 129.15.6.1 ESP
| |
+-----+ |
| NAT | SAs
+-----+ |
| 131.160.175.2 |
| | src = 131.160.175.2 dst = 129.15.6.1 ESP
| 129.15.5.1 |
+--------+ v
| Server | src = 131.160.175.2 dst = 129.15.6.1 clear text
+--------+
Figure 1
A drawback in this scheme is that the Client must either know its
public IP address, or it must rely on the Server to tell what address
to use. It must be noticed that if the NAT box is mapping several
internal IP addresses into a single public address, the public
address cannot be directly used. In that case the client and server
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need to agree on a unique address, to be used to internally represent
the client. It must be pointed out that such an address is
semantically very similar to a Mobile IP home address. The details
of such address agreement are beyound the scope of this document.
4.2 Mobile IP
In Mobile IP, the BEET mode could be used instead of the currently
defined wire formats. If the hosts would be using end-to-end ESP
anyway, this has the benefit of saving the space that would otherwise
be taken by the standard Mobile IP wire formats. Furthermore, in
BEET the inner IP header does not actually appear in the wire format.
Effectively, this makes BEET as space efficient for mobile nodes as
the standard ESP transport mode is today between fixed hosts.
Instead of having a separate Binding Cache, the nodes could include
the address translation information into a pair of BEET mode security
associations.
4.2.1 Mobile IPv4
In the current Mobile IPv4, two different wire formats are used,
depending on whether there is a NAT device between the communicating
hosts or not. See Figure 2, below.
Mobile IPv4 wire format without NAT traversal
IP(CoA->HA) | IP(HoA->CN) | payload
Mobile IPv4 wire format with NAT traversal
IP(CoA->HA) | UDP(any->434) | MIP header | IP(HoA->CN) | payload
(where the MIP header is a minimal 4 octet header)
[Figure courtesy to Sami Vaarala.]
Figure 2
It is required that the inner address representing the mobile node,
as seen by the application, is always the home address. That is,
from the application point of view, the packets flow between the home
address and the correspondent node address.
If IPsec is used to protect the traffic between the Mobile Node and
the Correspondent node, ESP transport mode can be used. However, the
transport mode ESP packet is enclosed into an IP-over-IP wrapper at
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the home agent, see Figure 3.
Current Mobile IPv4 wire format with end-to-end ESP transport mode:
CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer
HA -> MN: IP(HA->CoA) | IP(CN->HoA) | ESP | payload | ESP trailer
MN -> HA: IP(CoA->HA) | IP(HoA->CN) | ESP | payload | ESP trailer
HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer
Proposed Mobile IPv4 wire format with ESP BEET mode:
CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer
HA -> MN: IP(HA->CoA) | ESP | payload | ESP trailer
MN -> HA: IP(CoA->HA) | ESP | payload | ESP trailer
HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer
Figure 3
In this scenario, the correspondent node does not need to be aware
that the security association is in fact using the BEET mode. If the
home agent and the mobile node co-operate, and the mobile node
implements the BEET semantics, the change could be implemented
transparently to the correspondent node.
It should be noticed that the space savings are even larger in the
NAT traversal situation, as is illustrated in Figure Figure 4, below.
Current Mobile IPv4 NAT-traversal wire format with end-to-end transport ESP:
CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer
HA -> MN: IP(HA->CoA) | UDP | MIP | IP(CN->HoA) | ESP | payload | ESP trailer
MN -> HA: IP(CoA->HA) | UDP | MIP | IP(HoA->CN) | ESP | payload | ESP trailer
HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer
Proposed Mobile IPv4 NAT-traversal wire format with BEET ESP:
CN -> HA: IP(CN->HoA) | ESP | payload | ESP trailer
HA -> MN: IP(HA->CoA) | UDP | ESP | payload | ESP trailer
MN -> HA: IP(CoA->HA) | UDP | ESP | payload | ESP trailer
HA -> CN: IP(HoA->CN) | ESP | payload | ESP trailer
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XXX: Is this format identical to NAT traversal with the
exception of using BEET instead of tunnel mode?
Figure 4
4.2.2 Mobile IPv6
Triangular routing in Mobile IPv6 is similar to that of Mobile IPv4.
However, the tunnel between the home agent and the mobile node is an
ESP tunnel instead of being a plain IP-over-IP tunnel. However, if
BEET mode was used between the correspondent node and the mobile
node, the ESP tunnel between the home agent and the mobile node would
not bring any additional protection to the payload data. Thus, in
that case BEET could replace the ESP tunnel, similar to the IPv4
case, illustraged in Figure 3 above.
Mobile IPv6 Route Optimization uses a Type 2 Routing Header (RH) and
Home Address Option (HAO) in the packet wire format. However, it can
be argued that the semantics of these options is equivalent to a
optimized point-to-point tunnel. That is, the Type 2 RH defines the
real destination address of a packet, thereby effectively creating a
partial tunnel where the inner and outer source addresses are
identical but the destination addresses differ. Similarily, the Home
Address Option defines the real source address of the packet, again
creating a partial tunnel. The only difference is that this time the
inner and outer destination addresses are identical but the source
addresses differ.
Thus, for Mobile IPv6, BEET mode would define a different wire format
for the payload packets. Instead of using Type 2 RH and HAO, the
packets could be encapsulated into an BEET mode ESP tunnel. In the
case that ESP is used anyway, this has the advantage that the
standard Mobile IPv6 extra headers are not needed, thereby saving
bytes in the headers. Compared to tunnel mode ESP, BEET mode has the
advantage that the inner IP header is not needed.
In Mobile IPv6, mobility management can be implemented just as
before, using the HoTI/CoTI, HoT/CoT and Binding Update (BU)
messages. The difference would lay in handling Binding Updates. If
BEET mode was used, processing Binding Updates would change the outer
IP addresses in the BEET mode Security Associations instead of
changing the Binding Cache.
4.3 End-node multi-address multi-homing
The BEET mode provides for limited end-node multi-address
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multi-homing. It semantically provides a tunnel between the
end-hosts, with fixed inner IP addresses. This allows a multi-homed
host to use different outer IP addresses in different packets,
without any notice by the upper layer protocols. The upper layer
protocols see the inner IP address at all times. Thus, this limited
form of multi-homing has no affect on the applications, which
seemingly communicate over fixed IP addresses all the time.
Implementing this kind of limited multi-homing support would require
a small change to the current IPsec SPD and SA implementations.
Currently the incoming SA selection is based on the SPI and
destination address, with the implicit assumption that there is only
one possible destination address for each incoming SA. In a
multi-homed host it would be desirable to have multiple destination
addresses associated with the SA, thereby allowing the same SA to be
used independent on the actual destination address in the packets.
Removing the destination address from unicast SA lookup is already
being proposed in the current ESP draft [4].
If it is considered undesirable to change the implementations to
support multiple alternative destination addresses, it would still be
possible to support limited multi-homing by creating several parallel
SAs, one for each destination address. Each of these SAs would have
identical inner addresses. Effectively, this would distribute the
tunnel over multiple SAs.
In this latter implementation, the outgoing SA processing becomes
more complex. Selecting the outgoing SA does not depend only on the
inner IP addresses but also on the outer destination address.
Selecting the outer destination address depends on the current
multi-homing situation. This creates a situation where the SA
processing must be defered after selecting the actual outer address
to be used. This might be difficult in some implementations.
4.4 Host Identity Protocol
The Host Identity Protocol (HIP) as a piece of more recent
development. Its aim is to explore the possibilities created by
separating the end-host identifier and locator natures of IP
addresses. There are currently four implementations, and the
specificatoins are being finalized. [8] [9]
In HIP, the TCP and UDP sockets are not bound to IP addresses but to
Host Identifiers. The Host Identifiers (HI) create a new independent
name space.
The BEET mode could be used to support HIP by defining the inner
tunnel in terms of Host Identifiers and the outer tunnel in terms of
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standard IP addresses. In that way all processing prior to outgoing
ESP and after incoming ESP uses Host Identifiers. The wire format
packets use standard IP addresses and ESP transport packet format.
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5. Protocol definition
In this section we define the exact protocol formats and operations.
This section is normative.
5.1 Changes to Security Association data structures
An BEET mode Security Association contains the same data as a regular
tunnel mode Security Association, with the exception that the inner
selectors must be single addresses and cannot be subnets. The data
includes the following:
A pair of inner IP addresses.
A pair of outer IP addresses.
Cryptographic keys and other data as defined in RFC2401 [3]
Section 4.4.3.
A conforming implementation MAY store the data in a way different
than or similar to a regular tunnel mode Security Association.
Note that in a conforming implementation the inner and outer
addresses MAY belong to different address families. All
implementations that support both IPv4 and IPv6 SHOULD support both
IPv4-over-IPv6 and IPv6-over-IPv4 tunneling.
5.2 Packet format
The wire packet format is identical to the ESP transport mode wire
format as defined in [4] Section 3.1.1. However, the resulting
packet contains outer IP addresses instead the inner IP addresses as
received from the upper layer. The construction of the outer headers
is defined in RFC2401 [3] Section 5.1.2. The following diagram
illustrates ESP BEET mode positioning for typical IPv4 and IPv6
packets.
IPv4 INNER ADDRESSES
--------------------
BEFORE APPLYING ESP
------------------------------
| inner IP hdr | | |
| (any options) | TCP | Data |
------------------------------
AFTER APPLYING ESP, OUTER v4 ADDRESSES
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----------------------------------------------------
| outer IP hdr | | | | ESP | ESP |
| (any options) | ESP | TCP | Data | Trailer | ICV |
----------------------------------------------------
|<---- encryption ---->|
|<-------- integrity ------->|
AFTER APPLYING ESP, OUTER v6 ADDRESSES
------------------------------------------------------
| outer | new ext | | | | ESP | ESP |
| IP hdr | hdrs. | ESP | TCP | Data | Trailer| ICV |
------------------------------------------------------
|<--- encryption ---->|
|<------- integrity ------->|
IPv6 INNER ADDRESSES
--------------------
BEFORE APPLYING ESP
------------------------------------------
| | ext hdrs | | |
| inner IP hdr | if present | TCP | Data |
------------------------------------------
AFTER APPLYING ESP, OUTER v6 ADDRESSES
--------------------------------------------------------------
| outer | new ext | | dest | | | ESP | ESP |
| IP hdr | hdrs.* | ESP | opts.| TCP | Data | Trailer | ICV |
--------------------------------------------------------------
|<---- encryption ---->|
|<------- integrity ------>|
AFTER APPLYING ESP, OUTER v4 ADDRESSES
----------------------------------------------------
| outer | | dest | | | ESP | ESP |
| IP hdr | ESP | opts.| TCP | Data | Trailer | ICV |
----------------------------------------------------
|<------- encryption -------->|
|<----------- integrity ----------->|
5.3 Cryptographic processing
The outgoing packets MUST be protected exactly as in ESP transport
mode [4]. That is, the upper layer protocol packet is wrapped into
an ESP header, encrypted, and authenticated exactly as if regular
transport mode was used. The resulting ESP packet is subject to IP
header processing as defined in Section 5.4 and Section 5.5. The
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incoming ESP protected messages are verified and decrypted exactly as
if regular transport mode was used. The resulting cleartext packet
is subject to IP header processing as defined in Section 5.4 and
Section 5.6.
5.4 IP header processing
The biggest difference between the BEET mode and the other two modes
is in IP header processing. In the regular transport mode the IP
header is kept intact. In the regular tunnel mode an outer IP header
is created on output and discarded on input. In the BEET mode the IP
header is replaced with another one on both input and output.
On the BEET mode output side, the IP header processing MUST first
ensure that the IP addresses in the original IP header contain the
inner addresses as specified in the SA. This MAY be ensured by
proper policy processing, and it is possible that no checks are
needed at the SA processing time. Once the IP header has been
verified to contain the right IP inner addresses, it is discarded. A
new IP header is created, using the discarded inner header as a hint
for other fields but the IP addresses. The IP addresses in the new
header MUST be the outer tunnel addresses.
On input side, the received IP header is simply discarded. Since the
packet has been decrypted and verified, no further checks are
necessary. A new IP header, corresponding to a tunnel mode inner
header, is created, using the discarded outer header as a hint for
other fields but the IP addresses. The IP addresses in the new header
MUST be the inner addresses.
5.5 Handling of outgoing packets
The outgoing BEET mode packets are processed as follows:
1. The system MUST verify that the IP header contains the inner
source and destination addresses, exactly as defined in the SA.
This verification MAY be explicit, or it MAY be implicit, for
example, as a result of prior policy processing. Note that in
some implementations there may be no real IP header at this time
but the source and destination addresses may be carried
out-of-band. In the case where the source address is still
unassigned, it SHOULD be made sure that the designated inner
source address would have been selected at a later stage.
2. The IP payload (the contents of the packet beyond the IP header)
is wrapped into an ESP header as defined in [4] Section 3.3.
3. A new IP header is constructed, replacing the original one. The
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new IP header MUST contain the outer source and destination
addresses, as defined in the SA. Note that in some
implementations there may be no real IP header at this time but
the source and destination addresses may be carried out-of-band.
In the case where the source address must be left unassigned, it
SHOULD be made sure that the right source address is selected at
a later stage. Other than the addresses, it is RECOMMENDED that
the new IP header copies the fields from the original IP header.
4. If there are any IPv4 header options in the original packet, they
are simply discarded.
Instead of literally discarding the IP header and constructing a new
one a conforming implementation MAY simply replace the addresses in
an existing header. However, if the RECOMMENED feature of allowing
the inner and outer addresses from different address families is
used, this simple strategy does not work.
5.6 Handling of incoming packets
The incoming BEET mode packets are processed as follows:
1. The system MUST verify and decrypt the incoming packet
successfully, as defined in [4] section 3.4. If the verification
or decryption fails, the packet MUST be discarded.
2. The original IP header is simply discarded, without any checks.
Since the ESP verification succeeded, the packet can be safely
assumed to have arrived from the right sender.
3. A new IP header is constructed, replacing the original one. The
new IP header MUST contain the inner source and destination
addresses, as defined in the SA. Note that in some
implementations the real IP header may have already been
discarded and the source and destination addresses are carried
out-of-band. In such case the out-of-band addresses MUST be the
inner addresses. Other than the addresses, it is RECOMMENDED
that the new IP header copies the fields from the original IP
header.
Instead of literally discarding the IP header and constructing a new
one a conforming implementation MAY simply replace the addresses in
an existing header. However, if the RECOMMENED feature of allowing
the inner and outer addresses from different address families is
used, this simple strategy does not work.
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6. Policy considerations
In this section we describe how the BEET mode affects on IPsec policy
processing. This section is normative.
An BEET Security Association MUST NOT be used with NULL
authentication. The only exception to this is testing where NULL
authentication MAY be used for diagnostive purposes.
On the output side, the IPsec policy processing mechanism SHOULD take
care that only packets with IP addresses matching with the inner
addresses of a Security Association are passed to that Security
Association. If the policy mechanism do not provide full assurance
on this, the SA processing MUST check the addresses. Further policy
distinction may be specified based on IP version, upper layer
protocol, and ports. If such restrictions are defined, they MUST be
enforced.
On the output side, the policy rules SHOULD prevent any packets
containing the inner IP addresses pair from escaping to the wire in
clear text.
On the input side, there is no policy processing necessary on
encrypted packets. The SA is found based on the SPI and destination
address. A single SA MAY be associated with several destination
addresses. Since the outer IPsec addresses are discarded, and since
the packet authenticity and integrity is protected by ESP, there is
no need to check the outer addresses. Since the inner addresses are
fixed and restored from the SA, there is no need to check them.
There MAY be futher policy rules specifying allowed upper layer
protocols and ports. If such restrictions are defined, they MUST be
enforced.
On the input side, there SHOULD be a policy rule that filters out
clear text packets that contain the inner addresses.
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7. PF_KEY extensions
This section defines the necessary extensions to the PF_KEYv2 API [2]
to support the BEET mode. This section is informative.
An BEET mode Security Association is created by specifying the inner
IP addresses in the PF_KEYv2 Identity extensions, using two new
identity types. The identity types of the source and destination
identity extensions MUST be identical, i.e. either IPv4 or IPv6.
(#define SADB_X_IDENTTYPE_ADDR 4)
#define SADB_IDENTTYPE_BEET_IPV4_ADDRESS 5
#define SADB_IDENTTYPE_BEET_IPV6_ADDRESS 6
#define SADB_IDENTTYPE_MAX 6
When these new identity types are used, the contents of the identity
field in the PF_KEY messages MUST be a binary address, in network
byte order.
For SADB_IDENTTYPE_BEET_IPV4_ADDRESS the length of the identity MUST
be exactly four octets. For SADB_IDENTTYPE_BEET_IPV6_ADDRESS the
length must be exactly 16 octets.
Additionally, a new IPsec mode is defined in ipsec.h, and used in the
unspecified but commonly used the PF_KEY extension SADB_X_EXT_SA2
field sadb_x_sa2_mode.
#define IPSEC_MODE_BEET 3
If an SA is specified using SADB_X_EXT_SA2, if the sadb_x_sa2_mode is
IPSEC_MODE_BEET, and if the source and destination identities are
defined in terms of SADB_IDENTTYPE_BEET_IPV4_ADDRESS or
SADB_IDENTTY_BEET_IPV6_ADDRESS, then the BEET mode MUST be used. If
an SA is specified using SADB_X_EXT_SA2, if the sadb_x_sa2_mode is
IPSEC_MODE_BEET, and if the identities are defined in terms (other
than the new types defined above) that exactly match to single IPv4
or IPv6 addresses, then the BEET mode SHOULD be used.
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8. New requirements on Key Management protocols
In this section we discuss the requirements that the new mode places
upon existing and new key management protocols. This section is
informative.
In order to provide support for the BEET mode, key agreement protocol
implementations must understand the existence of such a mode. In
some situations it is sufficient that the BEET mode is implemented at
the IPsec ESP level only at one end as long as the key management is
aware of its usage. For example, the NAT scenario described in
Section 4.1 does not require BEET ESP implementation at the server
end. It is suffient that the client implements the BEET mode; in
fact, if the client somehow knows it public IP address it may be able
to set up the BEET mode security associations without any explicit
concent on the server end. On the other hand, if the client does not
know its public IP address, it needs help from the server in order to
determine it.
More generally, one can get benefit from the BEET mode only to the
extend the key management protocol supports it. If the key
management protocol is fully aware of mobility and multi-homing
issues, and provides facilities for signalling changes in the current
connectivity situation, it is relatively easy to implement end-node
mobility and multi-address multi-homing with BEET. An example of
such usage is HIP [9]. Additionally, current IPsec NAT traversal
with IKEv1 includes a "SHOULD" statement for the stationary end to
update the remote IP address of the peer whenever a valid IPsec
packet arrives. Unfortunately, such practise MAY be vulnerable to
various flooding attacks, cf. [11].
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9. Implementing the functionality with other means
It is currently possible to implement the equivalent of BEET mode by
using transport mode ESP and explicit network address translation at
the end-hosts themselves. In this section we briefly compare these
two alternativites. The purpose of this section is to give
background information for security considerations. This section is
informative.
In an implementation using the BEET mode, the input side IP address
translation is integrated with the decryption and integrity
verification processing. The packet is passed and given the inner
addresses if and only if it is correctly decrypted and verified. A
typical IPsec SPD implementation would prohibit receiving unprotected
IP packets that use the inner addresses on the wire, as it is done in
the regular tunnel mode. At the same time, any other uses of the
outer addresses would be trivial; passing a packet to the SA requires
both that the packet has an ESP header and that the SPI matches.
In an implementation based on separate address translation and
transport mode ESP, the address translation and cryptographic
processing are completely separate. In practise, the host must
translate the outer IP address into the inner IP addresses before the
packet is passed to IPsec. (The other way around may not be secure,
since there would be no way for the address translation process to
know if the packet was correctly decrypted and verified or if it was
received via some other means.) Using the outer addresses for other
purposes may be hard, depending on the implementation of the address
translation mechanism. In particular, using the outer addresses on
other ESP SAs may be hard, since the typical address translation
mechanisms could be only configured with the protocol level (ESP vs.
not) and do not understand SPIs.
At the output side, a BEET mode implementation takes care of
translating the inner addresses to outer addresses, as a part of the
encryption process. The IPsec SPD contains necessary entries that
make sure that the inner addresses never leak.
In a implementation based on separate components, the output packets
would be passed with inner addresses from IPsec to the address
translation mechanism. The address translation mechanism will then
translate the inner addresses to outer addresses. While this does
not prevent usage of the outer addresses for other purposes, the
configuration is brittle and error prone. If there are mistakes at
the IPsec configuration, the address translation mechanism may
translate unprotected packets, leading to potential confusion. If
there are mistakes at the address translation side, the inner
addresses may leak to the network.
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10. Security Considerations
In this section we discuss the security properties of the BEET mode,
discussing some limitations [10]. This section is normative.
There are no known new vulnerabilities that the introduction of the
BEET mode would create.
It is currently possible to implement the equivalent of BEET mode by
using transport mode ESP and explicit network address translation at
the end-hosts themselves. However, such an implementation is more
complex, less flexible, and potentially more vulnerable to security
problems that are caused by misconfigurations; see Section 9.
The main security benefit is an operational one. To implement the
same functionality without the BEET mode typically requires
configuring three different, unrelated components in the hosts.
The transport mode ESP SAs must be configured.
A host based NAT function must be configured to properly translate
between the inner and outer addresses.
A host firewall must be configured to properly filter out packets
so that inner addresses do not leak in or out.
While it may be possible to configure these components to achieve the
same functionality, such a configuration is error prone, increasing
the probability of security vulnerabilities. An integrated BEET mode
implementation is less prone to configuration mistakes. Furthermore,
it would be fairly hard to implement portable key management
protocols that would be able to configure all of the required
components at the same time. On the other hand, it would be easy to
provide a portable key management protocol implementation that would
be able to configure BEET mode SAs through the specified PF_KEY
extensions.
Since the BEET security associations have the semantics of a fixed,
point-to-point tunnel between two IP addresses, it is possible to
place one or both of the tunnel end points into other nodes but those
that actually "possess" the inner IP addresses, i.e., to implement a
BEET mode proxy. However, since such usage defeats the security
benefits of combined ESP and hostNAT processing, as discussed above,
the implementations SHOULD NOT support such usage.
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11. IANA Considerations
The PF_KEYv2 interface should probably have an IANA registry.
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12. Acknowledgments
During the 56th IETF meeting in San Francisco and afterwards, the
following people made comments on the ideas, helping the author to
write the draft: Jari Arkko, Steven Bellovin, Charlie Kaufman, Tero
Kivinen, Cheryl Madson, Andrew McGrecor, Robert Moskowitz, Michael
Richardson, Timothy Shepard, Jukka Ylitalo, Sami Vaarala.
The author ows special thanks to Derek Atkins and Steve Kent, who
strongly opposed the idea during the San Francisco IETF, and thereby
forced writing a high quality initial draft.
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Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] McDonald, D., Metz, C. and B. Phan, "PF_KEY Key Management API,
Version 2", RFC 2367, July 1998.
[3] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[4] Kent, S., "IP Encapsulating Security Payload (ESP)",
draft-ietf-ipsec-esp-v3-06 (work in progress), July 2003.
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Informative references
[5] Arkko, J. and P. Nikander, "Limitations in IPsec Policy",
Security Protocols 11th International Workshop, Cambridge, UK,
April 2-4, 2003, LNCS to be published, Springer, April 2003.
[6] Ionnadis, J., "Why we still don't have IPsec", Network and
Distributed Systems Security Symposium (NDSS'03), Internet
Society, February 2003.
[7] Bellovin, S., "EIDs, IPsec, and HostNAT", IETF 41th, March
1998.
[8] Moskowitz, R., "Host Identity Protocol Architecture",
draft-moskowitz-hip-arch-03 (work in progress), May 2003.
[9] Moskowitz, R., Nikander, P. and P. Jokela, "Host Identity
Protocol", draft-moskowitz-hip-07 (work in progress), June
2003.
[10] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", draft-iab-sec-cons-03 (work in
progress), February 2003.
[11] Nikander, P., "Mobile IP version 6 Route Optimization Security
Design Background", draft-nikander-mobileip-v6-ro-sec-01 (work
in progress), July 2003.
Author's Address
Pekka Nikander
Ericsson Research Nomadic Lab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: pekka.nikander@nomadiclab.com
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Appendix A. Implementation experiences
We have implemented the BEET mode to the NetBSD 1.6 KAME stack. Our
implementation uses the PF_KEYv2 identity extension, as described in
Section 7.
The current implementation is based on four hooks placed at the
stragical locations at the ESP and ip_output processing, and a
separate loadable kernel module that performs the actual processing.
We support full IPv4/IPv6 conversions, allowing both IPv4-over-IPv6
and IPv6-over-IPv4 tunneling. The number of lines changed in the
KAME policy processing is 36 lines; these changes were necessary to
fully support the identity extension, which is currently partly
unimplemented in the NetBSD 1.6 KAME stack. The hooks themselves
take 83 lines, and the protocol processing code in the kernel module
is 450 lines long. About 90% of the protocol processing code was
copied and pasted from the IPsec tunnel mode and transport mode
routines, with minimal changes. About 70% of the code is needed to
implement v4-over-v6 and v6-over-v4 tunneling. The number of actual
functional lines for the simple v4-over-v4 and v6-over-v6 cases is
mere 62 lines. The implementation effort took three days from two
programmers, including writing simple test cases and performing
rudimentary testing on the implementation to see that it works.
The current implentation is tailored for experimentation. A more
proper implementation would implement all of the processing as an
integral part of the IPsec processing. The current KAME code
supports only two modes. Once the necessary cleanups, such as
replacing "if" statements with "switch" statements, and additions for
supporting v4-over-v6 and v6-over-v4 tunneling is in place, we expect
the extra protocol processing code required by the BEET mode to take
less than 100 lines.
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Appendix B. Garden beets
Commonly known as the garden beet, this firm, round root vegetable
has leafy green tops, which are also edible and highly nutritious.
The most common color for beets (called "beetroots" in the British
Isles) is a garnet red. However, they can range in color from deep
red to white, the most intriguing being the Chioggia (also called
"candy cane"), with its concentric rings of red and white. Beets are
available year-round and should be chosen by their firmness and
smooth skins.
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