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IP Security Protocol Working Group (IPsec) T. Kivinen
INTERNET-DRAFT SafeNet A. Huttunen
draft-ietf-ipsec-nat-t-ike-08.txt B. Swander
Expires: 10 July 2004 Microsoft F-Secure Corporation
V. Volpe
Cisco Systems
10 Feb 2004

Negotiation of NAT-Traversal in the IKE

Status of This Memo

This document is a submission to the IETF IP Security Protocol
(IPSEC) Working Group. Comments are solicited and should be
addressed to the working group mailing list (ipsec@lists.tislabs.com)
or to the editor.

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

This document describes how to detect one or more network address trans-
lation devices (NATs) between IPsec hosts, and how to negotiate the use
of UDP encapsulation of IPsec packets through NAT boxes in Internet Key
Exchange (IKE).

T. Kivinen, et. al. [page 1]

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Specification of Requirements . . . . . . . . . . . . . . . . . 3
3. Phase 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Detecting support of Nat-Traversal . . . . . . . . . . . . . 3
3.2. Detecting the presence of NAT . . . . . . . . . . . . . . . 3
4. Changing to new ports . . . . . . . . . . . . . . . . . . . . . 5
5. Quick Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Negotiation of the NAT-Traversal encapsulation . . . . . . . 8
5.2. Sending the original source and destination addresses . . . 8
6. Initial contact notifications . . . . . . . . . . . . . . . . . 10
7. Recovering from the expiring NAT mappings . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 11
10. Intellectual property rights . . . . . . . . . . . . . . . . . 12
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 12
12. Normative References . . . . . . . . . . . . . . . . . . . . . 13
13. Non-Normative References . . . . . . . . . . . . . . . . . . . 13
14. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
15. Full copyright statement . . . . . . . . . . . . . . . . . . . 14

1. Introduction

This document is split in two parts. The first part describes what is
needed in IKE Phase 1 for NAT-Traversal support. This includes detecting
if the other end supports NAT-Traversal, and detecting if there is one
or more NAT between the peers.

The second part describes how to negotiate the use of UDP encapsulated
IPsec packets in IKE's Quick Mode. It also describes how to transmit the
original source and destination addresses to the peer if required. The
original source and destination addresses are used in transport mode to
incrementally update the TCP/IP checksums so that they will match after
the NAT transform (The NAT cannot do this, because the TCP/IP checksum
is inside the UDP encapsulated IPsec packet).

The document [Hutt03] describes the details of UDP encapsulation and
[Aboba03] provides background information and motivation of NAT-
Traversal in general. This document, in combination with [Hutt03]
represents an "unconditionally compliant" solution to the requirements
as defined by [Aboba03].

The basic scenario for this document is the case where the initiator is
behind NA(P)T and the responder has a fixed static IP address.

This document defines a protocol that will work even if both ends are
behind NAT, but the process of how to locate the other end is out of the
scope of this document. In one scenario, the responder is behind a
static host NAT (only one responder per IP as there is no way to use any
other destination ports than 500/4500), i.e. it is known by the

T. Kivinen, et. al. [page 2]

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configuration.

2. Specification of Requirements

This document shall use the keywords "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED, "MAY", and
"OPTIONAL" to describe requirements. They are to be interpreted as
described in [RFC-2119] document.

3. Phase 1

The detection of support for NAT-Traversal and detection of NAT along
the path between the two IKE peers occurs in IKE [RFC-2409] Phase 1.

The NAT may change the IKE UDP source port, and recipients MUST be able
to process IKE packets whose source port is different than 500. There
are cases where the NAT does not have to change the source port:

o only one IPsec host behind the NAT

o for the first IPsec host the NAT can keep the port 500, and the NAT
will only change the port number for later connections

Recipients MUST reply back to the source address from the packet (See
[Aboba03] section 2.1, case d). This also means that when the original
responder is doing rekeying, or sending notifications etc. to the
original initiator it MUST send the packets using the same set of port
and IP numbers that was used when the IKE SA was last time used.

For example, when the initiator sends a packet having source and
destination port 500, the NAT may change that to a packet which has
source port 12312 and destination port 500. The responder must be able
to process the packet whose source port is that 12312. It must reply
back with a packet whose source port is 500 and destination port 12312.
The NAT will then translate this packet to have source port 500 and
destination port 500.

3.1. Detecting support of Nat-Traversal

The NAT-Traversal capability of the remote host is determined by an
exchange of vendor ID payloads. In the first two messages of Phase 1,
the vendor id payload for this specification of NAT-Traversal (MD5 hash
of "RFC XXXX" - ["XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX"]) MUST be sent if
supported (and it MUST be received by both sides) for the NAT-Traversal
probe to continue.

[Note to the RFC Editor: The XXXX is replaced with the RFC number of
this document when the number is known. The XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX will be replaced with MD5 hash of the text "RFC XXXX" (the
exact hex string will be provided by the authors when the rfc number is
known). This instruction is to be removed from the final RFC].

3.2. Detecting the presence of NAT

T. Kivinen, et. al. [page 3]

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The purpose of the NAT-D payload is twofold, It not only detects the
presence of NAT between the two IKE peers, it also detects where the NAT
is. The location of the NAT device is important in that the keepalives
need to initiate from the peer "behind" the NAT.

To detect NAT between the two hosts, we need to detect if the IP address
or the port changes along the path. This is done by sending the hashes
of the IP addresses and ports of both IKE peers from each end to the
other. If both ends calculate those hashes and get same result they know
there is no NAT between. If the hashes do not match, somebody has
translated the address or port, meaning that we need to do NAT-Traversal
to get IPsec packets through.

If the sender of the packet does not know his own IP address (in case of
multiple interfaces, and the implementation does not know which IP
address is used to route the packet out), the sender can include
multiple local hashes to the packet (as separate NAT-D payloads). In
this case, NAT is detected if and only if none of the hashes match.

The hashes are sent as a series of NAT-D (NAT discovery) payloads. Each
payload contains one hash, so in case of multiple hashes, multiple NAT-D
payloads are sent. In the normal case there are only two NAT-D payloads.

The NAT-D payloads are included in the third and fourth packet of Main
Mode, and in second and third packet in the Aggressive Mode.

The format of the NAT-D packet is

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload length |
+---------------+---------------+---------------+---------------+
~ HASH of the address and port ~
+---------------+---------------+---------------+---------------+

The payload type for the NAT discovery payload is 15.

The HASH is calculated as follows:

HASH = HASH(CKY-I | CKY-R | IP | Port)

using the negotiated HASH algorithm. All data inside the HASH is in the
network byte-order. The IP is 4 octets for an IPv4 address and 16 octets
for an IPv6 address. The port number is encoded as a 2 octet number in
network byte-order. The first NAT-D payload contains the remote end's IP
address and port (i.e. the destination address of the UDP packet). The
remaining NAT-D payloads contain possible local end IP addresses and
ports (i.e. all possible source addresses of the UDP packet).

If there is no NAT between the peers, the first NAT-D payload received
should match one of the local NAT-D payloads (i.e. the local NAT-D
payloads this host is sending out), and one of the other NAT-D payloads
must match the remote end's IP address and port. If the first check

T. Kivinen, et. al. [page 4]

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fails (i.e. first NAT-D payload does not match any of the local IP
addresses and ports), then it means that there is dynamic NAT between
the peers, and this end should start sending keepalives as defined in
the [Hutt03] (this end is behind the NAT).

The CKY-I and CKY-R are the initiator and responder cookies. They are
added to the hash to make precomputation attacks for the IP address and
port impossible.

An example of a Phase 1 exchange using NAT-Traversal in Main Mode
(authentication with signatures) is:

Initiator Responder
------------ ------------
HDR, SA, VID -->
<-- HDR, SA, VID
HDR, KE, Ni, NAT-D, NAT-D -->
<-- HDR, KE, Nr, NAT-D, NAT-D
HDR*#, IDii, [CERT, ] SIG_I -->
<-- HDR*#, IDir, [ CERT, ], SIG_R

An example of Phase 1 exchange using NAT-Traversal in Aggressive Mode
(authentication with signatures) is:

Initiator Responder
------------ ------------
HDR, SA, KE, Ni, IDii, VID -->
<-- HDR, SA, KE, Nr, IDir,
[CERT, ], VID, NAT-D,
NAT-D, SIG_R
HDR*#, [CERT, ], NAT-D, NAT-D,
SIG_I -->

The '#' sign identifies that those packets are sent to the changed port
if NAT is detected.

4. Changing to new ports

IPsec-aware NATs can cause problems (See [Aboba03] section 2.3). Some
NATs will not change IKE source port 500 even if there are multiple
clients behind the NAT (See [Aboba03] section 2.3, case n). They can
also use IKE cookies to demultiplex traffic instead of using the source
port (See [Aboba03] section 2.3, case m). Both of these are problematic
for generic NAT transparency since it is difficult for IKE to discover
the capabilities of the NAT. The best approach is to simply move the IKE
traffic off port 500 as soon as possible to avoid any IPsec-aware NAT
special casing.

Take the common case of the initiator behind the NAT. The initiator must
quickly change to port 4500 once the NAT has been detected to minimize
the window of IPsec-aware NAT problems.

In Main Mode, the initiator MUST change ports when sending the ID

T. Kivinen, et. al. [page 5]

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payload if there is NAT between the hosts. The initiator MUST set both
UDP source and destination ports to 4500. All subsequent packets sent to
this peer (including informational notifications) MUST be sent on port
4500. In addition, the IKE data MUST be prepended with a non-ESP marker
allowing for demultiplexing of traffic as defined in [Hutt03].

Thus, the IKE packet now looks like:

IP UDP(4500,4500) <non-ESP marker> HDR*, IDii, [CERT, ] SIG_I

assuming authentication using signatures. The 4 bytes of non-ESP marker
is defined in the [Hutt03].

When the responder gets this packet, the usual decryption and processing
of the various payloads is performed. If this is successful, the
responder MUST update local state so that all subsequent packets
(including informational notifications) to the peer use the new port,
and possibly the new IP address obtained from the incoming valid packet.
The port will generally be different since the NAT will map UDP(500,500)
to UDP(X,500), and UDP(4500,4500) to UDP(Y,4500). The IP address will
seldom be different from the pre-changed IP address. The responder MUST
respond with all subsequent IKE packets to this peer using UDP(4500,Y).

Similarly, if the responder needs to rekey the Phase 1 SA, then the
rekey negotiation MUST be started using UDP(4500,Y). Any implementation
that supports NAT traversal MUST support negotiations that begin on port
4500. If a negotiation starts on port 4500, then it doesn't need to
change anywhere else in the exchange.

Once port change has occurred, if a packet is received on port 500, that
packet is old. If the packet is an informational packet, it MAY be
processed if local policy allows. If the packet is a Main Mode or
Aggressive Mode packet (with same cookies than previous packets), it
SHOULD be discarded. If the packet is new Main Mode or Aggressive
exchange then it is processed normally (the other end might have
rebooted, and this is starting new exchange).

Here is an example of a Phase 1 exchange using NAT-Traversal in Main
Mode (authentication with signatures) with changing port:

Initiator Responder
------------ ------------
UDP(500,500) HDR, SA, VID -->
<-- UDP(500,X) HDR, SA, VID
UDP(500,500) HDR, KE, Ni,
NAT-D, NAT-D -->
<-- UDP(500,X) HDR, KE, Nr,
NAT-D, NAT-D
UDP(4500,4500) HDR*#, IDii,
[CERT, ]SIG_I -->
<-- UDP(4500,Y) HDR*#, IDir,
[ CERT, ], SIG_R

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The procedure for Aggressive Mode is very similar. After the NAT has
been detected, the initiator sends: IP UDP(4500,4500) <4 bytes of non-
ESP marker> HDR*, [CERT, ], NAT-D, NAT-D, SIG_I. The responder does
similar processing to the above, and if successful, MUST update it's
internal IKE ports. The responder MUST respond with all subsequent IKE
packets to this peer using UDP(4500,Y).

Initiator Responder
------------ ------------

UDP(500,500) HDR, SA, KE,
Ni, IDii, VID -->
<-- UDP(500,X) HDR, SA, KE,
Nr, IDir, [CERT, ],
VID, NAT-D, NAT-D,
SIG_R
UDP(4500,4500) HDR*#, [CERT, ],
NAT-D, NAT-D,
SIG_I -->

<-- UDP(4500, Y) HDR*#, ...

If the support of the NAT-Traversal is enabled the port in the ID
payload in Main Mode/Aggressive Mode MUST be set to 0.

The most common case for the responder behind the NAT is if the NAT is
simply doing 1-1 address translation. In this case, the initiator still
changes both ports to 4500. The responder uses the identical algorithm
as above, although in this case Y will equal 4500, since no port
translation is happening.

A different port change case involves out-of-band discovery of the ports
to use. Those discovery methods are out of scope of this document. For
instance, if the responder is behind a port translating NAT, and the
initiator needs to contact it first, then the initiator will need to
determine which ports to use, usually by contacting some other server.
Once the initiator knows which ports to use to traverse the NAT,
generally something like UDP(Z,4500), it initiates using these ports.
This is similar to the responder rekey case above in that the ports to
use are already known upfront, and no additional change need take place.
Also, the first keepalive timer starts after the change to the new port,
no keepalives are sent to the port 500.

5. Quick Mode

After the Phase 1 both ends know if there is a NAT present between them.
The final decision of using NAT-Traversal is left to Quick Mode. The
use of NAT-Traversal is negotiated inside the SA payloads of Quick Mode.
In Quick Mode, both ends can also send the original addresses of the
IPsec packets (in case of the transport mode) to the other end, so the
other end has possibility to fix the TCP/IP checksum field after the NAT
transform.

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5.1. Negotiation of the NAT-Traversal encapsulation

The negotiation of the NAT-Traversal happens by adding two new
encapsulation modes. These encapsulation modes are:

UDP-Encapsulated-Tunnel 3
UDP-Encapsulated-Transport 4

It is not normally useful to propose both normal tunnel or transport
mode and UDP-Encapsulated modes. UDP encapsulation is required to fix
the inability to handle non-UDP/TCP traffic by NATs (See [Aboba03]
section 2.2, case i).

If there is a NAT box between hosts, normal tunnel or transport
encapsulations may not work and in that case UDP-Encapsulation SHOULD be
used.

If there is no NAT box between, there is no point of wasting bandwidth
by adding UDP encapsulation of packets, thus UDP-Encapsulation SHOULD
NOT be used.

Also, the initiator SHOULD NOT include both normal tunnel or transport
mode and UDP-Encapsulated-Tunnel or UDP-Encapsulated-Transport in its
proposals.

5.2. Sending the original source and destination addresses

In order to perform incremental TCP checksum updates, both peers may
need to know the original IP addresses used by their peer when that peer
constructed the packet (See [Aboba03] section 2.1, case b). For the
initiator, the original Initiator address is defined to be the
Initiator's IP address. The original Responder address is defined to be
the perceived peer's IP address. For the responder, the original
Initiator address is defined to be the perceived peer's address. The
original Responder address is defined to be the Responder's IP address.

The original addresses are sent using NAT-OA (NAT Original Address)
payloads.

The Initiator NAT-OA payload is first. The Responder NAT-OA payload is
second.

Example 1:

Initiator <---------> NAT <---------> Responder
^ ^ ^
Iaddr NatPub Raddr

The initiator is behind a NAT talking to the publicly available
responder. Initiator and Responder have IP addresses Iaddr, and Raddr.
NAT has public IP address NatPub.

Initiator:

T. Kivinen, et. al. [page 8]

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NAT-OAi = Iaddr
NAT-OAr = Raddr

Responder:
NAT-OAi = NATPub
NAT-OAr = Raddr

Example 2:

Initiator <------> NAT1 <---------> NAT2 <-------> Responder
^ ^ ^ ^
Iaddr Nat1Pub Nat2Pub Raddr

Here, NAT2 "publishes" Nat2Pub for Responder and forwards all traffic to
that address to Responder.

Initiator:
NAT-OAi = Iaddr
NAT-OAr = Nat2Pub

Responder:
NAT-OAi = Nat1Pub
NAT-OAr = Raddr

In case of transport mode both ends MUST send the both original
Initiator and Responder addresses to the other end. For tunnel mode both
ends SHOULD NOT send original addresses to the other end.

The NAT-OA payloads are sent inside the first and second packets of
Quick Mode. The initiator MUST send the payloads if it proposes any UDP-
Encapsulated-Transport mode and the responder MUST send the payload only
if it selected UDP-Encapsulated-Transport mode, i.e. it is possible that
the initiator sends the NAT-OA payload, but proposes both UDP-
Encapsulated transport and tunnel mode. Then the responder selects the
UDP-Encapsulated tunnel mode and does not send the NAT-OA payload back.

The format of the NAT-OA packet is

The payload type for the NAT original address payload is 16.

The ID type is defined in the [RFC-2407]. Only ID_IPV4_ADDR and
ID_IPV6_ADDR types are allowed. The two reserved fields after the ID
Type must be zero.

T. Kivinen, et. al. [page 9]

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An example of Quick Mode using NAT-OA payloads is:

Initiator Responder
------------ ------------
HDR*, HASH(1), SA, Ni, [, KE]
[, IDci, IDcr ]
[, NAT-OAi, NAT-OAr] -->
<-- HDR*, HASH(2), SA, Nr, [, KE]
[, IDci, IDcr ]
[, NAT-OAi, NAT-OAr]
HDR*, HASH(3)

6. Initial contact notifications

The source IP and port address of the INITIAL-CONTACT notification for
the host behind NAT are not meaningful (NAT can change them), so the IP
and port numbers MUST NOT be used for determining which IKE/IPsec SAs to
remove (See [Aboba03] section 2.1, case c). The ID payload sent from the
other end SHOULD be used instead, i.e. when an INITIAL-CONTACT
notification is received from the other end, the receiving end SHOULD
remove all the SAs associated with the same ID payload.

7. Recovering from the expiring NAT mappings

There are cases where NAT box decides to remove mappings that are still
alive (for example, the keepalive interval is too long, or the NAT box
is rebooted). To recover from this, ends which are NOT behind NAT SHOULD
use the last valid authenticated packet from the other end to determine
which IP and port addresses should be used. The host behind dynamic NAT
MUST NOT do this as otherwise it opens a DoS attack possibility, and
there is no need for that, because the IP address or port of the other
host will not change (it is not behind NAT).

Keepalives cannot be used for this purposes as they are not
authenticated, but any IKE authenticated IKE packet or ESP packet can be
used to detect that the IP address or the port has changed.

8. Security Considerations

Whenever changes to some fundamental parts of a security protocol are
proposed, the examination of security implications cannot be skipped.
Therefore, here are some observations on the effects, and whether or not
these effects matter.

o IKE probes reveal NAT-Traversal support to anyone watching the
traffic. Disclosure that NAT-Traversal is supported does not
introduce new vulnerabilities.

o The value of authentication mechanisms based on IP addresses
disappears once NATs are in the picture. That is not necessarily a
bad thing (for any real security, authentication measures other than
IP addresses should be used). This means that authentication using
pre-shared-keys cannot be used in Main Mode without using group

T. Kivinen, et. al. [page 10]

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shared keys for everybody behind the NAT box. Using group shared keys
is huge risk because it allows anyone in the group to authenticate to
any other party and claim to be anybody in the group, i.e. a normal
user could be impersonating a vpn-gateway, and acting as a man in the
middle, and read/modify all traffic to/from others in the group. Use
of group shared keys is NOT RECOMMENDED.

o As the internal address space is only 32 bits, and it is usually very
sparse, it might be possible for the attacker to find out the
internal address used behind the NAT box by trying all possible IP-
addresses and trying to find the matching hash. The port numbers are
normally fixed to 500, and the cookies can be extracted from the
packet. This limits the hash calculations down to 2^32. If an
educated guess of the private address space is done, then the number
of hash calculations needed to find out the internal IP address goes
down to 2^24 + 2 * (2^16).

o Neither NAT-D payloads or Vendor ID payloads are authenticated at all
in Main Mode nor in Aggressive Mode. This means that attacker can
remove those payloads, modify them or add them. By removing or adding
them, the attacker can cause Denial Of Service attacks. By modifying
the NAT-D packets the attacker can cause both ends to use UDP-
Encapsulated modes instead of directly using tunnel or transport
mode, thus wasting some bandwidth.

o The sending of the original source address in the Quick Mode reveals
the internal IP address behind the NAT to the other end. In this case
we have already authenticated the other end, and sending of the
original source address is only needed in transport mode.

o Updating the IKE SA / ESP UDP encapsulation IP addresses and ports
for each valid authenticated packet can cause DoS in the case where
we have an attacker who can listen to all traffic in the network, and
can change the order of the packets and inject new packets before the
packet he has already seen, i.e. the attacker can take an
authenticated packet from the host behind NAT, change the packet UDP
source or destination ports or IP addresses and sent it out to the
other end before the real packet reaches there. The host not behind
the NAT will update its IP address and port mapping and sends further
traffic to the wrong host or port. This situation is fixed
immediately when the attacker stops modifying the packets as the
first real packet will fix the situation back to normal.
Implementations SHOULD AUDIT the event every time the mapping is
changed, as in the normal case it should not happen that often.

9. IANA Considerations

This documents contains two new "magic numbers" which are allocated from
the existing IANA registry for IPsec. This document also renames
existing registered port 4500. This document also defines 2 new payload
types for IKE, and there is no registry for those in the IANA.

New items to be added in the "Internet Security Association and Key

T. Kivinen, et. al. [page 11]

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Management Protocol (ISAKMP) Identifiers" Encapsulation Mode registry:

Name Value Reference
---- ----- ---------
UDP-Encapsulated-Tunnel 3 [RFC XXXX]
UDP-Encapsulated-Transport 4 [RFC XXXX]

Change in the registered port registry:

Keyword Decimal Description Reference
------- ------- ----------- ---------
ipsec-nat-t 4500/tcp IPsec NAT-Traversal [RFC XXXX]
ipsec-nat-t 4500/udp IPsec NAT-Traversal [RFC XXXX]

New IKE payload numbers are (There is no IANA registry related to this,
and no need to create new one, but if one is added these should be added
to there):

NAT-D 15 NAT Discovery Payload
NAT-OA 16 NAT Original Address Payload

10. Intellectual property rights

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; nor does it represent that it has made any
independent effort to identify any such rights. Information on the
IETF's procedures with respect to rights in IETF Documents can be found
in RFC XX and RFC XY. [note to RFC Editor - replace XX with the number
of IETF IPR and replace XY with number of IETF SUB.]

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can be
obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
that may cover technology that may be required to implement this
standard. Please address the information to the IETF at ietf-
ipr@ietf.org.

11. Acknowledgments

Thanks to Markus Stenberg, Larry DiBurro and William Dixon who
contributed actively to this document.

Thanks to Tatu Ylonen, Santeri Paavolainen, and Joern Sierwald who
contributed to the document used as the base for this document.

T. Kivinen, et. al. [page 12]

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12. Normative References

[RFC-2409] Harkins D., Carrel D., "The Internet Key Exchange (IKE)",
November 1998

[RFC-2407] Piper D., "The Internet IP Security Domain Of Interpretation
for ISAKMP", November 1998

[Hutt03] Huttunen, A. et. al., "UDP Encapsulation of IPsec Packets",
draft-ietf-ipsec-udp-encaps-06.txt, January 2003

[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", March 1997

[IETF SUB] Bradner, S., "IETF Rights in Contributions", draft-ietf-ipr-
submission-rights-08.txt, October 2003

[IETF IPR] Bradner, S., "Intellectual Property Rights in IETF
Technology", draft-ietf-ipr-technology-rights-12.txt, October 2003

13. Non-Normative References

[Aboba03] Aboba, B. et. al., "IPsec-NAT Compatibility Requirements",
draft-ietf-ipsec-nat-reqts-06.txt, October 2003.

14. Authors' Addresses

Tero Kivinen
SafeNet, Inc.
Fredrikinkatu 47
FIN-00100 HELSINKI
Finland
E-mail: kivinen@safenet-inc.com

Ari Huttunen
F-Secure Corporation
Tammasaarenkatu 7,
FIN-00181 HELSINKI
Finland
E-mail: Ari.Huttunen@F-Secure.com

Brian Swander
Microsoft
One Microsoft Way
Redmond WA 98052
E-mail: briansw@microsoft.com

Victor Volpe
Cisco Systems
124 Grove Street
Suite 205
Franklin, MA 02038
E-mail: vvolpe@cisco.com

T. Kivinen, et. al. [page 13]

INTERNET-DRAFT 10 Feb 2004

15. Full copyright statement

Copyright (C) The Internet Society (year). This document is subject to
the rights, licenses and restrictions contained in RFC XXXX and except
as set forth therein, the authors retain all their rights.

[Note to the RFC Editor - XXXX above to be replaced with the number of
[IETF SUB]]

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

T. Kivinen, et. al. [page 14]