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Versions: 00 01 02 03 04 05 06 07 08 RFC 3947

 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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
 
 
 
 T. Kivinen, et. al. [page 6]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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.
 
 
 
 T. Kivinen, et. al. [page 7]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
                   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
 
               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 |
             +---------------+---------------+---------------+---------------+
             | ID Type | RESERVED | RESERVED |
             +---------------+---------------+---------------+---------------+
             | IPv4 (4 octets) or IPv6 address (16 octets) |
             +---------------+---------------+---------------+---------------+
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
         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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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
 
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 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]
 

 INTERNET-DRAFT 10 Feb 2004
 
 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]

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