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INFORMATIONAL
Errata Exist
Network Working Group                                           B. Aboba
Request for Comments: 3715                                      W. Dixon
Category: Informational                                        Microsoft
                                                              March 2004


   IPsec-Network Address Translation (NAT) Compatibility Requirements

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document describes known incompatibilities between Network
   Address Translation (NAT) and IPsec, and describes the requirements
   for addressing them.  Perhaps the most common use of IPsec is in
   providing virtual private networking capabilities.  One very popular
   use of Virtual Private Networks (VPNs) is to provide telecommuter
   access to the corporate Intranet.  Today, NATs are widely deployed in
   home gateways, as well as in other locations likely to be used by
   telecommuters, such as hotels.  The result is that IPsec-NAT
   incompatibilities have become a major barrier in the deployment of
   IPsec in one of its principal uses.





















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Requirements Language. . . . . . . . . . . . . . . . . .  2
   2.  Known Incompatibilities between NA(P)T and IPsec . . . . . . .  3
       2.1.  Intrinsic NA(P)T Issues. . . . . . . . . . . . . . . . .  3
       2.2.  NA(P)T Implementation Weaknesses . . . . . . . . . . . .  7
       2.3.  Helper Incompatibilities . . . . . . . . . . . . . . . .  8
   3.  Requirements for IPsec-NAT Compatibility . . . . . . . . . . .  8
   4.  Existing Solutions . . . . . . . . . . . . . . . . . . . . . . 12
       4.1.  IPsec Tunnel Mode. . . . . . . . . . . . . . . . . . . . 12
       4.2.  RSIP . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.3.  6to4 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 14
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
       6.1.  Normative References . . . . . . . . . . . . . . . . . . 15
       6.2.  Informative References . . . . . . . . . . . . . . . . . 16
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
   9 . Full Copyright Statement . . . . . . . . . . . . . . . . . . . 18

1.  Introduction

   Perhaps the most common use of IPsec [RFC2401] is in providing
   virtual private networking (VPN) capabilities.  One very popular use
   of VPNs is to provide telecommuter access to the corporate Intranet.
   Today, Network Address Translations (NATs) as described in [RFC3022]
   and [RFC2663], are widely deployed in home gateways, as well as in
   other locations likely to be used by telecommuters, such as hotels.
   The result is that IPsec-NAT incompatibilities have become a major
   barrier in the deployment of IPsec in one of its principal uses.
   This document describes known incompatibilities between NAT and
   IPsec, and describes the requirements for addressing them.

1.1.  Requirements Language

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [RFC2119].

   Please note that the requirements specified in this document are to
   be used in evaluating protocol submissions.  As such, the
   requirements language refers to capabilities of these protocols; the
   protocol documents will specify whether these features are required,
   recommended, or optional.  For example, requiring that a protocol
   support confidentiality is not the same thing as requiring that all
   protocol traffic be encrypted.




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   A protocol submission is not compliant if it fails to satisfy one or
   more of the MUST or MUST NOT requirements for the capabilities that
   it implements.  A protocol submission that satisfies all the MUST,
   MUST NOT, SHOULD, and SHOULD NOT requirements for its capabilities is
   said to be "unconditionally compliant"; one that satisfies all the
   MUST and MUST NOT requirements, but not all the SHOULD or SHOULD NOT
   requirements for its protocols is said to be "conditionally
   compliant."

2.  Known Incompatibilities between NA(P)T and IPsec

   The incompatibilities between NA(P)T and IPsec can be divided into
   three categories:

   1) Intrinsic NA(P)T issues.  These incompatibilities derive directly
      from the NA(P)T functionality described in [RFC3022].  These
      incompatibilities will therefore be present in any NA(P)T device.

   2) NA(P)T implementation weaknesses.  These incompatibilities are not
      intrinsic to NA(P)T, but are present in many NA(P)T
      implementations.  Included in this category are problems in
      handling inbound or outbound fragments.  Since these issues are
      not intrinsic to NA(P)T, they can, in principle, be addressed in
      future NA(P)T implementations.  However, since the implementation
      problems appear to be wide spread, they need to be taken into
      account in a NA(P)T traversal solution.

   3) Helper issues.  These incompatibilities are present in NA(P)T
      devices which attempt to provide for IPsec NA(P)T traversal.
      Ironically, this "helper" functionality creates further
      incompatibilities, making an already difficult problem harder to
      solve.  While IPsec traversal "helper" functionality is not
      present in all NA(P)Ts, these features are becoming sufficiently
      popular that they also need to be taken into account in a NA(P)T
      traversal solution.

2.1.  Intrinsic NA(P)T Issues

   Incompatibilities that are intrinsic to NA(P)T include:

   a) Incompatibility between IPsec AH [RFC2402] and NAT.  Since the AH
      header incorporates the IP source and destination addresses in the
      keyed message integrity check, NAT or reverse NAT devices making
      changes to address fields will invalidate the message integrity
      check.  Since IPsec ESP [RFC2406] does not incorporate the IP
      source and destination addresses in its keyed message integrity
      check, this issue does not arise for ESP.




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   b) Incompatibility between checksums and NAT.  TCP and UDP checksums
      have a dependency on the IP source and destination addresses
      through inclusion of the "pseudo-header" in the calculation.  As a
      result, where checksums are calculated and checked upon receipt,
      they will be invalidated by passage through a NAT or reverse NAT
      device.

      As a result, IPsec Encapsulating Security Payload (ESP) will only
      pass through a NAT unimpeded if TCP/UDP protocols are not involved
      (as in IPsec tunnel mode or IPsec protected GRE), or checksums are
      not calculated (as is possible with IPv4 UDP).  As described in
      [RFC793], TCP checksum calculation and verification is required in
      IPv4.  UDP/TCP checksum calculation and verification is required
      in IPv6.

      Stream Control Transmission Protocol (SCTP), as defined in
      [RFC2960] and [RFC3309], uses a CRC32C algorithm calculated only
      on the SCTP packet (common header + chunks), so that the IP header
      is not covered.  As a result, NATs do not invalidate the SCTP CRC,
      and the problem does not arise.

      Note that since transport mode IPsec traffic is integrity
      protected and authenticated using strong cryptography,
      modifications to the packet can be detected prior to checking
      UDP/TCP checksums.  Thus, checksum verification only provides
      assurance against errors made in internal processing.

   c) Incompatibility between IKE address identifiers and NAT.  Where IP
      addresses are used as identifiers in Internet Key Exchange
      Protocol (IKE) Phase 1 [RFC2409] or Phase 2, modification of the
      IP source or destination addresses by NATs or reverse NATs will
      result in a mismatch between the identifiers and the addresses in
      the IP header.  As described in [RFC2409], IKE implementations are
      required to discard such packets.

      In order to avoid use of IP addresses as IKE Phase 1 and Phase 2
      identifiers, userIDs and FQDNs can be used instead.  Where user
      authentication is desired, an ID type of ID_USER_FQDN can be used,
      as described in [RFC2407].  Where machine authentication is
      desired, an ID type of ID_FQDN can be used.  In either case, it is
      necessary to verify that the proposed identifier is authenticated
      as a result of processing an end-entity certificate, if
      certificates are exchanged in Phase 1.  While use of USER_FQDN or
      FQDN identity types is possible within IKE, there are usage
      scenarios (e.g.  Security Policy Database (SPD) entries describing
      subnets) that cannot be accommodated this way.





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      Since the source address in the Phase 2 identifier is often used
      to form a full 5-tuple inbound SA selector, the destination
      address, protocol, source port and destination port can be used in
      the selector so as not to weaken inbound SA processing.

   d) Incompatibility between fixed IKE source ports and NAPT.  Where
      multiple hosts behind the NAPT initiate IKE SAs to the same
      responder, a mechanism is needed to allow the NAPT to demultiplex
      the incoming IKE packets from the responder.  This is typically
      accomplished by translating the IKE UDP source port on outbound
      packets from the initiator.  Thus responders must be able to
      accept IKE traffic from a UDP source port other than 500, and must
      reply to that port.  Care must be taken to avoid unpredictable
      behavior during re-keys.  If the floated source port is not used
      as the destination port for the re-key, the NAT may not be able to
      send the re-key packets to the correct destination.

   e) Incompatibilities between overlapping SPD entries and NAT.  Where
      initiating hosts behind a NAT use their source IP addresses in
      Phase 2 identifiers, they can negotiate overlapping SPD entries
      with the same responder IP address.  The responder could then send
      packets down the wrong IPsec SA.  This occurs because to the
      responder, the IPsec SAs appear to be equivalent, since they exist
      between the same endpoints and can be used to pass the same
      traffic.

   f) Incompatibilities between IPsec SPI selection and NAT.  Since
      IPsec ESP traffic is encrypted and thus opaque to the NAT, the NAT
      must use elements of the IP and IPsec header to demultiplex
      incoming IPsec traffic.  The combination of the destination IP
      address, security protocol (AH/ESP), and IPsec SPI is typically
      used for this purpose.

      However, since the outgoing and incoming SPIs are chosen
      independently, there is no way for the NAT to determine what
      incoming SPI corresponds to what destination host merely by
      inspecting outgoing traffic.  Thus, were two hosts behind the NAT
      to attempt to create IPsec SAs at the same destination
      simultaneously, it is possible that the NAT will deliver the
      incoming IPsec packets to the wrong destination.

      Note that this is not an incompatibility with IPsec per se, but
      rather with the way it is typically implemented.  With both AH and
      ESP, the receiving host specifies the SPI to use for a given SA, a
      choice which is significant only to the receiver.  At present, the
      combination of Destination IP, SPI, and Security Protocol (AH,
      ESP) uniquely identifies a Security Association.  Also, SPI values
      in the range 1-255 are reserved to IANA and may be used in the



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      future.  This means that, when negotiating with the same external
      host or gateway, the internal hosts behind the same NAPT can
      select the same SPI value, such that one host inbound SA is
        (SPI=470, Internal Dest IP=192.168.0.4)
      and a different host inbound SA is
        (SPI=470, Internal Dest IP=192.168.0.5).
      The receiving NAPT will not be able to determine which internal
      host an inbound IPsec packet with SPI=470 should be forwarded to.

      It is also possible for the receiving host to allocate a unique
      SPI to each unicast Security Association.  In this case, the
      Destination IP Address need only be checked to see if it is "any
      valid unicast IP for this host", not checked to see if it is the
      specific Destination IP address used by the sending host.  Using
      this technique, the NA(P)T can be assured of a low but non-zero
      chance of forwarding packets to the wrong internal host, even when
      two or more hosts establish SAs with the same external host.

      This approach is completely backwards compatible, and only
      requires the particular receiving host to make a change to its SPI
      allocation and IPsec_esp_input() code.  However, NA(P)T devices
      may not be able to detect this behavior without problems
      associated with parsing IKE payloads.  And a host may still be
      required to use a SPI in the IANA reserved range for the assigned
      purpose.

   g) Incompatibilities between embedded IP addresses and NAT.  Since
      the payload is integrity protected, any IP addresses enclosed
      within IPsec packets will not be translatable by a NAT.  This
      renders ineffective Application Layer Gateways (ALGs) implemented
      within NATs.  Protocols that utilize embedded IP addresses include
      FTP, IRC, SNMP, LDAP, H.323, SIP, SCTP (optionally), and many
      games.  To address this issue, it is necessary to install ALGs on
      the host or security gateway that can operate on application
      traffic prior to IPsec encapsulation and after IPsec
      decapsulation.

   h) Implicit directionality of NA(P)T.  NA(P)Ts often require an
      initial outbound packet to flow through them in order to create an
      inbound mapping state.  Directionality prohibits unsolicited
      establishment of IPsec SAs to hosts behind the NA(P)T.

   i) Inbound SA selector verification. Assuming IKE negotiates phase 2
      selectors, inbound SA processing will drop the decapsulated
      packet, since [RFC2401] requires a packet's source address match
      the SA selector value, which NA(P)T processing of an ESP packet
      would change.




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2.2.  NA(P)T Implementation Weaknesses

   Implementation problems present in many NA(P)Ts include:

   j) Inability to handle non-UDP/TCP traffic.  Some NA(P)Ts discard
      non-UDP/TCP traffic or perform address-only translation when only
      one host is behind the NAT.  Such NAPTs are unable to enable SCTP,
      ESP (protocol 50), or AH (protocol 51) traffic.

   k) NAT mapping timeouts.  NA(P)Ts vary in the time for which a UDP
      mapping will be maintained in the absence of traffic.  Thus, even
      where IKE packets can be correctly translated, the translation
      state may be removed prematurely.

   l) Inability to handle outgoing fragments.  Most NA(P)Ts can properly
      fragment outgoing IP packets in the case where the IP packet size
      exceeds the MTU on the outgoing interface.  However, proper
      translation of outgoing packets that are already fragmented is
      difficult and most NAPTs do not handle this correctly.  As noted
      in Section 6.3 of [RFC3022], where two hosts originate fragmented
      packets to the same destination, the fragment identifiers can
      overlap.  Since the destination host relies on the fragmentation
      identifier and fragment offset for reassembly, the result will be
      data corruption.  Few NA(P)Ts protect against identifier
      collisions by supporting identifier translation.  Identifier
      collisions are not an issue when NATs perform the fragmentation,
      since the fragment identifier need only be unique within a
      source/destination IP address pair.

      Since a fragment can be as small as 68 octets [RFC791], there is
      no guarantee that the first fragment will contain a complete TCP
      header.  Thus, a NA(P)T looking to recalculate the TCP checksum
      may need to modify a subsequent fragment.  Since fragments can be
      reordered, and IP addresses can be embedded and possibly even
      split between fragments, the NA(P)T will need to perform
      reassembly prior to completing the translation.  Few NA(P)Ts
      support this.

   m) Inability to handle incoming fragments.  Since only the first
      fragment will typically contain a complete IP/UDP/SCTP/TCP header,
      NAPTs need to be able to perform the translation based on the
      source/dest IP address and fragment identifier alone.  Since
      fragments can be reordered, the headers to a given fragment
      identifier may not be known if a subsequent fragment arrives prior
      to the initial one, and the headers may be split between
      fragments.  As a result, the NAPT may need to perform reassembly
      prior to completing the translation.  Few NAPTs support this.
      Note that with NAT, the source/dest IP address is enough to



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      determine the translation so that this does not arise.  However,
      it is possible for the IPsec or IKE headers to be split between
      fragments, so that reassembly may still be required.

2.3.  Helper Incompatibilities

   Incompatibilities between IPsec and NAT "helper" functionality
   include:

   n) Internet Security Association and Key Management Protocol (ISAKMP)
      header inspection.  Today some NAT implementations attempt to use
      IKE cookies to de-multiplex incoming IKE traffic.  As with
      source-port de-multiplexing, IKE cookie de-multiplexing results in
      problems with re-keying, since Phase 1 re-keys typically will not
      use the same cookies as the earlier traffic.

   o) Special treatment of port 500.  Since some IKE implementations are
      unable to handle non-500 UDP source ports, some NATs do not
      translate packets with a UDP source port of 500.  This means that
      these NATs are limited to one IPsec client per destination
      gateway, unless they inspect details of the ISAKMP header to
      examine cookies which creates the problem noted above.

   p) ISAKMP payload inspection.  NA(P)T implementations that attempt to
      parse ISAKMP payloads may not handle all payload ordering
      combinations, or support vendor_id payloads for IKE option
      negotiation.

3.  Requirements for IPsec-NAT Compatibility

   The goal of an IPsec-NAT compatibility solution is to expand the
   range of usable IPsec functionality beyond that available in the
   NAT-compatible IPsec tunnel mode solution described in Section 2.3.

   In evaluating a solution to IPsec-NAT incompatibility, the following
   criteria should be kept in mind:

   Deployment

      Since IPv6 will address the address scarcity issues that
      frequently lead to use of NA(P)Ts with IPv4, the IPsec-NAT
      compatibility issue is a transitional problem that needs to be
      solved in the time frame prior to widespread deployment of IPv6.
      Therefore, to be useful, an IPsec-NAT compatibility solution MUST
      be deployable on a shorter time scale than IPv6.






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      Since IPv6 deployment requires changes to routers as well as
      hosts, a potential IPsec-NAT compatibility solution, which
      requires changes to both routers and hosts, will be deployable on
      approximately the same time scale as IPv6.  Thus, an IPsec-NAT
      compatibility solution SHOULD require changes only to hosts, and
      not to routers.

      Among other things, this implies that communication between the
      host and the NA(P)T SHOULD NOT be required by an IPsec-NAT
      compatibility solution, since that would require changes to the
      NA(P)Ts, and interoperability testing between the host and NA(P)T
      implementations.  In order to enable deployment in the short term,
      it is necessary for the solution to work with existing router and
      NA(P)T products within the deployed infrastructure.

   Protocol Compatibility

      An IPsec NAT traversal solution is not expected to resolve issues
      with protocols that cannot traverse NA(P)T when unsecured with
      IPsec.  Therefore, ALGs may still be needed for some protocols,
      even when an IPsec NAT traversal solution is available.

   Security

      Since NA(P)T directionality serves a security function, IPsec
      NA(P)T traversal solutions should not allow arbitrary incoming
      IPsec or IKE traffic from any IP address to be received by a host
      behind the NA(P)T, although mapping state should be maintained
      once bidirectional IKE and IPsec communication is established.

   Telecommuter Scenario

      Since one of the primary uses of IPsec is remote access to
      corporate Intranets, a NA(P)T traversal solution MUST support
      NA(P)T traversal, via either IPsec tunnel mode or L2TP over IPsec
      transport mode [RFC3193].  This includes support for traversal of
      more than one NA(P)T between the remote client and the VPN
      gateway.

      The client may have a routable address and the VPN gateway may be
      behind at least one NA(P)T, or alternatively, both the client and
      the VPN gateway may be behind one or more NA(P)Ts.  Telecommuters
      may use the same private IP address, each behind their own NA(P)T,
      or many telecommuters may reside on a private network behind the
      same NA(P)T, each with their own unique private address,
      connecting to the same VPN gateway.  Since IKE uses UDP port 500
      as the destination, it is not necessary to enable multiple VPN
      gateways operating behind the same external IP address.



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   Gateway-to-Gateway Scenario

      In a gateway-gateway scenario, a privately addressed network (DMZ)
      may be inserted between the corporate network and the Internet.
      In this design, IPsec security gateways connecting portions of the
      corporate network may be resident in the DMZ and have private
      addresses on their external (DMZ) interfaces.  A NA(P)T connects
      the DMZ network to the Internet.

   End-to-End Scenario

      A NAT-IPsec solution MUST enable secure host-host TCP/IP
      communication via IPsec, as well as host-gateway communications.
      A host on a private network MUST be able to bring up one or
      multiple IPsec-protected TCP connections or UDP sessions to
      another host with one or more NA(P)Ts between them.  For example,
      NA(P)Ts may be deployed within branch offices connecting to the
      corporate network, with an additional NA(P)T connecting the
      corporate network to the Internet.  Likewise, NA(P)Ts may be
      deployed within a corporate network LAN or WAN to connect wireless
      or remote location clients to the corporate network.  This may
      require special processing of TCP and UDP traffic on the host.

   Bringing up SCTP connections to another host with one or more NA(P)Ts
   between them may present special challenges.  SCTP supports multi-
   homing.  If more than one IP address is used, these addresses are
   transported as part of the SCTP packet during the association setup
   (in the INIT and INIT-ACK chunks).  If only single homed SCTP end-
   points are used, [RFC2960] section 3.3.2.1 states:

         Note that not using any IP address parameters in the INIT and
         INIT-ACK is an alternative to make an association more likely
         to work across a NAT box.

   This implies that IP addresses should not be put into the SCTP packet
   unless necessary.  If NATs are present and IP addresses are included,
   then association setup will fail.  Recently [AddIP] has been proposed
   which allows the modification of the IP address once an association
   is established.  The modification messages have also IP addresses in
   the SCTP packet, and so will be adversely affected by NATs.

   Firewall Compatibility

      Since firewalls are widely deployed, a NAT-IPsec compatibility
      solution MUST enable a firewall administrator to create simple,
      static access rule(s) to permit or deny IKE and IPsec NA(P)T
      traversal traffic.  This implies, for example, that dynamic
      allocation of IKE or IPsec destination ports is to be avoided.



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   Scaling

      An IPsec-NAT compatibility solution should be capable of being
      deployed within an installation consisting of thousands of
      telecommuters.  In this situation, it is not possible to assume
      that only a single host is communicating with a given destination
      at a time.  Thus, an IPsec-NAT compatibility solution MUST address
      the issue of overlapping SPD entries and de-multiplexing of
      incoming packets.

   Mode Support

      At a minimum, an IPsec-NAT compatibility solution MUST support
      traversal of the IKE and IPsec modes required for support within
      [RFC2409] and [RFC2401].  For example, an IPsec gateway MUST
      support ESP tunnel mode NA(P)T traversal, and an IPsec host MUST
      support IPsec transport mode NA(P)T traversal.  The purpose of AH
      is to protect immutable fields within the IP header (including
      addresses), and NA(P)T translates addresses, invalidating the AH
      integrity check.  As a result, NA(P)T and AH are fundamentally
      incompatible and there is no requirement that an IPsec-NAT
      compatibility solution support AH transport or tunnel mode.

   Backward Compatibility and Interoperability

      An IPsec-NAT compatibility solution MUST be interoperable with
      existing IKE/IPsec implementations, so that they can communicate
      where no NA(P)T is present.  This implies that an IPsec-NAT
      compatibility solution MUST be backwards-compatible with IPsec as
      defined in [RFC2401] and IKE as defined in [RFC2409].  In
      addition, it SHOULD be able to detect the presence of a NA(P)T, so
      that NA(P)T traversal support is only used when necessary.  This
      implies that it MUST be possible to determine that an existing IKE
      implementation does not support NA(P)T traversal, so that a
      standard IKE conversation can occur, as described in [RFC2407],
      [RFC2408], and [RFC2409].  Note that while this implies initiation
      of IKE to port 500, there is no requirement for a specific source
      port, so that UDP source port 500 may or may not be used.

   Security

      An IPsec-NAT compatibility solution MUST NOT introduce additional
      IKE or IPsec security vulnerabilities.  For example, an acceptable
      solution must demonstrate that it introduces no new denial of
      service or spoofing vulnerabilities.  IKE MUST be allowed to re-
      key in a bi-directional manner as described in [RFC2408].





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4.  Existing Solutions

4.1.  IPsec Tunnel Mode

   In a limited set of circumstances, it is possible for an IPsec tunnel
   mode implementation, such as that described in [DHCP], to traverse
   NA(P)T successfully.  However, the requirements for successful
   traversal are sufficiently limited so that a more general solution is
   needed:

   1) IPsec ESP.  IPsec ESP tunnels do not cover the outer IP header
      within the message integrity check, and so will not suffer
      Authentication Data invalidation due to address translation.
      IPsec tunnels also need not be concerned about checksum
      invalidation.

   2) No address validation.  Most current IPsec tunnel mode
      implementations do not perform source address validation so that
      incompatibilities between IKE identifiers and source addresses
      will not be detected.  This introduces security vulnerabilities as
      described in Section 5.

   3) "Any to Any" SPD entries.  IPsec tunnel mode clients can negotiate
      "any to any" SPDs, which are not invalidated by address
      translation.  This effectively precludes use of SPDs for the
      filtering of allowed tunnel traffic.

   4) Single client operation.  With only a single client behind a NAT,
      there is no risk of overlapping SPDs.  Since the NAT will not need
      to arbitrate between competing clients, there is also no risk of
      re-key mis-translation, or improper incoming SPI or cookie
      de-multiplexing.

   5) No fragmentation.  When certificate authentication is used, IKE
      fragmentation can be encountered.  This can occur when certificate
      chains are used, or even when exchanging a single certificate if
      the key size, or the size of other certificate fields (such as the
      distinguished name and other extensions), is large enough.
      However, when pre-shared keys are used for authentication,
      fragmentation is less likely.

   6) Active sessions.  Most VPN sessions typically maintain ongoing
      traffic flow during their lifetime so that UDP port mappings are
      less likely be removed due to inactivity.







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4.2.  RSIP

   RSIP, described in [RSIP] and [RSIPFrame], includes mechanisms for
   IPsec traversal, as described in [RSIPsec].  By enabling host-NA(P)T
   communication, RSIP addresses issues of IPsec SPI de-multiplexing, as
   well as SPD overlap.  It is thus suitable for use in enterprises, as
   well as home networking scenarios.  By enabling hosts behind a NAT to
   share the external IP address of the NA(P)T (the RSIP gateway), this
   approach is compatible with protocols including embedded IP
   addresses.

   By tunneling IKE and IPsec packets, RSIP avoids changes to the IKE
   and IPsec protocols, although major changes are required to host IKE
   and IPsec implementations to retrofit them for RSIP-compatibility.
   It is thus compatible with all existing protocols (AH/ESP) and modes
   (transport and tunnel).

   In order to handle de-multiplexing of IKE re-keys, RSIP requires
   floating of the IKE source port, as well as re-keying to the floated
   port.  As a result, interoperability with existing IPsec
   implementations is not assured.

   RSIP does not satisfy the deployment requirements for an IPsec-NAT
   compatibility solution because an RSIP-enabled host requires a
   corresponding RSIP-enabled gateway in order to establish an IPsec SA
   with another host.  Since RSIP requires changes only to clients and
   routers and not to servers, it is less difficult to deploy than IPv6.
   However, for vendors, implementation of RSIP requires a substantial
   fraction of the resources required for IPv6 support.  Thus, RSIP
   solves a "transitional" problem on a long-term time scale, which is
   not useful.

4.3.  6to4

   6to4, as described in [RFC3056] can form the basis for an IPsec-NAT
   traversal solution.  In this approach, the NAT provides IPv6 hosts
   with an IPv6 prefix derived from the NAT external IPv4 address, and
   encapsulates IPv6 packets in IPv4 for transmission to other 6to4
   hosts or 6to4 relays.  This enables an IPv6 host using IPsec to
   communicate freely to other hosts within the IPv6 or 6to4 clouds.

   While 6to4 is an elegant and robust solution where a single NA(P)T
   separates a client and VPN gateway, it is not universally applicable.
   Since 6to4 requires the assignment of a routable IPv4 address to the
   NA(P)T in order to allow formation of an IPv6 prefix, it is not
   usable where multiple NA(P)Ts exist between the client and VPN





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   gateway.  For example, an NA(P)T with a private address on its
   external interface cannot be used by clients behind it to obtain an
   IPv6 prefix via 6to4.

   While 6to4 requires little additional support from hosts that already
   support IPv6, it does require changes to NATs, which need to be
   upgraded to support 6to4.  As a result, 6to4 may not be suitable for
   deployment in the short term.

5.  Security Considerations

   By definition, IPsec-NAT compatibility requires that hosts and
   routers implementing IPsec be capable of securely processing packets
   whose IP headers are not cryptographically protected.  A number of
   issues arise from this that are worth discussing.

   Since IPsec AH cannot pass through a NAT, one of the side effects of
   providing an IPsec-NAT compatibility solution may be for IPsec ESP
   with null encryption to be used in place of AH where a NAT exists
   between the source and destination.  However, it should be noted that
   ESP with null encryption does not provide the same security
   properties as AH.  For example, there are security risks relating to
   IPv6 source routing that are precluded by AH, but not by ESP with
   null encryption.

   In addition, since ESP with any transform does not protect against
   source address spoofing, some sort of source IP address sanity
   checking needs to be performed.  The importance of the anti-spoofing
   check is not widely understood.  There is normally an anti-spoofing
   check on the Source IP Address as part of IPsec_{esp,ah}_input().
   This ensures that the packet originates from the same address as that
   claimed within the original IKE Phase 1 and Phase 2 security
   associations.  When a receiving host is behind a NAT, this check
   might not strictly be meaningful for unicast sessions, whereas in the
   Global Internet this check is important for tunnel-mode unicast
   sessions to prevent a spoofing attack described in [AuthSource],
   which can occur when access controls on the receiver depend upon the
   source IP address of verified ESP packets after decapsulation.
   IPsec-NAT compatibility schemes should provide anti-spoofing
   protection if it uses source addresses for access controls.

   Let us consider two hosts, A and C, both behind (different) NATs, who
   negotiate IPsec tunnel mode SAs to router B.  Hosts A and C may have
   different privileges; for example, host A might belong to an employee
   trusted to access much of the corporate Intranet, while C might be a
   contractor only authorized to access a specific web site.





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   If host C sends a tunnel mode packet spoofing A's IP address as the
   source, it is important that this packet not be accorded the
   privileges corresponding to A.  If authentication and integrity
   checking is performed, but no anti-spoofing check (verifying that the
   originating IP address corresponds to the SPI) then host C may be
   allowed to reach parts of the network that are off limits.  As a
   result, an IPsec-NAT compatibility scheme MUST provide some degree of
   anti-spoofing protection.

6.  References

6.1.  Normative References

   [RFC791]     Postel, J., "Internet Protocol", STD 5, RFC 791,
                September 1981.

   [RFC793]     Postel, J., "Transmission Control Protocol", STD 7, RFC
                793, September 1981.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2401]    Atkinson, R. and S. Kent, "Security Architecture for the
                Internet Protocol", RFC 2401, November 1998.

   [RFC2402]    Kent, S. and R. Atkinson, "IP Authentication Header",
                RFC 2402, November 1998.

   [RFC2406]    Kent,S. and R. Atkinson, "IP Encapsulating Security
                Payload (ESP)", RFC 2406, November 1998.

   [RFC2407]    Piper, D., "The Internet IP Security Domain of
                Interpretation for ISAKMP", RFC 2407, November 1998.

   [RFC2409]    Harkins, D. and D. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.

   [RFC2663]    Srisuresh, P. and M. Holdredge, "IP Network Address
                Translator (NAT) Terminology and Considerations", RFC
                2663, August 1999.

   [RFC3022]    Srisuresh, P. and K. Egevang, "Traditional IP Network
                Address Translator (Traditional NAT)", RFC 3022, January
                2001.







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6.2.  Informative References

   [RFC2408]    Maughan, D., Schertler, M., Schneider, M. and J. Turner,
                "Internet Security Association and Key Management
                Protocol (ISAKMP)", RFC 2408, November 1998.

   [RFC2960]    Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
                Zhang, M. and V. Paxson, "Stream Control Transmission
                Protocol", RFC 2960, October 2000.

   [RFC3056]    Carpenter, B. and K. Moore, "Connection of IPv6 Domains
                via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3193]    Patel, B., Aboba, B., Dixon, W., Zorn, G. and S. Booth,
                "Securing L2TP using IPsec", RFC 3193, November 2001.

   [RFC3309]    Stone, J., Stewart, R. and D. Otis, "Stream Control
                Transmission Protocol (SCTP) Checksum Change", RFC 3309,
                September 2002.

   [RSIPFrame]  Borella, M., Lo, J., Grabelsky, D. and G. Montenegro,
                "Realm Specific IP: Framework", RFC 3102, October 2001.

   [RSIP]       Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi,
                "Realm Specific IP: Protocol Specification", RFC 3103,
                October 2001.

   [RSIPsec]    Montenegro, G. and M. Borella, "RSIP Support for End-
                to-End IPsec", RFC 3104, October 2001.

   [DHCP]       Patel, B., Aboba, B., Kelly, S. and V. Gupta, "Dynamic
                Host Configuration Protocol (DHCPv4) Configuration of
                IPsec Tunnel Mode", RFC 3456, January 2003.

   [AuthSource] Kent, S., "Authenticated Source Addresses", IPsec WG
                Archive (ftp://ftp.ans.net/pub/archive/IPsec), Message-
                Id:  <v02130517ad121773c8ed@[128.89.0.110]>, January 5,
                1996.

   [AddIP]      Stewart, R., et al., "Stream Control Transmission
                Protocol (SCTP) Dynamic Address Reconfiguration", Work
                in Progress.








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7.  Acknowledgments

   Thanks to Steve Bellovin of AT&T Research, Michael Tuexen of Siemens,
   Peter Ford of Microsoft, Ran Atkinson of Extreme Networks, and Daniel
   Senie for useful discussions of this problem space.

8.  Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329
   EMail: bernarda@microsoft.com


   William Dixon
   V6 Security, Inc.
   601 Union Square, Suite #4200-300
   Seattle, WA 98101

   EMail: ietf-wd@v6security.com



























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9.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 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.

Intellectual Property

   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 procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   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
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   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









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