IPSEC Working Group                                        Bernard Aboba
INTERNET-DRAFT                                             William Dixon
Category: Informational                                        Microsoft
20 October 2003

                  IPsec-NAT Compatibility Requirements

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
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Copyright Notice

Copyright (C) The Internet Society (2003).  All Rights Reserved.


Perhaps the most common use of IPsec is in providing virtual private
networking capabilities. One very popular use of 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 to deployment of IPsec in
one of its principal uses.  This draft describes known incompatibilities
between NAT and IPsec, and describes the requirements for addressing

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Table of Contents

1.     Introduction ..........................................    3
   1.1       Requirements language ...........................    3
2.     Known incompatibilities between NA(P)T and IPsec ......    3
   2.1       Intrinsic NA(P)T issues .........................    4
   2.2       NA(P)T implementation weaknesses ................    7
   2.3       Helper incompatibilities ........................    8
3.     Requirements for IPsec-NAT compatibility ..............    8
4.     Existing solutions ....................................   11
   4.1       IPsec tunnel mode ...............................   11
   4.2       RSIP ............................................   12
   4.3       6to4 ............................................   13
5.     Security considerations ...............................   13
6.     References ............................................   14
   6.1       Normative references ............................   14
   6.2       Informative references ..........................   15
ACKNOWLEDGMENTS ..............................................   16
AUTHORS' ADDRESSES ...........................................   16
Full Copyright Statement .....................................   16
Intellectual property statement ..............................   17

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1.  Introduction

Perhaps the most common use of IPsec [RFC2401] is in providing virtual
private networking capabilities.  One very popular use of VPNs is to
provide telecommuter access to the corporate Intranet.  Today 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 to deployment of IPsec in
one of its principal uses.  This draft describes known incompatibilities
between NAT and IPsec, and describes the requirements for addressing

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.

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

[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

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     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

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 [4] does not incorporate the IP source and
   destination addresses in its keyed message integrity check,
   this issue does not arise for ESP.

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 on receipt, they will be invalidated by passage through
   a NAT or reverse NAT device.

   As a result, IPsec ESP will only pass unimpeded through a NAT if
   TCP/UDP protocols are not involved (as in IPsec tunnel
   mode or IPsec/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.

   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

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   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 IKE MM [RFC2409]
   or QM, 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 MM and QM 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 identity matches that enclosed in the certificate.
   However, while use of USER_FQDN or FQDN identity types is possible
   within IKE, there are usage scenarios (e.g. SPD entries
   describing subnets) that cannot be accommodated this way.

d) Incompatibility between fixed IKE destination 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

e) Incompatibilities between overlapping SPD entries and NAT.
   Where hosts behind a NAT negotiate overlapping SPD entries
   with the same destination in IKE QM, packets may be sent
   down the wrong IPsec SA.  This occurs because to the
   sender, 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

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   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 bring up IPsec SAs to
   the same destination simultaneously, it is possible that
   the NAT will send 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.  At present, the combination of
   Destination IP, SPI, and Security Protocol (AH, ESP) uniquely
   identifies a Security Association.  This means that the
   receiving host can select SPIs such that
   it has one Security Association (SA) with (SPI=470,
   Dest IP= and  a different Security Association with
   (SPI=470, Dest IP=

   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.  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.

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 which 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 inbound mapping state.  Directionality prohibits
   unsolicited establishment of IPsec SAs to hosts behind the NA(P)T.

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2.2.  NA(P)T implementation weaknesses

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

i) Inability to handle non-UDP/TCP traffic.  Some NAPTs discard
   non-UDP/TCP traffic.  Such NAPTs are unable to pass
   SCTP,  ESP (protocol 50) or AH (protocol 51) traffic.

j) 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.

k) 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 [RFC790],
   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.

l) 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 determine the translation,

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   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:

m) 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.

n) 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.

o) 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

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 an NAT-compatible
IPsec tunnel mode solution described above.

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

          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.

          Since IPv6 deployment requires changes to routers as well as
          hosts, a 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

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          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

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

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

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 [RFC 3193].  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.

          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

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          portions of the corporate network will have private addresses
          on their external interfaces.  A NA(P)T connects the DMZ
          network to the Internet.

          A NAT-IPsec solution MUST enable secure host-host
          communication via IPsec as well as host-gateway
          communications.  A host on a private network MUST be able to
          bring up 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.  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 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

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

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          de-multiplexing of incoming packets.

Mode support
          At a minimum, an IPsec-NAT compatibility solution MUST support
          traversal of the IPsec modes required for support within
          [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].

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 limiting that a more general solution is needed:

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[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 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-

[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 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.

4.2.  RSIP

RSIP, described in [RSIP] and [RSIPFrame], includes mechanisms for IPsec
traversal, as described in [RSIPsec].  By enabling host-gateway
communication, RSIP addresses issues of IPsec SPI de-multiplexing as
well as SPD overlap.  It is thus suitable for use in enterprise as well
as home networking scenarios.  By enabling hosts behind a NAT to share
the external IP address of the 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

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(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 a 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 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 gateway.  For example,
a 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

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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 was claimed within
the original IKE MM and QM 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

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.

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

6.  References

6.1.  Normative references

[RFC791]  ISI, "Internet Protocol Specification", RFC 791, September

[RFC793]  Information Sciences Institute, "Transmission Control
          Protocol", 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.

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INTERNET-DRAFT       IPsec-NAT Compatibility Reqts.      20 October 2003

[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
          of 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.

6.2.  Informative references

[RFC2661] Townsley, W., et al., "Layer Two Tunneling Protocol L2TP", RFC
          2661, August 1999.

[RFC2960] Stewart, R., et al., "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.

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

          Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm
          Specific IP: A 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, "DHCPv4
          Configuration of IPsec Tunnel Mode", RFC 3456, January 2003.

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

[AddIP]   Stewart, R., et al., "Stream Control Transmission Protocol
          (SCTP) Dynamic Address Reconfiguration", Internet draft (work
          in progress), draft-ietf-tsvwg-addip-sctp-06.txt, September


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.

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
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: w_dixon@comcast.net

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INTERNET-DRAFT       IPsec-NAT Compatibility Reqts.      20 October 2003

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