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IPsec Working Group                                        Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Informational
<draft-aboba-nat-ipsec-04.txt>
30 May 2001

                  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 RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
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Copyright Notice

Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

Perhaps the most common use of IPsec is in providing virtual private
networking capabilities. One very popular use of VPNs is to provide
tele-commuter access to the corporate Intranet.  Today NATs are widely
deployed in home gateways, as well as in other locations likely to be
used by tele-commuters, 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
them.

1.  Introduction

Perhaps the most common use of IPsec [6] is in providing virtual private
networking capabilities. One very popular use of VPNs is to provide



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tele-commuter access to the corporate Intranet.  Today NATs [8]-[9] are
widely deployed in home gateways, as well as in other locations likely
to be used by tele-commuters, 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 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 [2].

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 NAT and IPsec

The incompatibilities between NAT and IPsec are numerous, ranging from
the obvious to the subtle. Some of the known incompatibilities include:

a) Incompatibility between IPsec AH [3] 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/UDP/SCTP
   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



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   a NAT or reverse NAT device.

   As a result, IPsec ESP will only pass unimpeded through a NAT if
   TCP/UDP/SCTP 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 [13], TCP checksum
   calculation and verification is required in IPv4. UDP/TCP
   checksum calculation and verification is required in IPv6.

   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/SCTP
   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 [7]
   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 [7], 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
   [5]. 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. This is typically accomplished by translating
   the IKE UDP source port. While it is permissible to float
   the IKE UDP source port, this can result in unpredictable
   behavior during re-keys. Unless the floated source port is
   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 IKE cookie usage and NAT.
   Today some NAT implementations attempt to use IKE cookies
   to de-multiplex incoming IKE traffic. As with source-port



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   de-multiplexing, IKE cookie de-multiplexing also results
   in problems with re-keying, since re-keys typically will
   not use the same cookies as the earlier traffic.

f) 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.

g) 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 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=1.2.3.4) and  a different Security Association with
   (SPI=470, Dest IP=2.3.4.5).

   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.




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h) 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. Protocols that utilize embedded IP addresses include
   FTP, IRC, SNMP, LDAP, H.323, SIP and many games.

2.1.  What works

Given the incompatibilities described above, it might seem unlikely that
any IPsec/IKE conversation could survive passage through a NAT. However,
IPsec tunnel mode clients [15] are capable of traversing NATs under
limited conditions:

[1]  IPsec ESP. IPsec ESP tunnels do not cover the outer IP header
     within the authentication hash, and so will not suffer hash
     invalidation due to address translation. IPsec tunnels also need
     not be concerned about checksum invalidation (unlike L2TP).

[2]  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 the
     security considerations section.

[3]  SPD entries. Most IPsec tunnel mode clients 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-
     multiplexing.

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:

Deployability
          Since IPv6 will address the address scarcity issues that
          frequently lead to use of NATs with IPv4, the IPsec-NAT
          compability issue is a transitional problem that needs to be



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          solved in the timeframe 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
          compatibility solution SHOULD require changes only to hosts,
          and not to routers.

          Among other things, this implies that communication between
          the host and the NAT MUST NOT be required by an IPsec-NAT
          compatibility solution, since existing NATs cannot meet such a
          requirement.

Telecommuter scenario
          Since a typical telecommuter is only interested in obtaining
          access to the corporate Intranet for itself, an IPsec-NAT
          compatibility solution need not enable gateway-gateway
          connectivity. Thus, it can be assumed that negotiated SPD
          entries will only refer to the communication endpoints, and
          there is no need to support negotiation of SPD entries
          involving subnets.  to

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
          passage of IPsec ESP tunnel mode through a NAT.  Since IPsec
          transport mode is used for tunneling protocols such as L2TP
          [1], an IPsec-NAT compatibility solution SHOULD support IPsec
          transport mode using ESP, at least for protection of UDP
          traffic where no embedded IP addresses are present.  Since
          passage of AH through a NAT is not possible in any mode, there
          is no need for an IPsec-NAT compatibility solution to attempt
          to address this.

Interoperability
          An IPsec-NAT compatibility solution MUST be interoperable with
          existing IPsec implementations.  Thus, existing IPsec
          implementations MUST be able to communicate with an IPsec-NAT



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          compatible implementation in the case where no NAT is present.
          This implies that an IPsec-NAT compatibility solution MUST be
          backwards-compatible with IPsec as defined in [3]-[7], and
          SHOULD be able to detect the presence of a NAT so that the
          required changes for NAT compatibility will only be used when
          necessary.

          For example, it may be possible to enable ISAKMP to pass
          information about each host's perception of its own IP
          address, in order to make key management aware of the presence
          of the NAT and facilitate the use of standard key management
          methods through a NAT to support ESP/AH.

Security  An IPsec-NAT compatibility solution MUST NOT introduce
          additional security vulnerabilities into IKE. For example, an
          acceptable solution must demonstrate that it introduces no new
          denial of service or spoofing vulnerabilities.

4.  Existing solutions


4.1.  RSIP

RSIP, described in [10]-[11], includes mechanisms for IPsec traversal,
as described in [12]. 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
(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, inter-operability with existing IPsec implementations
is not assured.

RSIP does not satisfy the deployability 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



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fraction of the resources required for IPv6 support. Thus, RSIP solves a
"transitional" problem on a long-term time scale, which is not useful.

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 IP 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
[14].

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
protection.







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6.  Acknowledgments

Thanks to Steve Bellovin of AT&T Research, William Dixon of Microsoft,
Ran Atkinson of Extreme Networks and Daniel Senie for useful discussions
of this problem space.

7.  References

[1]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and
     Palter, B., "Layer Two Tunneling Protocol L2TP", RFC 2661, August
     1999.

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

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

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

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

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

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

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

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

[10] Borella, M., Lo, J., Grabelsky, D., Montenegro, G., "Realm Specific
     IP: A Framework", Internet draft (work in progress), draft-ietf-
     nat-rsip-framework-05.txt, July 2000.

[11] Borella, M., Grabelsky, D., Lo, J., Taniguchi, K., "Realm Specific
     IP: Protocol Specification", Internet draft (work in progress),
     draft-ietf-nat-rsip-protocol-07.txt, July 2000.

[12] Montenegro, G., Borella, M., "RSIP Support for End-to-End IPsec",
     Internet draft (work in progress), draft-ietf-nat-rsip-
     IPsec-04.txt, July 2000.




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[13] Information Sciences Institute, "Transmission Control Protocol",
     RFC 793, September 1981.

[14] 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.

[15] Patel, B., Aboba, B., Kelly, S., Gupta, V., "DHCPv4 Configuration
     of IPsec Tunnel Mode", Internet draft (work in progress), draft-
     ietf-ipsec-dhcp-12.txt, May 2001.

8.  Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

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

9.  Intellectual Property Statement

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The IETF invites any interested party to bring to its attention any
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10.  Full Copyright Statement

Copyright (C) The Internet Society (2000).  All Rights Reserved.

This document and translations of it may be copied and furnished to



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others, and derivative works that comment on or otherwise explain it or
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11.  Expiration Date

This memo is filed as <draft-aboba-nat-IPsec-04.txt>, and expires
December 1, 2001.




























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