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Versions: 00 01 02 03 04 05 RFC 6978

TCPM WG                                                        J. Touch
Internet Draft                                                  USC/ISI
Intended status: Proposed Standard                     January 31, 2010
Expires: July 2010



                 A TCP Authentication Option NAT Extension
                       draft-touch-tcp-ao-nat-01.txt


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   This Internet-Draft will expire on July 31, 2010.

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Abstract

   This document describes an extension to the TCP Authentication
   Option to support its use over connections that pass through network
   address and/or port translators (NATs/NAPTs). This document
   describes this extension, and is intended that if desired, would be
   incorporated into the TCP-AO description currently under
   development. This document is not intended to remain stand-alone.

Table of Contents


   1. Introduction...................................................2
   2. Conventions used in this document..............................2
   3. Background.....................................................3
   4. Extension to Allow NAT Use.....................................3
   5. Intended Use...................................................4
   6. Security Considerations........................................4
   7. IANA Considerations............................................5
   8. References.....................................................5
      8.1. Normative References......................................5
      8.2. Informative References....................................5
   9. Acknowledgments................................................5

1. Introduction

   This document assumes detailed familiarity with TCP-AO [ID-TCP-AO].
   It is intended as an extension to TCP-AO, and is described
   separately to allow its separate consideration. If approved, it is
   expected that this document would be integrated with TCP-AO. As a
   result, no background is included herein.

   TCP-AO can be extended to support use through NAT/NAPT devices
   [RFC2663]. These devices translate the source address and/or the
   source port number of a TCP connection. TCP-AO without these
   extensions would be sensitive to these modifications, and would
   discard authenticated segments.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [RFC2119].
   When used in lower case, these words have their conventional meaning
   and do not convey the interpretations in RFC-2119.




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3. Background

   TCP-AO generates traffic keys that are specific to a socket pair
   [ID-TCP-AO]. Using the TCP-AO convention (local = source for
   outgoing segments, destination for incoming segments), the following
   information is used to create a connection's traffic keys:

   o  IP local address

   o  IP remote address

   o  TCP local port

   o  TCP remote port

   o  TCP local Initial Sequence Number (ISN)

   o  TCP remote Initial Sequence Number (ISN)

   Of these fields, the remote ISN is not known when for SYN segments,
   and is excluded from the traffic key used to authentication them.
   Otherwise, all fields are used in the traffic keys of all other
   segments.

   NATs and NAPTs (here just "NATs", even if port translation is
   included) would interfere with these uses, because they alter the
   local IP address and local TCP port [RFC2663].

4. Extension to Allow NAT Use

   It might be useful to allow TCP-AO use in the presence of NATs,
   e.g., to protect client/server communication where clients are
   behind NATs.

   We propose an extension to TCP-AO that enables its use in the
   presence of NATs. This extension requires no modification to the
   TCP-AO header. The change is limited to the way in which traffic
   keys are generated. We call this TCP-AO-NAT.

   For TCP-AO-NAT, there are two additional flags for each TCP
   connection. These flags, which could be copied from parameters of
   the MKT or set on a per-connection basis, are:

   o  localNAT

   o  remoteNAT



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   These flags indicate whether a segment's local or remote
   (respectively) IP address and TCP port are zeroed before MAC
   calculation, either for creating the MAC to insert (for outgoing
   segments) or for calculating a MAC to validate against the value in
   the option. I.e., these would modify the processing rules as
   follows:

   o  Traffic keys are computed by zeroing the local/remote IP address
      and TCP port as indicated by the localNAT and remoteNAT flags.

   o  MAC values are computed by zeroing the local/remote IP address
      and TCP port as indicated by the localNAT and remoteNAT flags.

5. Intended Use

   A client behind a NAT, or that suspects being behind a NAT, would
   set localNAT=TRUE. A server willing to support incoming TCP-AO-NAT
   connections would set remoteNAT=TRUE. Peer-to-peer applications with
   dual NAT support, e.g., those traversing symmetric NATs, would set
   both localNAT=TRUE and remoteNAT=TRUE [RFC5389].

6. Security Considerations

   TCP-AO-NAT does not affect the security of connections that set
   neither of the localNAT or remoteNAT flags. Such connections are not
   affected themselves, and are not affected by segments in other
   connections that set those flags.

   Setting either the localNAT or remoteNAT flags reduces the entropy
   of the input to the KDF used to generate the traffic keys. The
   largest impact occurs when using IPv4, which reduces the entropy
   from 2 IPv4 addresses, 2 ISNs, and both ports down to 1 IPv4
   address, 2 ISNs, and only the dynamic port. The amount of entropy in
   the IPv4 addresses and service port is likely to be small, and the
   entropy of the dynamic port is under debate and should not be
   considered substantial [La09]; the KDF input entropy is thus
   expected to be dominated by that of the ISNs, so reducing it by one
   IPv4 address and the service port may not have a significant impact.
   IPv6 addresses can have substantially more entropy because they
   usually include the device's MAC address, which itself has more
   entropy than IPv6 prefixes or IPv4 address in total.

   TCP-AO-NAT SHOULD NOT be used with both flags set in IPv4, however,
   as the result would rely entirely on the ISNs alone.



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7. IANA Considerations

   There are no IANA considerations for this document. This section can
   be removed upon publication as an RFC.

8. References

8.1. Normative References

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

   [ID-TCP-AO] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
             Option",draft-ietf-tcpm-tcp-auth-opt-09.txt, Jan. 2010.

8.2. Informative References

   [La09]    Larsen, M., F. Gont, "Port Randomization," draft-ietf-
             tsvwg-port-randomization-05, Nov. 09.

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

   [RFC5389] Rosenberg, J., R. Mahy, P. Matthews, D. Wing, "Session
             Traversal Utilities for NAT (STUN)", RFC 5389, Oct. 2008.

9. Acknowledgments

   This extension was inspired by discussions with Dan Wing.

   This document was prepared using 2-Word-v2.0.template.dot.

Author's Address

   Joe Touch
   USC/ISI
   4676 Admiralty Way
   Marina del Rey, CA 90292
   USA

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu






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