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Versions: 00 01 02 03 04 05 RFC 6978

TCPM WG                                                        J. Touch
Internet Draft                                                  USC/ISI
Intended status: Experimental                              May 23, 2013
Expires: November 2013

          A TCP Authentication Option Extension for NAT Traversal

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on November 23, 2013.

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   This document describes an extension to the TCP Authentication
   Option (TCP-AO) to support its use over connections that pass
   through network address and/or port translators (NATs/NAPTs). This
   extension changes the data used to compute traffic keys, but does
   not alter TCP-AO's packet processing or key generation algorithms.

Table of Contents

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

1. Introduction

   This document describes an extension to the TCP Authentication
   Option (TCP-AO) [RFC5925] called TCP-AO-NAT to support its use in
   the presence of network address and/or port translators (NAT/NAPT)
   [RFC2663]. These devices translate the source address and/or the
   source port number of a TCP connection. TCP-AO without TCP-AO-NAT
   extensions would be sensitive to these modifications, and would
   discard authenticated segments.

   At least one potential application of TCP-AO-NAT is to support the
   experimental multipath TCP protocol [RFC6824], which uses multiple
   IP addresses to support a single TCP transfer.

   This document assumes detailed familiarity with TCP-AO [RFC5925]. As
   a preview, this document focuses on how TCP-AO generates traffic
   keys, and does not otherwise alter the TCP-AO mechanism or that of
   its key generation [RFC5926].

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

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   When used in lower case, these words have their conventional meaning
   and do not convey the interpretations in RFC-2119.

3. Background

   TCP-AO generates traffic keys that are specific to a socket pair
   [RFC5925]. 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 for SYN segments, and
   is excluded from the traffic key used to authenticate them.
   Otherwise, all fields are used in the traffic keys of all other

   NATs and NAPTs (both referred to herein as "NATs", even if port
   translation is included) would interfere with these uses, because
   they alter the IP address and TCP port of the endpoint behind the
   NAT [RFC2663].

4. Extension to Allow NAT Traversal

   The premise of TCP-AO-NAT is that 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.

   This document describes TCP-AO-NAT, an extension to TCP-AO that
   enables its use in the presence of NATs. This extension requires no
   modification to the TCP-AO header or packet processing, and requires
   no modification to the algorithms used to generate traffic keys
   [RFC5926]. The change is limited to the data used to generate
   traffic keys only.

   In TCP-AO, "a Master Key Tuple (MKT) describes the TCP-AO properties
   to be associated with one or more connections" [RFC5925]. This

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   includes the TCP connection identifier, the TCP option flag
   (indicating whether TCP options other than TCP-AO are included in
   the MAC calculation), keying information, and other parameters. TCP-
   AO-NAT augments the MKT with two additional flags:

   o  localNAT

   o  remoteNAT

   TCP-AO implementations supporting TCP-AO-NAT MUST support both
   localNAT and remoteNAT flags.

   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 TCP-AO processing rules as

   o  In TCP-AO-NAT, traffic keys are computed by zeroing the
      local/remote IP address and TCP port as indicated by the localNAT
      or remoteNAT flags.

   o  In TCP-AO-NAT, MAC values are computed by zeroing the
      local/remote IP address and TCP port as indicated by the localNAT
      or remoteNAT flags.

   The use of these flags needs to match on both ends of the
   connection, just as with all other MKT parameters.

5. Intended Use

   A host MAY use TCP-AO-NAT when it is behind a NAT, as determined
   using NAT discovery techniques, or when TCP-AO protection is desired
   but conventional TCP-AO fails to establish connections.

   A client behind a NAT MAY set localNAT=TRUE for MKTs supporting TCP-
   AO-NAT for outgoing connections. A server MAY set remoteNAT=TRUE for
   MKTs supporting TCP-AO-NAT for incoming connections. Peer-to-peer
   applications with dual NAT support, e.g., those traversing so-called
   'symmetric NATs' [RFC5389], MAY set both localNAT=TRUE and
   remoteNAT=TRUE for MKTs supporting TCP-AO-NAT bidirectionally. Once
   these flags are set in an MKT, they affect all connections that
   match that MKT.

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   TCP-AO-NAT is intended for use only where coordinated between
   endpoints for connections that match the shared MKT parameters, as
   with all other MKT parameters.

   Note that TCP-AO-NAT is not intended for use with services
   transiting application layer gateways (ALGs), i.e., NATs that also
   translate in-band addresses, such as used in FTP or SIP. TCP-AO-NAT
   protects the contents of the TCP segments from modification, and
   would (correctly) interpret with such alterations as an attack on
   those contents.

6. Security Considerations

   TCP-AO-NAT does not affect the security of connections that do not
   set either 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
   randomness of the input to the KDF used to generate the traffic
   keys. The largest impact occurs when using IPv4, which reduces the
   randomness from 2 IPv4 addresses, 2 ISNs, and both ports down to
   just the two ISNs when both flags are set. The amount of randomness
   in the IPv4 addresses and service port is likely to be small, and
   the randomness of the dynamic port is under debate and should not be
   considered substantial [RFC6056]. The KDF input randomness is thus
   expected to be dominated by that of the ISNs, so reducing it by
   either or both the IPv4 addresses and ports is not expected to have
   a significant impact.

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.

   [RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
             Option", RFC 5925, Jun. 2010.

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8.2. Informative References

   [RFC6824]  Ford, A., C. Raiciu, M. Handley, O. Bonaventure, "TCP
             Extensions for Multipath Operation with Multiple
             Addresses", RFC 6824, Jan. 2013.

   [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.

   [RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
             for the TCP Authentication Option (TCP-AO)", RFC 5926,
             June 2010.

   [RFC6056] Larsen, M., F. Gont, "Port Randomization," RFC 6056, Jan.

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
   4676 Admiralty Way
   Marina del Rey, CA 90292

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

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