Independent Submission J. Touch
Request for Comments: 6978 USC/ISI
Category: Experimental July 2013
ISSN: 2070-1721
A TCP Authentication Option Extension for NAT Traversal
Abstract
This document describes an extension to the TCP Authentication Option
(TCP-AO) to support its use over connections that pass through
Network Address Translators and/or Network Address and Port
Translators (NATs/NAPTs). This extension changes the data used to
compute traffic keys, but it does not alter TCP-AO's packet
processing or key generation algorithms.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6978.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
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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. References ......................................................5
7.1. Normative References .......................................5
7.2. Informative References .....................................5
8. 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 Translators and/or Network Address and
Port Translators (NATs/NAPTs) [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 it 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",
"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
[RFC5925]. The following information is used to create a
connection's traffic keys. (Note that 'local' and 'remote' are
interpreted as in TCP-AO [RFC5925].)
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 segments.
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 it
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
includes the TCP connection identifier, the TCP option flag
(indicating whether TCP options other than TCP-AO are included in the
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RFC 6978 TCP-AO NAT Extension July 2013
Message Authentication Code (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. These flags modify TCP-AO processing rules as follows:
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.
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.
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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 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 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 Key Derivation Function (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 of the IPv4 addresses and ports is not
expected to have a significant impact.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
7.2. Informative References
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
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[RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
for the TCP Authentication Option (TCP-AO)", RFC 5926,
June 2010.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
January 2011.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013.
8. Acknowledgments
This extension was inspired by discussions with Dan Wing.
This document was initially 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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