Internet DRAFT - draft-touch-tcp-ao-nat
draft-touch-tcp-ao-nat
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
draft-touch-tcp-ao-nat-05.txt
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Abstract
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
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 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
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.
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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.
2011.
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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