TCPM WG J. Touch
Internet Draft USC/ISI
Intended status: Experimental May 30, 2012
Expires: November 2012
A TCP Authentication Option NAT Extension
draft-touch-tcp-ao-nat-03.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 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 describes an extension to the TCP Authentication
Option (TCP-AO) [RFC5925] 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 these extensions would be
sensitive to these modifications, and would discard authenticated
segments.
At least one potential application of this extension is to support
the experimental multipath TCP protocol [Fo11], 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 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.
This document describes an extension to TCP-AO that enables its use
in the presence of NATs called TCP-AO-NAT. 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.
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:
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o localNAT
o remoteNAT
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 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
randonmess 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 1
IPv4 address, 2 ISNs, and only the dynamic port. 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 one IPv4 address and the service port may not have a
significant impact. IPv6 addresses can have substantially more
randomness because they usually include the device's MAC address,
which itself has more randomness than IPv6 prefixes or IPv4 address
in total.
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TCP-AO-NAT SHOULD NOT be used with both flags set in IPv4, however,
as the result would rely entirely on the ISNs alone.
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.
8.2. Informative References
[Fo11] Ford, A., C. Raiciu, M. Handley, O. Bonaventure, "TCP
Extensions for Multipath Operation with Multiple
Addresses", (work in progress), draft-ietf-mptcp-
multiaddressed, Mar. 2011.
[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.
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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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