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 Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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Touch Expires November 30, 2012 [Page 1] Internet-Draft TCP-AO NAT Extension May 2012 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]. Touch Expires November 30, 2012 [Page 2] Internet-Draft TCP-AO NAT Extension May 2012 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: Touch Expires November 30, 2012 [Page 3] Internet-Draft TCP-AO NAT Extension May 2012 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. Touch Expires November 30, 2012 [Page 4] Internet-Draft TCP-AO NAT Extension May 2012 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. Touch Expires November 30, 2012 [Page 5] Internet-Draft TCP-AO NAT Extension May 2012 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 Touch Expires November 30, 2012 [Page 6]