Network Working Group P. Hoffman Internet-Draft Cybersecurity Association Expires: December 3, 2005 June 2005 NAT Behavioral Requirements for Unicast TCP draft-hoffman-behave-tcp-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 3, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document defines a set of requirements for NATs that handle unicast TCP that would allow many applications, such as multimedia communications or on-line gaming, to work consistently. Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. 1. Introduction [BEHAVE-UDP] defines many terms relating to NATs, lays out general Hoffman Expires December 3, 2005 [Page 1] Internet-Draft NAT TCP Unicast Requirements June 2005 requirements for all NATs, and sets requirements for NATs that handle unicast UDP traffic. This document is an adjunct to [BEHAVE-UDP] that sets requirements for NATs that handle unicast TCP traffic (that is, almost every NAT). All definitions and requirements in [BEHAVE- UDP] are inherited here. 2. Incoming SYN packets on an open TCP connection Some TCP-based protocols attempt to start multiple TCP connections for a single flow of traffic. Such protocols can sometimes work through NATs. Normal TCP stacks accept incoming SYN packets, of course. REQ-1 A NAT MUST allow SYN packets in either direction on any open NAT session that is a TCP connection. 3. Incoming SYN packets to non-existent sessions TCP-handling have two basic choices when they receive a SYN packet for a session that they do not intend to set up: ignore it or respond to it with a RST packet. A NAT that receives a SYN packet from the outside for a connection that does not exist must decide whether or not to respond with a RST packet. Ignoring such a SYN packet causes no traffic overhead, and does not expose the NAT to any known security vulnerability. Ignoring the SYN instead of responding with an RST can help some applications that start multiple TCP connections. REQ-2 A NAT MUST NOT reply to a SYN packet sent from the outside to a connection that is not established. 4. Resource exhaustion and timers A NAT maintains state associated with the sessions that it maintains. Because of this, a NAT is susceptible to a resource-exhaustion attack whereby the attacker attempts to cause the NAT to create more state that it has resources. To prevent such an attack, a NAT needs to abandon sessions in order to free the state resources. A common method that is applicable only to TCP connections is to look for partially-open or partially closed TCP connections and abandon those based on timers specific to those states. There is a serious problem with this method: the timers used by the NAT are unlikely to be the same as the timers that are used by the two end systems. This is particularly true for end systems that do not use timers for partially-open or partially-closed TCP connections, but instead use Hoffman Expires December 3, 2005 [Page 2] Internet-Draft NAT TCP Unicast Requirements June 2005 resource-based methods. A NAT that uses such timers is inherently non-deterministic. REQ-3 A NAT MUST NOT abandon a TCP connection simply because it has been in a partially-open or partially-closed state for a long time, or because it has been open for a long time without traffic. A NAT that is facing resource exhaustion MAY abandon TCP connections. This is the only time that a NAT may abandon a TCP connection, even one that is partially-open. REQ-4 In order to cause the least unexpected behavior, a NAT that is abandoning TCP connections to prevent resource exhaustion SHOULD first abandon partially-closed TCP sessions; then, only if needed, it SHOULD abandon partially-open TCP sessions; then, only if needed, it SHOULD abandon open TCP sessions that have had no traffic for a relatively long time. A NAT is allowed to change the order of the sessions to abandon based on particular attack patterns. REQ-5 When a NAT closes an open TCP session to avoid resource exhaustion, it MUST send RST packets to each end of the session. 5. Security considerations The security considerations for this document are the same as for [BEHAVE-UDP]. The fact that this document covers unicast TCP does not change any of the security considerations there. [[ Is this statement actually true? ]] 6. IANA considerations This document does not change or create any IANA-registered values. 7. Normative References [BEHAVE-UDP] Audet, F. and C. Jennings, "NAT Behavioral Requirements for Unicast UDP", draft-ietf-behave-nat-udp (work in progress). Hoffman Expires December 3, 2005 [Page 3] Internet-Draft NAT TCP Unicast Requirements June 2005 Author's Address Paul Hoffman Cybersecurity Association 127 Segre Place Santa Cruz, CA 95060 USA Email: paul.hoffman@cybersecurity.org Hoffman Expires December 3, 2005 [Page 4] Internet-Draft NAT TCP Unicast Requirements June 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Hoffman Expires December 3, 2005 [Page 5]