BEHAVE WG P. Srisuresh Internet Draft Consultant Expires: December 8, 2006 B. Ford M.I.T. S. Sivakumar Cisco Systems S. Guha Cornell U. May 2006 NAT Behavioral Requirements for ICMP protocol 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. Abstract This document identifies the behavioral properties required of the Network Address Translator devices (NATs) in conjunction with the ICMP protocol. The objective of this memo is to make NAT devices more predictable and compatible with diverse application protocols that traverse the devices. Companion documents provide behavioral recommendations specific to TCP and UDP. Srisuresh, et. al [Page 1] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 Table of Contents 1. Introduction and Scope ........................................ 2. Terminology ................................................... 3. ICMP Query Handling ........................................... 3.1. ICMP Query Mapping .....,,................................ 3.2. ICMP Query Session Timeouts .............................. 4. ICMP Error Forwarding ......................................... 4.1. ICMP Error Payload Validation .....,,..................... 4.2. ICMP Error Packet Translation ............................ 4.2.1. ICMP Error Packet Received from External Realm .... 4.2.2. ICMP Error Packet Received from Private Realm ..... 4.3. NAT Sessions Pertaining to ICMP Error Payload ............ 5. Rejection of Outbound Flows Disallowed by NAT ................. 6. Conformance to RFC 1812 ....................................... 6.1. IP packet fragmentation .................................. 6.1.1. Generating "Packet too Big" ICMP error Message .... 6.1.2. Forwarding "Packet too big" ICMP Error Message .... 6.2. Generating "Time Exceeded" Error Message ................. 6.3. RFC 1812 Conformance Requirements summary ................ 7. Summary of Requirements ....................................... 8. Security Considerations ....................................... 9. Acknowledgements .............................................. 1. Introduction and Scope This document is an adjunct to [BEH-UDP] and [BEH-TCP], which define requirements for NATs when handling unicast UDP and TCP traffic. The purpose of this document is to set requirements for NATs with regard to ICMP messages. The requirements of this specification apply to Traditional NATs as described in RFC 2663 [2]. This document only covers the ICMP aspects of NAT traversal. Traditional NAT inherently mandates a certain level of firewall like functionality. However, firewall functionality in general or any other middlebox functionality is out of the scope of this specification. Application and Operating System (OS) aspects of ICMP NAT traversal are out of scope. NAT traversal strategies that involve explicit signaling between applications and NAT devices, namely [SOCKS, RSIP, MIDCOM, UPNP] are also out of the scope of this document. This document focuses strictly on the behavior of the NAT device, Srisuresh, et. al [Page 2] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 and not on the behavior of applications that traverse NATs. A separate companion document [BEH-APP] provides recommendations for application designers on how to make applications work robustly over NATs that follow the behavioral requirements specified here and the companion Behave documents. Even though ICMP is a transport protocol on top of IP, ICMP message processing is often considered an integral of IP and is independent of other transport protocols. As such, many of the ICMP behavioral requirements for NAT devices apply to all IP protocols. In case a requirement in this document conflicts with protocol specific behave requirement(s), protocol specific behave documents will take precedence. Note, the authors are not aware of any conflicts between this and any other IETF document at the time of this writing. Section 2 describes the terminology used throughout the document. Sections 3, 4 and 5 discuss the behavioral requirements for a NAT device when processing ICMP packets. Section 3 summarizes all the requirements in one place. 2. Terminology Definitions for the NAT terms used throughout the document may be found in [RFC2663] and [BEH-UDP]. 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 [RFC2119]. ICMP messages are broadly grouped into two classes, namely "ICMP Query" messages and "ICMP Error" messages in section 3.2.2 of [RFC1122]. The following explanations further illustrate these ICMP message classes. ICMP Query Messages - All ICMP query messages are characterized by the fact that have an Identifier field in the ICMP header. The Identifier field used by the ICMP Query messages is also referred as "Query Identifier" or "Query Id", for short throughout the document. A Query Id is used by query senders and responders as the equivalent of a TCP/UDP port to identify an ICMP Query session. ICMP Error Messages - All ICMP error messages are characterized by the fact that they embed an IP packet (a minimum of 64 bits) that triggered the ICMP error message. 3. ICMP Query Handling Srisuresh, et. al [Page 3] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 This section lists the behavioral requirements for a NAT device when processing ICMP Query packets. The following sub sections discuss requirements specific to ICMP Query handling in detail. 3.1. ICMP Query Mapping A NAT device MUST transparently forward any ICMP Query packets initiated from the nodes behind NAT devices and the responses to these Query packets in the opposite direction. This requires translating the IP header. A NAPT device SHOULD additionally modify the ICMP Query Identifier (also referred simply as 'Query Id') and the associated checksum in the ICMP header prior to forwarding. The mapping of ICMP Query identifier within the NAT device SHOULD be external endpoint independent. Say, an internal host A sent an ICMP query out to an external host B using Query Id X. And, say, the NAT assigned this an external mapping of Query id X' on NAT's public address. If host A reused the Query Id X to send ICMP queries to the same or different external host, the NAT device SHOULD reuse the same Query Id mapping (i.e., map private host's Query id X to Query id X' on NAT's public IP address) instead of assigning a different mapping. This is similar to the "endpoint independent mapping" requirement specified in the TCP and UDP drafts([BEH-TCP, BEH-UDP]). REQ-1: A NAT device MUST permit ICMP query based applications to be initiated from private hosts to the external hosts. a) NAT mapping of ICMP Query identifiers SHOULD be external host independent. 3.2. ICMP Query Session Timeouts It is to RECOMMENDED that the administrator be allowed to configure the ICMP Query session timeout on the NAT devices. Typically, the ICMP NAT Session timeout is set to 60 seconds. Setting the ICMP NAT Session timeout to a very large duration (say, much larger than 60 secs) could potentially tie up NAT resources. Caution is warranted as applications such as ICMP Ping and ICMP traceroute [RFC 1147], built on top of ICMP query messages typically complete within a few seconds. By setting the timeout to a large value, the NAT device could be holding up precious NAT resources such as mappings and NAT Sessions for the whole duration. REQ-2: An ICMP Query session mapping timer MUST NOT expire in less than 60 seconds. a) The value of NAT ICMP mapping timer MAY be configurable. Srisuresh, et. al [Page 4] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 4. ICMP Error Forwarding Applications depend on ICMP error messages from end hosts and intermediate devices being forwarded reliably by the NAT devices. A NAT device MUST confirm to a number of requirements to ensure reliable forwarding. The following sub-sections discuss the requirements in detail. 4.1. ICMP Error Payload Validation Appendix C of [ICMP-ATK] points out that newer revision end host TCP stacks donot accept ICMP error messages with a mismatched IP or TCP checksum in the embedded payload. NAT devices should ensure that the embedded payload is not corrupted. Only after the embedded payload is validated, should the NAT proceed to consider the error packet for forwarding. The NAT device SHOULD verify the IP checksum of the embedded payload and if the IP checksum does not match, the NAT MUST simply drop the error packet. [ICMP] stipulates that an ICMP error message should embed IP header and a minimum of 8 bytes of the IP payload. Section 4.3.2.3 of [RFC1812] further recommends that an ICMP error originator SHOULD include as much of the original packet as possible in the payload without the length of the ICMP datagram exceeding 576 bytes. If the embedded payload is a complete IP packet, the NAT device SHOULD further validate the applicable transport checksum. If the transport checksum fails, the NAT MUST once again drop the error packet. REQ-3: When an ICMP error packet is received, the NAT device SHOULD verify that the checksum(s) of the embedded IP packet is not corrupted. a) If the IP checksum of the embedded payload fails, the NAT MUST drop the error packet. b) If the embedded payload is a complete TCP segment, the NAT device SHOULD validate the TCP checksum. If the transport checksum fails, the NAT device MUST drop the error packet. c) If the embedded payload is a complete UDP datagram and the UDP datagram contains non-zero checksum, the NAT device SHOULD validate the UDP checksum. If the transport checksum fails, the NAT device MUST drop the error packet. 4.2. ICMP Error Packet Translation Section 4.3 of the RFC 3022 describes the various fields within an ICMP error message a NAT device MUST translate. In this section, Srisuresh, et. al [Page 5] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 we describe the requirements a NAT device must conform to while doing the translations. Consider the following scenario in figure 1. Say, NAT-xy is a traditional NAT device connecting hosts in private and external networks. Router-x and Host-x are in the external network. Router-y and Host-y are in the private network. The subnets in the external network are routable from the private as well as the external domains. Whereas, the subnets in the private network are only routable within the private domain. When Host-y initiated a session to Host-x, let us say that the NAT device assigned an IP address of Host-y' to associate with Host-y in the external network. Host-x | ---------------+------------------- | +-------------+ | Router-x | +-------------+ External Network | --------------------+--------+------------------- | ^ | | | (Host-y', Host-x) | | | v +-------------+ | NAT-xy | +-------------+ | | Private Network ----------------+------------+---------------- | +-------------+ | Router-y | +-------------+ | ----------------+-------+-------- | ^ | | | (Host-y, Host-x) | | | v Host-y Figure 1. NAT topology with routers in private & external realms 4.2.1. ICMP Error Packet Received from External Realm Srisuresh, et. al [Page 6] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 Say, a packet from Host-y to Host-x triggered an ICMP error message from one of Router-x or Host-x (both of which are in the external domain). Such an ICMP error packet will have one of Router-x or Host-x as the source IP address and Host-y' as the destination IP address. When the NAT device receives the ICMP error packet, the NAT device must use the packet embedded within the ICMP error message (i.e., the IP packet from Host-y to Host-x) to look up the NAT Session the embedded packet belongs to and use the NAT Session to translate the embedded payload. The NAT device must also use the NAT Session to translate the destination IP address in the outer IP header. In the outer header, the source IP address will remain unchanged because the originator of the ICMP error message (Host-x or Router-x) is in external domain and routable from the private domain. The destination IP address Host-y' must however be translated to Host-y using the NAT Session parameters. REQ-4: If a NAT device receives an ICMP error packet from external realm, and the NAT does not have an active mapping for the embedded payload, the NAT SHOULD silently drop the ICMP error packet. If the the NAT has active mapping for the embedded payload, then the NAT MUST transparently forward the ICMP error message, without modifying the ICMP error type or code as follows. a) The NAT MUST revert the IP and transport headers of the embedded IP packet to their original form, using the matching mapping. b) The NAT device MUST modify the destination IP address of the outer IP header to be same as the source IP address of the embedded IP packet. c) Lastly, the NAT MUST update the IP and ICMP checksums in the outer headers to reflect the above changes. 4.2.2. ICMP Error Packet Received from Private Realm Now, say, a packet from Host-x to Host-y triggered an ICMP error message from one of Router-y or Host-y (both of which are in the private domain). Such an ICMP error packet will have one of Router-y or Host-y as the source IP address and Host-x as the destination IP address. When the NAT device receives the ICMP error packet, the NAT device must use the packet embedded within the ICMP error message (i.e., the IP packet from Host-x to Host-y) to look up the NAT Session the embedded packet belongs to and use the NAT Session to translate the embedded payload. Srisuresh, et. al [Page 7] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 In the outer header, the destination IP address will remain unchanged, as the IP addresses for Host-x is already in the external domain. If the ICMP error message is generated by Host-y, the NAT device must simply use the NAT Session to translate the source IP address Host-y to Host-y'. However, if the ICMP error message is generated by the intermediate node Router-y, and the NAT device does not have a translation entry for Router-y within NAT, the NAT device must simply use its own IP address in the external domain to translate the source IP address. REQ-5: If a NAT device receives an ICMP error packet from private realm, and the NAT does not have an active mapping for the embedded payload, the NAT SHOULD silently drop the ICMP error packet. If the the NAT has active mapping for the embedded payload, then the NAT MUST transparently forward the ICMP error message, without modifying the ICMP error type or code as follows. a) The NAT MUST revert the IP and transport headers of the embedded IP packet to their original form, using the matching mapping. b) If the NAT device has active mapping for the ICMP error packet originator, the NAT MUST translate the source IP address of the ICMP error packet with the public IP address in the mapping. Otherwise, the NAT MUST translate the source IP address of the ICMP error packet with its own public IP address. c) Lastly, the NAT MUST update the IP and ICMP checksums in the outer headers to reflect the above changes. 4.3. NAT Sessions Pertaining to ICMP Error Payload While processing an ICMP error packet, a NAT device MUST NOT refresh or delete the NAT Session that pertains to the embedded payload within the ICMP error packet. This is in spite of the fact that the NAT device uses the NAT Session to translate the embedded payload. By not effecting the NAT Sessions, the NAT device is able to retain them, even as someone spoofs ICMP error messages pertaining to the NAT Sessions. REQ-6: While processing an ICMP error packet, a NAT device MUST NOT refresh or delete the NAT Session that pertains to the embedded payload within the ICMP error packet. 5. Rejection of Outbound Flows Disallowed by NAT A NAT device typically permits all outbound sessions. However, a NAT device may disallow some outbound sessions due to resource constraints. For example, a NAT device may not permit the first packet of a new outbound session, if the NAT device is out of Srisuresh, et. al [Page 8] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 resources (out of addresses or TCP/UDP ports or a NAT Session resource) to set up a state for the session, or, the specific session is administratively restricted by the NAT device. When the first packet of an outbound flow is prohibited by a NAT device due to resource or administration considerations, the NAT device SHOULD send ICMP destination unreachable message. Section 5.2.7.1 of [RFC1812] recommends routers to use ICMP code 13 (Communication administratively prohibited) when they administratively filter packets. As such, a NAT device MUST use ICMP code 13 when generating an ICMP error message. REQ-7: When an outbound flow is prohibited by a NAT device due to resource or authorization consideration, the NAT device SHOULD send ICMP destination unreachable message, with a code of 13 (Communication administratively prohibited) to the sender prior to dropping the original packet. 6. Conformance to RFC 1812 A NAT device is inherently an intermediate router that forwards IP packets between private and public realms. As such, the NAT device MUST confirm to all the requirements of a router, as specified in [RFC1812]. Section 5.2.7.1 of [RFC1812] states that a router MUST also be able to generate ICMP Destination Unreachable messages and SHOULD choose a response code that most closely matches the reason the message is being generated. Note, however, NAT devices also function as hosts on the Internet and are bound by the conformance requirements in [RFC1122]. Protocol specific Behave documents ([BEH-UDP], [BEH-TCP]) identify instances where a NAT device should deviate from RFC 1122. As such, the host behavior requirements of NAT devices specified in the protocol specific behave drafts take precedence over RFC 1122. The focus of this section is on conformance to router requirements. The following sub sections identify specific instances where a NAT device would be expected to confirm to RFC 1812. 6.1. IP packet fragmentation IP fragmentation by intermediate nodes often results in degraded performance. In some cases, IP fragmentation by the intermediate nodes could even cause end-to-end communication to entirely fail. Many applications avoid fragmentation in the network by originating IP packets that fit within the maximum Path MTU enroute and setting the DF (Don't Fragment) bit so the intermediate nodes enroute do not fragment the Srisuresh, et. al [Page 9] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 packets. Path discovery is not restricted to TCP or UDP based application and applies to all IP protocols. For instance, TCP connections often set the DF bit set in the IP header of the TCP segments they transmit. Likewise, IP based VPN tunnels also often set the DF bit on the external IP encapsulation. 6.1.1. Generating "Packet too Big" ICMP error Message When a router is unable to forward a datagram because it exceeds the MTU of the nexthop network and its Don't Fragment (DF) bit is set, the router is required to return an ICMP Destination Unreachable message to the source of the datagram, with the Code indicating "fragmentation needed and DF set". Further, the router MUST include the MTU of that nexthop network in the low order 16 bits of the ICMP header, as specified in RFC 1191. A NAT device MUST honor the DF bit in the IP header of the packets transiting the device. If the DF bit is set and the MTU on the forwarding interface of the NAT device is such that the IP datagram cannot be forwarded without fragmentation, the NAT device MUST issue a "packet too big" ICMP message (ICMP type 3, Code 4) with a suggested MTU back to the sender and drop the original IP packet. The sender will resend after taking the appropriate corrective action. If the DF bit is not set and the MTU on the forwarding interface of the NAT device mandates fragmentation, the NAT device must simply send this fragmented, just as any router does [RFC1812]. 6.1.2. Forwarding "Packet too big" ICMP Error Message This is flip side of the argument for the above section. By virtue of the address translation NAT performs, NAT may end up being the recipient of "Packet too big" message. When NAT device is the recipient of "Packet too big" ICMP message from the network, the NAT device MUST forward the ICMP message back to the intended recipient, pursuant to the previosuly stated requirements REQ-3, REQ-4, REQ-5 and REQ-6. 6.2. Generating "Time Exceeded" Error Message Section 5.2.7.3 of RFC 1812 says that a router MUST generate "Time Exceeded" ICMP error message when it discards a packet due to an expired TTL field. A router MAY have a per interface option to disable origination of these messages on that interface, but that option MUST default to allowing the messages to be originated. Srisuresh, et. al [Page 10] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 6.3. RFC 1812 Conformance Requirements summary REQ-8: A NAT device MUST confirm to RFC 1812 in IP packet handling. Below are specific instances where a NAT device MUST confirm to RFC 1812. a) If DF bit is set on a transit IP packet and the NAT device cannot forward the packet without fragmentation, the NAT device MUST send a "Packet too big" ICMP message (ICMP type 3, Code 4) with a suggested MTU back to the sender and drop the original IP packet. b) A NAT device MUST generate "Time Exceeded" ICMP error message when it discards a packet due to an expired TTL field. 7. Summary of Requirements This section summarizes the requirements discussed in the preceding sections. REQ-1: A NAT device MUST permit ICMP query based applications to be initiated from private hosts to the external hosts. a) NAT mapping of ICMP Query identifiers SHOULD be external host independent. REQ-2: An ICMP Query session mapping timer MUST NOT expire in less than 60 seconds. a) The value of NAT ICMP mapping timer MAY be configurable. REQ-3: When an ICMP error packet is received, the NAT device SHOULD verify that the checksum(s) of the embedded IP packet is not corrupted. a) If the IP checksum of the embedded payload fails, the NAT MUST drop the error packet. b) If the embedded payload is a complete TCP segment, the NAT device SHOULD validate the TCP checksum. If the transport checksum fails, the NAT device MUST drop the error packet. c) If the embedded payload is a complete UDP datagram and the UDP datagram contains non-zero checksum, the NAT device SHOULD validate the UDP checksum. If the transport checksum fails, the NAT device MUST drope the error packet. REQ-4: If a NAT device receives an ICMP error packet from external realm, and the NAT does not have an active mapping for the embedded payload, the NAT SHOULD silently drop the ICMP error packet. If the the NAT has active mapping for the embedded payload, then the NAT MUST transparently forward the ICMP error message, without modifying the ICMP error type or code as follows. a) The NAT MUST revert the IP and transport headers of the embedded Srisuresh, et. al [Page 11] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 IP packet to their original form, using the matching mapping. b) The NAT device MUST modify the destination IP address of the outer IP header to be same as the source IP address of the embedded IP packet. c) Lastly, the NAT MUST update the IP and ICMP checksums in the outer headers to reflect the above changes. REQ-5: If a NAT device receives an ICMP error packet from private realm, and the NAT does not have an active mapping for the embedded payload, the NAT SHOULD silently drop the ICMP error packet. If the the NAT has active mapping for the embedded payload, then the NAT MUST transparently forward the ICMP error message, without modifying the ICMP error type or code as follows. a) The NAT MUST revert the IP and transport headers of the embedded IP packet to their original form, using the matching mapping. b) If the NAT device has active mapping for the ICMP error packet originator, the NAT MUST translate the source IP address of the ICMP error packet with the public IP address in the mapping. Otherwise, the NAT MUST translate the source IP address of the ICMP error packet with its own public IP address. c) Lastly, the NAT MUST update the IP and ICMP checksums in the outer headers to reflect the above changes. REQ-6: While processing an ICMP error packet, a NAT device MUST NOT refresh or delete the NAT Session that pertains to the embedded payload within the ICMP error packet. REQ-7: When an outbound flow is prohibited by a NAT device due to resource or authorization consideration, the NAT device SHOULD send ICMP destination unreachable message, with a code of 13 (Communication administratively prohibited) to the sender prior to dropping the original packet. REQ-8: A NAT device MUST confirm to RFC 1812 in IP packet handling. Below are specific instances where a NAT device MUST confirm to RFC 1812. a) If DF bit is set on a transit IP packet and the NAT device cannot forward the packet without fragmentation, the NAT device MUST send a "Packet too big" ICMP message (ICMP type 3, Code 4) with a suggested MTU back to the sender and drop the original IP packet. b) A NAT device MUST generate "Time Exceeded" ICMP error message when it discards a packet due to an expired TTL field. 8. Security Considerations This document does not introduce any new security concerns related Srisuresh, et. al [Page 12] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 to ICMP error message handling in the NAT devices. However, the document does propose cunter measures to mitigate security concerns that already exist with ICMP error messages. [ICMP-ATK] lists a number of ICMP attacks that can be directed against end host TCP stacks and suggests remedies to counter the attacks. [TCP-SOFT] describes improvements to the handling of ICMP error messages in many of the existing TCP/IP stacks, including Linux. Section 4 of this document decribes a number of measures by which NAT devices should validate and update the embedded payload in ICMP error messages prior to forwarding. These measure ensure that NATs forward the ICMP error messages reliably, as stipulated in [ICMP-ATK]. For example, a rogue entity could bombard the NAT device with a large number of ICMP errors. If the NAT device did not validate the legitimacy of the ICMP error packets, the ICMP errors would be forwarded directly to the end nodes. End hosts not capable of defending themselves against such bogus ICMP error attacks could be adversely impacted by such attacks. Req-3 recommends validating embedded payload prior to forwarding. Checksum validation by itself does not protect end hosts from attacks. However, checksum validation mitigates endhosts from malformed ICMP error attacks. Req-4 and Req-5 further mandate that when a NAT device does not find a mapping selection for the embedded payload, the NAT should drop the ICMP error packets, without forwarding. A rogue source could also try and send bogus ICMP error messages for the active NAT sessions, with an intent to destroy the sessions. Req-6 averts such an attack by ensuring that an ICMP error message does not effect the state of a session on the NAT device. 9. Acknowledgements The authors wish to thank Fernando Gont and Dan Wing for providing valuable input and offering generous amount of their time in shaping the ICMP requirements. Their valuable feedback makes this document a better read. The authors highly appreciate that. Normative References [BEH-UDP] F. Audet and C. Jennings, "NAT Behavioral Requirements for Unicast UDP", draft-ietf-behave-nat-udp-04.txt (Work In Progress), September 2005. [BEH-TCP] Guha, S., Biswas, K., Ford, B., Francis, P., Sivakumar, S., Srisuresh, et. al [Page 13] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 and Srisuresh, P., "NAT Behavioral Requirements for Unicast TCP", draft-ietf-behave-tcp-00.txt (Work In Progress), February 2006. [ICMP] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [NAT-TERM] P. Srisuresh and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [NAT-TRAD] P. Srisuresh and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [PMTU] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [PRIV-ADDR] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and E. Lear, "Address Allocation for Private Internets", RFC 1918, February 1996. [RFC1191] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191, November 1990. [RFC1147] Stine, R., "FYI on a Network Management Tool Catalog: Tools for Monitoring and Debugging TCP/IP Internets and Interconnected Devices", RFC 1147, April 1990. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. Informative References [BEH-APP] Ford, B., Srisuresh, P., and Kegel, D., "Application Design Guidelines for Traversal through Network Address Translators", draft-ford-behave-app-02.txt (Work In Progress), March 2006. [RFC1122] Braden, R., "Requirements for Internet Hosts -- Communication Layers", RFC 1122, October 1989. [UNSAF] Daigle, L., and IAB, "IAB Considerations for UNilateral Srisuresh, et. al [Page 14] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [ICMP-ATK] Gont, F., "ICMP attacks against TCP", draft-ietf-tcpm-icmp-attacks-00.txt (Work In Progress), February 2006. [TCP-SOFT] Gont, F., "TCP's Reaction to Soft Errors", draft-ietf-tcpm-tcp-soft-errors-00.txt (Work In Progress), February 2006. Author's Addresses: Pyda Srisuresh Consultant 20072 Pacifica Dr. Cupertino, CA 95014 U.S.A. Phone: (408)836-4773 E-mail: srisuresh@yahoo.com Bryan Ford Computer Science and Artificial Intelligence Laboratory Massachusetts Institute of Technology 77 Massachusetts Ave. Cambridge, MA 02139 U.S.A. Phone: (617) 253-5261 E-mail: baford@mit.edu Web: http://www.brynosaurus.com/ Senthil Sivakumar Cisco Systems, Inc. 7100-8 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709-4987 U.S.A. Phone: +1 919 392 5158 Email: ssenthil@cisco.com Saikat Guha Cornell University 331 Upson Hall Ithaca, NY 14853 U.S.A. Email: saikat@cs.cornell.edu Intellectual Property Statement Srisuresh, et. al [Page 15] Internet-Draft NAT Behavioral Requirements for ICMP May 2006 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. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Srisuresh, et. al [Page 16]