BEHAVE WG P. Srisuresh Internet Draft Consultant Expires: September 5, 2006 B. Ford M.I.T. S. Sivakumar Cisco Systems S. Guha Cornell U. March 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 middleboxes 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 March 2006 Table of Contents 1. Introduction & Scope .......................................... 2. ICMP Behavioral Requirements .................................. 2.1. ICMP Query Packet Handling .....,,........................ 2.2. ICMP Error Packet Handling .....,,........................ 2.2.1. NAT Sessions Pertaining to ICMP Error Packets ..... 2.3. Rejection of Outbound IP Packets Disallowed by NAT ....... 2.4. Path MTU Discovery ....................................... 2.4.1. Honoring the DF Bit in IP header .................. 2.4.2. Honoring the "Packet too big" ICMP Error Message .. 3. Summary of Requirements ....................................... 4. Security Considerations ....................................... 1. Introduction & Scope This document is adjunct to [BEH-UDP] and [BEH-TCP], which define requirements for NATs that handle 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 the application and the NAT [SOCKS, RSIP, MIDCOM, UPNP] are out of the scope of this document. This document focuses strictly on the behavior of the NAT device, 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. The following section lists the behavioral recommendations to the NAT device vendors in conjunction with handling ICMP protocol. Section 3 summarizes all the requirements in one place. Srisuresh et. al. [Page 2] Internet-Draft NAT Behavioral requirements for ICMP March 2006 2. ICMP Behavioral Requirements This section lists the behavioral requirements for a NAT device when processing ICMP packets. Even though ICMP is a transport protocol on top of IP, ICMP packet processing is often considered an integral of IP and is independent of other transport protocols. In the following sub sections, requirements are discussed in detail along with the rationale behind them. 2.1. ICMP Query Packet Handling A NAT device SHOULD 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 involves translating the IP header. The NAPT device SHOULD additionally modify the query ID and the associated checksum in the ICMP header if there is an outstanding query with the same ID. It is to RECOMMENDED that the administrator be allowed to configure the ICMP session timeout. Typically, the ICMP NAT Session timeout is set to 30 seconds or longer. Setting the ICMP NAT Session timeout to a very large duration (say, much larger than 30 secs) longer could potentially tie up NAT resources. Caution is warranted as applications (such as ping and traceroute) built on top of ICMP query messages 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-1: A NAT device SHOULD transparently forward any ICMP query packets initiated from the nodes behind NAT devices and the responses to these query packets in the opposite direction. It is RECOMMENDED that the administrator be allowed to configure the ICMP session timeout. 2.2. ICMP Error Packet Handling A NAT device MUST transparently forward ICMP error messages ([ICMP]) it receives from intermediate or end nodes in either realm to the intended end node. Unlike other IP packets, the basis for translation of an ICMP error packet is the NAT Session to which the packet embedded within the ICMP error message payload belongs to, not the IP and ICMP headers in the outer layer. 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 Srisuresh et. al. [Page 3] Internet-Draft NAT Behavioral requirements for ICMP March 2006 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 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 Srisuresh et. al. [Page 4] Internet-Draft NAT Behavioral requirements for ICMP March 2006 NAT Session to translate 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. 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. The NAT device must also use the NAT Session to translate the outer IP header. 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, the NAT device will not have had a translation entry for Router-y within the NAT Session. The NAT may also not have an Address mapping in place for Router-y. In such a case, the NAT device must simply use its own IP address in the external domain to translate the source IP address. Changes to ICMP error message ([ICMP]) MUST include changes to IP and ICMP headers on the outer layer as well as changes to the relevant IP and transport headers of the packet embedded within the ICMP-error message payload. Section 4.3 of the RFC 3022 describes the various items within the ICMP error message that MUST be translated by the NAT device. REQ-2: A NAT device MUST transparently forward ICMP error packets to the target end node, so long as the NAT has active mapping for for the embedded payload. If the NAT does not have an active mapping for the embedded payload, the NAT should silently drop the ICMP error packet. In the case the ICMP error packet is originated by a node in the private realm for which the NAT device has no mapping, the NAT device MUST use its own IP address in the public realm to translate the originating node IP address in the outer IP header. 2.2.1. NAT Sessions Pertaining to ICMP Error Packets While processing an ICMP error packet, a NAT device MUST NOT Srisuresh et. al. [Page 5] Internet-Draft NAT Behavioral requirements for ICMP March 2006 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-3: 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. 2.3. Rejection of Outbound IP Packets 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 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 may be administratively restricted by the NAT device. When a packet is prohibited by a NAT device due to resource or administration considerations, the NAT device SHOULD send ICMP destination unreachable message, with a code of 10 (Communication with destination host administratively prohibited) to the sender. Unfortunately, there is not another ICMP code currently defined to indicate "Communication with destination host port administratively prohibited". So, the same code should be used for host as well as port filtering. REQ-4: When an outbound packet is prohibited by a NAT device due to resource/authorization consideration, the NAT device SHOULD send ICMP destination unreachable message, with a code of 10 (Communication with destination host administratively prohibited) to the sender prior to dropping the packet. 2.4. Path MTU support 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 packets. For example, a number of TCP connections have the Srisuresh et. al. [Page 6] Internet-Draft NAT Behavioral requirements for ICMP March 2006 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. 2.4.1. Honoring the DF Bit in IP header A NAT device MUST honor the DF bit in the IP header of the packets that transit 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] REQ-5: 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. 2.4.2. Honoring the "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 simply forward the ICMP message back to the intended recipient, as stated in REQ-2. 3. Summary of Requirements This section summarizes the requirements discussed at length in the preceding section. REQ-1: A NAT device SHOULD transparently forward any ICMP query packets initiated from the nodes behind NAT devices and the responses to these query packets in the opposite direction. It is RECOMMENDED that the administrator be allowed to configure the ICMP session timeout. REQ-2: A NAT device MUST transparently forward ICMP error packets Srisuresh et. al. [Page 7] Internet-Draft NAT Behavioral requirements for ICMP March 2006 to the target end node, so long as the NAT has active mapping for for the embedded payload. If the NAT does not have an active mapping for the embedded payload, the NAT should silently drop the ICMP error packet. In the case the ICMP error packet is originated by a node in the private realm for which the NAT device has no mapping, the NAT device MUST use its own IP address in the public realm to translate the originating node IP address in the outer IP header. REQ-3: 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-4: When an outbound packet is prohibited by a NAT device due to resource/authorization consideration, the NAT device SHOULD send ICMP destination unreachable message, with a code of 10 (Communication with destination host administratively prohibited) to the sender prior to dropping the packet. REQ-5: 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. 4. Security Considerations This memo raises no new security issues. 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., 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. Srisuresh et. al. [Page 8] Internet-Draft NAT Behavioral requirements for ICMP March 2006 [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. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. Informative References [UNSAF] L. Daigle and IAB, "IAB Considerations for UNilateral Self- Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. 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 Srisuresh et. al. [Page 9] Internet-Draft NAT Behavioral requirements for ICMP March 2006 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. 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