Behavior Engineering for Hindrance R. Denis-Courmont Avoidance VideoLAN project Internet-Draft October 04, 2008 Intended status: Standards Track Expires: April 7, 2009 Network Address Translation (NAT) Behavioral Requirements for DCCP draft-ietf-behave-dccp-03.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 April 7, 2009. Abstract This document defines a set of requirements for NATs handling the Datagram Congestion Control Protocol (DCCP). Those allow DCCP applications, such as streaming applications to operate consistently. These requirements are very similar to the TCP requirements for NATs already published by the IETF Behavior Engineering for Hindrance Avoidance (BEHAVE) working group. Developing NATs that meet this set of requirements will greatly increase the likelihood that applications using DCCP will function properly. Denis-Courmont Expires April 7, 2009 [Page 1]
Internet-Draft NAT DCCP Requirements October 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Applicability statement . . . . . . . . . . . . . . . . . . . . 3 4. DCCP Connection Initiation . . . . . . . . . . . . . . . . . . 4 5. NAT Session Refresh . . . . . . . . . . . . . . . . . . . . . . 5 6. Application Level Gateways . . . . . . . . . . . . . . . . . . 6 7. Other Requirements Applicable to DCCP . . . . . . . . . . . . . 6 8. Requirements specific to DCCP . . . . . . . . . . . . . . . . . 7 9. DCCP without NAT support . . . . . . . . . . . . . . . . . . . 7 10. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 13.1. Normative References . . . . . . . . . . . . . . . . . . . 9 13.2. Informative References . . . . . . . . . . . . . . . . . . 9 Denis-Courmont Expires April 7, 2009 [Page 2]
Internet-Draft NAT DCCP Requirements October 2008 1. Introduction For historical reasons, NAT devices are not typically capable of handling datagrams and flows for applications using the Datagram Congestion Control Protocol (DCCP)[RFC4340]. This draft discusses the technical issues involved, and proposes a set of requirements for NAT devices to handle DCCP in a way that enables communications when either or both of the DCCP endpoints are located behind one or more NAT devices. All definitions and requirements in [RFC4787] are inherited here. The requirements are otherwise designed similarly to those in [I-D.ietf-behave-tcp], from which this memo borrows its structure and much of its content. Note however that, if both endpoints are hindered by NAT devices, the normal model of asymmetric connection model of DCCP will not work. A simultaneous open must be performed, as in [I-D.ietf-dccp-simul-open]. Also, a separate unspecified mechanism may be needed, such as Unilateral Self Address Fixing (UNSAF)[RFC3424] protocols, if an endpoint needs to learn its own external NAT mappings. 2. Definitions 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]. This documentation uses the term "DCCP connection" to refer to invidual DCCP flows, as uniquely identified by the quadruple (source and destination IP addresses and DCCP ports) at a given time. This document uses the term "NAT mapping" to refer to state at the NAT necessary for network address and port translation of DCCP connections. This document also uses the terms "endpoint-independent mapping", "address-dependent mapping", "address and port-dependent mapping", "filtering behavior", "endpoint-independent filtering", "address-dependent filtering", "address and port-dependent filtering", "port assignment", "port overloading", "hairpinning", and "external source IP address and port" as defined in [RFC4787]. 3. Applicability statement This document applies to NAT devices that want to handle DCCP datagrams. It is not the intent of this document to deprecate the overwhelming majority of deployed NAT devices. These NATs are simply not expected to handle DCCP, so this memo is not applicable to them. Denis-Courmont Expires April 7, 2009 [Page 3]
Internet-Draft NAT DCCP Requirements October 2008 Expected NAT behaviors applicable to DCCP connections are very similar to those applicable to TCP connections (with the exception of REQ-6 below). The following requirements are discussed and justified extensively in [I-D.ietf-behave-tcp]. These justifications are not reproduced here for the sake of brevity. In addition to the usual changes to the IP header (in particular the IP addresses), NAT devices need to mangle: o the DCCP source port, for outgoing packets, depending on the NAT mapping o the DCCP destination port, for incoming packets, depending on the NAT mapping o the DCCP checksum, to compensate for IP address and port number modifications. Because changing the source or destination IP address of a DCCP packet will normally invalidate the DCCP checksum, it is not possible to use DCCP through a NAT without dedicated support. Some NAT devices are known to provide a "generic" transport protocol support, whereby only the IP header is mangled. That scheme is not sufficient to support DCCP. 4. DCCP Connection Initiation 4.1. Address and Port Mapping Behavior A NAT uses a mapping to translate packets for each DCCP connection. A mapping is dynamically allocated for connections initiated from the internal side, and potentially reused for certain subsequent connections. NAT behavior regarding when a mapping can be reused differs for different NATs as described in [RFC4787]. REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior for DCCP. 4.2. Established Connections REQ-2: A NAT MUST support all valid sequences of DCCP packets (defined in [RFC4340] and its updates) for connections initiated both internally as well as externally when the connection is permitted by the NAT. In particular, in addition to handling the DCCP 3-way handshake mode of connection initiation, A NAT MUST handle the DCCP simultaneous-open mode of connection initiation, defined in [I-D.ietf-dccp-simul-open]. That mode updates DCCP by adding a new packet type, DCCP-Listen. The A DCCP-Listen packet communicates the Denis-Courmont Expires April 7, 2009 [Page 4]
Internet-Draft NAT DCCP Requirements October 2008 information necessary to uniquely identify a DCCP session. NATs may utilise the connection information (address, port, Service Code) to establish local forwarding state. 4.3. Externally Initiated Connections REQ-3: If application transparency is most important, it is RECOMMENDED that a NAT have an "Endpoint-independent filtering" behavior for DCCP. If a more stringent filtering behavior is most important, it is RECOMMENDED that a NAT have an "Address-dependent filtering" behavior. o The filtering behavior MAY be an option configurable by the administrator of the NAT. o The filtering behavior for DCCP MAY be independent of the filtering behavior for any other transport-layer protocol, such as UDP, UDP-Lite, TCP, SCTP. REQ-4: A NAT MUST wait for at least 6 seconds from the reception of an unsolicited inbound DCCP-Listen or DCCP-Sync packet before it may respond with an ICMP Port Unreachable error, an ICMP Protocol Unreachable error or a DCCP-Reset. If during this interval the NAT receives and translates an outbound DCCP-Request packet for the connection the NAT MUST silently drop the original unsolicited inbound DCCP-Listen packet. Otherwise the NAT SHOULD send an ICMP Port Unreachable error (Type 3, Code 3) for the original DCCP-Listen, unless the security policy forbids it. 5. NAT Session Refresh The "established connection idle-timeout" for a NAT is defined as the minimum time a DCCP connection in the established phase must remain idle before the NAT considers the associated session a candidate for removal. The "transitory connection idle-timeout" for a NAT is defined as the minimum time a DCCP connection in the CLOSEREQ or CLOSING phases must remain idle before the NAT considers the associated session a candidate for removal. DCCP connections in the TIMEWAIT state are not affected by the "transitory connection idle- timeout". REQ-5: If a NAT cannot determine whether the endpoints of a DCCP connection are active, it MAY abandon the session if it has been idle for some time. In such cases, the value of the "established connection idle-timeout" MUST be of 124 minutes or longer. The value of the "transitory connection idle-timeout" MUST be of 4 minutes or longer. The value of the NAT idle-timeouts MAY be configurable. Denis-Courmont Expires April 7, 2009 [Page 5]
Internet-Draft NAT DCCP Requirements October 2008 NAT behavior for handling DCCP-Reset packets, or connections in TIMEWAIT state is left unspecified. 6. Application Level Gateways Contrary to TCP, DCCP is a loss-tolerant protocol. Therefore, modifying the payload of DCCP packets may present a significant additional challenge in maintaining sane any application-layer state needed for an ALG to function. Additionaly, there are no known DCCP- capable Application Level Gateways (ALGs) at the time of writing this document. REQ-6: If a NAT includes ALGs, it MUST NOT affect DCCP. NOTE: This is not consistent with REQ-6 of [I-D.ietf-behave-tcp]. 7. Other Requirements Applicable to DCCP A list of general and UDP specific NAT behavioral requirements are described in [RFC4787]. A list of ICMP specific NAT behavioral requirements are described in [I-D.ietf-behave-nat-icmp]. The requirements listed below reiterate the requirements from these two documents that directly affect DCCP. The following requirements do not relax any requirements in [RFC4787] or [I-D.ietf-behave-nat-icmp]. 7.1. Port Assignment REQ-7: A NAT MUST NOT have a "Port assignment" behavior of "Port overloading" for DCCP. 7.2. Hairpinning Behavior REQ-8: A NAT MUST support "Hairpinning" for DCCP. Futhermore, A NAT's Hairpinning behavior MUST be of type "External source IP address and port". 7.3. ICMP Responses to DCCP Packets REQ-9: If a NAT translates DCCP, it SHOULD translate ICMP Destination Unreachable (Type 3) messages. REQ-10: Receipt of any sort of ICMP message MUST NOT terminate the NAT mapping or DCCP connection for which the ICMP was generated. Denis-Courmont Expires April 7, 2009 [Page 6]
Internet-Draft NAT DCCP Requirements October 2008 8. Requirements specific to DCCP 8.1. Partial checksum coverage DCCP supports partial checksum coverage. A NAT will usually need to perform incremental changes to the packet checksum field, as for other IETF-defined protocols. However, if it needs to recalculate a correct checksum value, it must take the checksum coverage into account, as described in section 9.2 of [RFC4340]. REQ-11: If a NAT translates a DCCP packet with a valid DCCP checksum, it MUST ensure that the DCCP checksum is translated such that it is valid after the translation. REQ-12: A NAT MUST NOT modify the value of the DCCP Checksum Coverage. The Checksum Coverage field in the DCCP header determines the parts of the packet that are covered by the Checksum field. This always includes the DCCP header and options, but some or all of the application data may be excluded as determined on a packet-by-packet basis by the application. Changing the Checksum Coverage in the network violates the integrity assumptions at the receiver and may result in unpredictable or incorrect application behaviour. 8.2. Services codes DCCP specifies a Service Code as a 4-byte value (32 bits) that describes the application-level service to which a client application wishes to connect [RFC4340]. REQ-13: If a NAT translates a DCCP packet, it MUST NOT modify its DCCP service code value. Further guidance on the use of Service Codes by middleboxes, including NATs, can be found in [I-D.ietf-dccp-serv-codes]. 9. DCCP without NAT support If the NAT device cannot be updated to support DCCP, DCCP datagrams can be encapsulated within an UDP transport header. Indeed, most NAT devices are already capable of handling UDP. This is however beyond the scope of this document. 10. Security Considerations [RFC4787] discusses security considerations for NATs that handle IP and unicast (UDP) traffic, all of which apply equally to this Denis-Courmont Expires April 7, 2009 [Page 7]
Internet-Draft NAT DCCP Requirements October 2008 document. Security concerns specific to handling DCCP packets are discussed in this section. REQ-1, and REQ-6 through REQ-13 do not introduce any new known security concerns. REQ-2 does not introduce any new known security concerns. While a NAT may elect to keep track of some DCCP-specific per-flow state (compared to UDP), it has no obligations to do so. REQ-3 allows a NAT to adopt either a more secure, or a more application-transparent filtering policy. This is already addressed in [RFC4787] and [I-D.ietf-behave-tcp]. Similar to [I-D.ietf-behave-tcp], REQ-4 of this document recommends a NAT to respond to unsolicited inbound Listen and Sync packets with an ICMP error delayed by a few seconds. Doing so may reveal the presence of a NAT to an external attacker. Silently dropping the Listen makes it harder to diagnose network problems. and forces applications to wait for the DCCP stack to finish several retransmissions before reporting an error. An implementer must therefore understand and carefully weigh the effects of not sending an ICMP error or rate-limiting such ICMP errors to a very small number. REQ-5 recommends that a NAT that passively monitors DCCP state keep idle sessions alive for at least 124 minutes or 4 minutes depending on the state of the connection. it may attempt to actively determine the liveliness of a DCCP connection or let the NAT administrator configure more conservative timeouts. 11. IANA Considerations This document raises no IANA considerations. 12. Acknowledgments The author would like to thank Gorry Fairhurst, Eddie Kohler, Dan Wing, Alfred Hoenes, Magnus Westerlund, for his comments and help on this document. This memo borrows heavily from draft-ietf-behave-tcp-07, by S. Guha (editor), K. Biswas, B. Ford, S. Sivakumar and P. Srisuresh. 13. References Denis-Courmont Expires April 7, 2009 [Page 8]
Internet-Draft NAT DCCP Requirements October 2008 13.1. Normative References [I-D.ietf-behave-nat-icmp] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT Behavioral Requirements for ICMP protocol", draft-ietf-behave-nat-icmp-09 (work in progress), September 2008. [I-D.ietf-dccp-simul-open] Fairhurst, G., "DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal", draft-ietf-dccp-simul-open-04 (work in progress), October 2008. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. 13.2. Informative References [I-D.ietf-behave-tcp] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, "NAT Behavioral Requirements for TCP", draft-ietf-behave-tcp-08 (work in progress), September 2008. [I-D.ietf-dccp-serv-codes] Fairhurst, G., "The DCCP Service Code", draft-ietf-dccp-serv-codes-08 (work in progress), September 2008. [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. Denis-Courmont Expires April 7, 2009 [Page 9]
Internet-Draft NAT DCCP Requirements October 2008 Author's Address Remi Denis-Courmont VideoLAN project EMail: rem@videolan.org URI: http://www.videolan.org/ Denis-Courmont Expires April 7, 2009 [Page 10]
Internet-Draft NAT DCCP Requirements October 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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. 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, THE IETF TRUST 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. Intellectual Property 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. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Denis-Courmont Expires April 7, 2009 [Page 11]
