Network Working Group Q. Xie Internet-Draft Motorola Expires: January 7, 2005 R. Stewart Cisco Systems, Inc. M. Holdrege Strix Systems July 9, 2004 SCTP NAT Transverse Considerations draft-xie-tsvwg-sctp-nat-00.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 January 7, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document provides guidelines and solutions for dealing with SCTP association transversing NAT and similar middleboxes. Xie, et al. Expires January 7, 2005 [Page 1] Internet-Draft SCTP NAT Transverse Considerations July 2004 Table of Contents 1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. SCTP NAT Transverse Scenarios . . . . . . . . . . . . . . . . 3 3.1 Single Point Transvers . . . . . . . . . . . . . . . . . . 3 3.2 Multi-point Traverse . . . . . . . . . . . . . . . . . . . 4 4. Considerations for SCTP NAT transverse . . . . . . . . . . . . 4 4.1 Simple NAT enhancement to support singly-homed SCTP associations . . . . . . . . . . . . . . . . . . . . . . . 4 4.2 NAT enhancement to support multi-homed SCTP associations through a single traverse point . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1 Normative References . . . . . . . . . . . . . . . . . . . . 6 6.2 Informative References . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . 8 Xie, et al. Expires January 7, 2005 [Page 2] Internet-Draft SCTP NAT Transverse Considerations July 2004 1. Conventions The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [2]. 2. Introduction It is the job of Network Address Translators (NAT) [3] and middleboxes [6] that utilize a NAT function to manipulate address and port information in the IP header. This poses a challenge for hosts that attempt to use certain end-to-end protocols [5]. This issue has drawn increasingly wide attention from the IP development and service community and much work has been done to ameliorate the situation for UDP, TCP and other protocols. The same issue not only exists for SCTP [4], but also may become a more difficult issue when SCTP associations are multi-homed. This document provides guidelines and solutions for dealing with SCTP and NAT traversal. In the following discussion, we will simply refer to NAT as a function, but note that many types of middleboxes employ NAT functions. 3. SCTP NAT Transverse Scenarios 3.1 Single Point Transvers In this case, all packets in the SCTP association go through a single NAT, as shown below: +------+ +-----+ +------+ |SCTP |==========| NAT |==========|SCTP | |end A | +-----+ |end B | +------+ +------+ A variation of this case is shown below, i.e., multiple NATs in a single transverse path: +------+ +-----+ +-----+ +------+ |SCTP |======| NAT |=::==| NAT |=======|SCTP | |end A | +-----+ +-----+ |end B | +------+ +------+ The two SCTP endpoints in this case can be either singly-homed or multi-homed. However, the important thing is that the NAT (or NATs) in this case sees ALL the packets of the SCTP association. Xie, et al. Expires January 7, 2005 [Page 3] Internet-Draft SCTP NAT Transverse Considerations July 2004 In this single traverse point scenario, we must acknowledge that while one of the main benefits of SCTP multi-homing is redundant paths, the NAT function represents a single point of failure in the path of the SCTP multi-home association. However, the rest of the path may still benefit from path diversity provided by SCTP multi-homing. 3.2 Multi-point Traverse This case involves multiple NATs and each NAT only sees some of the packets in the SCTP association. An example is shown below: +------+ /====|NAT A |====\ +------+ / +------+ \ +------+ |SCTP |/ ... \|SCTP | |end A |\ /|end B | +------+ \ +------+ / +------+ \====|NAT B |====/ +------+ This case does NOT apply to a singly-homed SCTP association (i.e., BOTH endpoints in the association use only one IP address). The advantage here is that the existance of multiple NAT traverse points can preserve the path diversity of a multi-homed association for the entire path. This in turn can improve the robustness of the communication. To make this work, however, all the NATs involved must recognize the packets they see as belonging to the same SCTP association and perform address translation in a consistent way. This can only be possible if there exists some inter-NAT protocol which can provide coordination amongst the NATs when the association is set up. Further study is needed for this scenario. In the rest of this document, we will only discuss the single traverse point scenario. 4. Considerations for SCTP NAT transverse In any type of traverse, the NAT must understand the SCTP protocol. Since SCTP is relatively new (compared to UDP or TCP), some older existing NATs that are capable of handling UDP or TCP traverse will need to be enhanced for SCTP. In this section we discuss what considerations should be made for that NAT enhancement. 4.1 Simple NAT enhancement to support singly-homed SCTP associations In a singly-homed SCTP association, each endpoint uses only one IP address and the association will always go through a single NAT Xie, et al. Expires January 7, 2005 [Page 4] Internet-Draft SCTP NAT Transverse Considerations July 2004 traverse point. In such cases, the enhancement needed for a NAT to support SCTP is relatively simple: 1) recognize the protocol number for SCTP, and 2) locate and change the transport ports (if needed). The location of the transport port numbers are the same as both TCP and UDP so these should be minor changes for NAT software. It is important that the endpoints not to list the address again within the INIT or INIT-ACK when setting up the association. Note, the use of a single address by the endpoint may be the result of either the endpoint only has a single address assigned to the host or due to user of the SCTP stack only specifies to bind one address to the endpoint. 4.2 NAT enhancement to support multi-homed SCTP associations through a single traverse point At the association setup, the NAT needs to recognize and intercept the INIT, or INIT-ACK chunk. The NAT will look into the chunk and find out all the listed IP addresses (including the source IP address in the packet's IP header). The NAT will map this address list to a _list_ of external addresses. Then, the NAT will reconstruct the INIT or INIT-ACK chunk, fill it with external addresses, re-calculate the checksum, and finally forward the packet on to the peer. This approach is transparent to the endpoints and can support multi-homing. The NAT function is capable of variety in translating one to many or many to one addresses in the chunk. Depending on network architectural requirements, NAT administrators may choose to map many internal addresses to a single external address. It should be noted that this will reduce path diversity in a portion of the network. Alternatively, by mapping 1 internal address to many external addresses can increase the path diversity in a portion of the network. Editor's Note: Add discussion about ASCONF handling by the NAT. Editor's Note: Add some scenario examples. 5. Security Considerations See RFC XXXX on SCTP security considerations. See RFC XXXX on NAT security considerations. See RFC XXXX for IAB comments on NAT. 6. References Xie, et al. Expires January 7, 2005 [Page 5] Internet-Draft SCTP NAT Transverse Considerations July 2004 6.1 Normative References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 6.2 Informative References [3] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [4] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October 2000. [5] Holdrege, M. and P. Srisuresh, "Protocol Complications with the IP Network Address Translator", RFC 3027, January 2001. [6] Swale, R., Mart, P., Sijben, P., Brim, S. and M. Shore, "Middlebox Communications (midcom) Protocol Requirements", RFC 3304, August 2002. Authors' Addresses Qiaobing Xie Motorola, Inc. 1501 W. Shure Drive, 2-F9 Arlington Heights, IL 60004 US Phone: +1-847-632-3028 EMail: qxie1@email.mot.com Randall R. Stewart Cisco Systems, Inc. 8725 West Higgins Road Suite 300 Chicago, IL 60631 USA Phone: +1-815-477-2127 EMail: rrs@cisco.com Xie, et al. Expires January 7, 2005 [Page 6] Internet-Draft SCTP NAT Transverse Considerations July 2004 Matt Holdrege Strix Systems Suite 110, 26610 Agoura Road Calabasas, CA 91302 USA EMail: matt@strixsystems.com Xie, et al. Expires January 7, 2005 [Page 7] Internet-Draft SCTP NAT Transverse Considerations July 2004 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 (2004). 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. Xie, et al. Expires January 7, 2005 [Page 8]