Network Working Group R. Stewart Internet-Draft Cisco Systems, Inc. Intended status: Standards Track M. Tuexen Expires: May 20, 2008 Muenster Univ. of Applied Sciences November 17, 2007 Stream Control Transmission Protocol (SCTP) Network Address Translation draft-stewart-behave-sctpnat-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 May 20, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Stream Control Transmission Protocol [RFC4960] provides a reliable communications channel between two end-hosts in many ways similar to TCP [RFC0793]. With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT for TCP that allows multiple hosts to reside behind a NAT and yet use only a single globally unique IPv4 address, even when two hosts (behind the NAT) choose the same port numbers for their connection. This Stewart & Tuexen Expires May 20, 2008 [Page 1] Internet-Draft SCTP Network Address Translation November 2007 additional code is sometimes classified as Network Address and Port Translation or NAPT. To date, specialized code for SCTP has NOT yet been added to most NAT's so that only pure NAT is available. The end result of this is that only one SCTP capable host can be behind a NAT. This document describes an SCTP specific variant of NAT which provides similar features of NAPT in the single point traversal scenario described in [I-D.xie-behave-sctp-nat-cons]. Furthermore both algorithms are compared. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. The SCTP specific variant of NAT . . . . . . . . . . . . . . . 4 4. Handling of local port number collisions . . . . . . . . . . . 7 5. Handling of local port number and verification tag collisions and lost state . . . . . . . . . . . . . . . . . . 7 6. Discussion of the SCTP specific variant of NAT . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Security considerations . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 11 Stewart & Tuexen Expires May 20, 2008 [Page 2] Internet-Draft SCTP Network Address Translation November 2007 1. Introduction Stream Control Transmission Protocol [RFC4960] provides a reliable communications channel between two end-hosts in many ways similar to TCP [RFC0793]. With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT for TCP that allows multiple hosts to reside behind a NAT and yet use only a single globally unique IPv4 address, even when both hosts (behind the NAT) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation or NAPT. To date, specialized code for SCTP has NOT yet been added to most NAT's so that only true NAT is available. The end result of this is that only one SCTP capable host can be behind a NAT. This document proposes an SCTP specific variant NAT that provides the NAPT functionality without changing SCTP port numbers. The authors feel it is possible and desirable to make these changes for a number of reasons. o It is desirable for SCTP end-hosts on multiple platforms to be able to share a global IP address behind a NAT, much as TCP does today. o If a NAT does not need to change any data within an SCTP packet it will reduce the processing burden of NAT'ing SCTP by NOT needing to execute the CRC32c checksum required by SCTP. o Not having to touch the IP payload makes the processing of ICMP messages in NATs easier. 2. Terminology For this discussion we will use several terms. For clarity we will first define these terms. o Global-Address - That address that a host behind a NAT is attempting to contact. o Global-Port - The port number of the peer process at the Global- Address. o Local-Address - The local address that is known to the host behind the NAT, aka a private address [RFC1918]. o Local-Port - The port number that is in use by the host holding the Local-Address. Normally this is the port that will be Stewart & Tuexen Expires May 20, 2008 [Page 3] Internet-Draft SCTP Network Address Translation November 2007 translated by the NAPT to a different port number. o Nat-Global-Address - The global address assigned to the NAT box which it uses as a source address when sending packets towards the Global-Address. o Natted-Port - The port number that the NAT is using to represent the Local-Port when send data packets toward the Global-Address and Global-Port. o Local-Vtag - The Verification Tag that the host inside the natted address space has chosen for its communication. The V-Tag is a unique 32 bit tag that must accompany any incoming SCTP packet for this association to the Local-Address. o Remote-Vtag - The Verification Tag that the host holding the Global-Address has chosen for its communication. The V-Tag is a unique 32 bit tag that must accompany any incoming SCTP packet for this association to the Global-Address. 3. The SCTP specific variant of NAT In this section we assume that we have multiple SCTP capable hosts behind a NAT which has one Nat-Global address. Furthermore we are considering only the single point traversal scenario described in [I-D.xie-behave-sctp-nat-cons]. The modification of SCTP packets sent to the public Internet is easy. The source address of the packet has to be replaced with the Nat- Global-Address. It may also be necessary to establish some state in the NAT box to handle incoming packets, which is discussed later. For SCTP packets coming from the public Internet the destination address of the packets has to be replaced with the Local-Address of the host the packet has to be delivered to. The lookup of the Local- Address is based on the Global-VTag, Global-Port, Global-Address, Local-Vtag and the Local-Port. For the SCTP NAT processing the NAT box has to maintain a table of Global-VTag, Global-Port, Global-Address, Local-VTag, Local-Port and Local-Address. An entry in that table is called a NAT state control block. The processing of outgoing SCTP packets containing an INIT-chunk is described in the following figure. Stewart & Tuexen Expires May 20, 2008 [Page 4] Internet-Draft SCTP Network Address Translation November 2007 Local-Network Global-Internet [From(Local-Address,Local-Port), To(Global-Address:Global-Port) INIT(Initiate-Tag)]-------> Create(Global-Port,Global-Address,Initiate-Tag,Local-Port,Local-Address) Returns(NAT-State control block) Translate To: [From(Nat-Global-Address:Local-Port), To(Global-Address:Global-Port) INIT(Initiate-Tag)]-------> It should be noted that normally no NAT control block will be created. However it is possible that that there is already a NAT control block with the same Global-Port, Global-Address, Initiate- Tag, Local-VTag but different Local-Address. In this case the INIT SHOULD be dropped and an ABORT MAY be sent back. The processing of outgoing SCTP packets containing no INIT-chunk is described in the following figure. Local-Network Global-Internet [From(Local-Address,Local-Port), To(Global-Address:Global-Port)]-------> Translate To: [From(Nat-Global-Address:Local-Port), To(Global-Address:Global-Port)]-------> The processing of incoming SCTP packets containing INIT-ACK chunks is described in the following figure. Stewart & Tuexen Expires May 20, 2008 [Page 5] Internet-Draft SCTP Network Address Translation November 2007 Local-Network Global-Internet <-----[From(Global-Address,Global-Port), To(Nat-Global-Address,Local-Port), SCTP(Local-VTag), INIT-ACK(Initiate-Tag)] Lookup(0,Global-Port,Global-Address,Local-VTag,Local-Port) Update(Initiate-Tag,Global-Port,Global-Address,Local-VTag,Local-Port) Returns(NAT-State control block containing Local-Address) <-----[From(Global-Address:Global-Port), To(Local-Address,Local-Port) SCTP(Local-VTag)] In the case Lookup fails, the SCTP packet is dropped. The Update routine inserts the Global-VTag in the NAT state control block. The processing of incoming SCTP packets containing and ABORT or SHUTDOWN-COMLETE chunk with the T-Bit set is described in the following figure. Local-Network Global-Internet <-----[From(Global-Address,Global-Port), To(Nat-Global-Address,Local-Port), SCTP(Global-VTag)] Lookup(Global-VTag,Global-Port,Global-Address,0,Local-Port) Returns(NAT-State control block containing Local-Address) <-----[From(Global-Address:Global-Port), To(Local-Address,Local-Port) SCTP(Global-VTag)] The processing of other incoming SCTP packets is described in the following figure. Stewart & Tuexen Expires May 20, 2008 [Page 6] Internet-Draft SCTP Network Address Translation November 2007 Local-Network Global-Internet <-----[From(Global-Address,Global-Port), To(Nat-Global-Address,Local-Port), SCTP(Local-VTag)] Lookup(0,Global-Port,Global-Address,Local-VTag,Local-Port) Returns(NAT-State control block contaning Local-Address) <-----[From(Global-Address:Global-Port), To(Local-Address,Local-Port) SCTP(Local-VTag)] 4. Handling of local port number collisions There is one drawback of the SCTP specific variant of NAT compared to a NAPT solution like the ones available for TCP. Consider the case where two hosts in the Local-Address space want to setup an SCTP association with the same server running on the same host in the Internet. This means that the Global-Port and the Global-Address are the same. If they both chose the same Local-Port the server can not distinguish both associations based on the address and port numbers. For the server it looks like the association is being restarted. To overcome this limitation the client sends a NAT_SUPPORTED parameter in the INIT-chunk which is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0xC007 | Length=4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ When the server receives this parameter it will also use the verification tag to look up the association. However, this will make it impossible to restart such associations. 5. Handling of local port number and verification tag collisions and lost state Consider the case where two hosts in the Local-Address space want to setup an SCTP association with the same server running on the same host in the Internet. This means that the Global-Port and the Global-Address are the same. If they both chose the same Local-Port and Local-VTag, the NAT box can not distinguish incoming packets Stewart & Tuexen Expires May 20, 2008 [Page 7] Internet-Draft SCTP Network Address Translation November 2007 anymore. But this is very unlikely. The Local-Vtags are chosen by random and if the Local-Ports are also chosen ephemeral an random this gives a 46 bit random number which has to match. In the TCP like NAPT case the NAT box can control the 16 bit Natted Port. However, if this unlikely happens the NAT box SHOULD respond to the INIT chunk by sending an ABORT chunk with the M-bit set. The source address of the ABORT chunk MUST be the destination address of the SCTP packet containing the INIT chunk. The ABORT chunk defined in [RFC4960] is therefore extended by using the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 6 | Reserved |M|T| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / zero or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ In case the NAT box has to handle an SCTP packet for which it does not have the necessary state, it SHOULD send an SCTP packet with an ABORT chunk as descibed above. 6. Discussion of the SCTP specific variant of NAT The advantages of using the SCTP specific variant of NAT is that the NAT engines do not have to modify the SCTP packet at all. This is important because modifying the packets requires a recalculation of the checksum over the complete packet. There is no way of just computing the different like it is possible for the IP, UDP and TCP checksum. It should also be noted that the processing of ICMP packets is easier. 7. IANA Considerations TBD 8. Security considerations State maintenance within a NAT is always a subject of possible Denial Of Service attack. This document recommends that at a minimum a NAT Stewart & Tuexen Expires May 20, 2008 [Page 8] Internet-Draft SCTP Network Address Translation November 2007 run a timer on any SCTP state so that old association state can be cleaned up. 9. Acknowledgments The authors wish to thank Henning Peters, Irene Ruengeler for their invaluable comments. 10. References 10.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. 10.2. Informative References [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [I-D.xie-behave-sctp-nat-cons] Xie, Q., Stewart, R., Holdrege, M., and M. Tuexen, "SCTP NAT Traversal Considerations", draft-xie-behave-sctp-nat-cons-03 (work in progress), November 2007. Stewart & Tuexen Expires May 20, 2008 [Page 9] Internet-Draft SCTP Network Address Translation November 2007 Authors' Addresses Randall R. Stewart Cisco Systems, Inc. 4875 Forest Drive Suite 200 Columbia, SC 29206 USA Phone: Email: rrs@cisco.com Michael Tuexen Muenster Univ. of Applied Sciences Stegerwaldstr. 39 48565 Steinfurt Germany Email: tuexen@fh-muenster.de Stewart & Tuexen Expires May 20, 2008 [Page 10] Internet-Draft SCTP Network Address Translation November 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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