Network Working Group R. Stewart Internet-Draft Adara Networks Intended status: Standards Track M. Tuexen Expires: April 12, 2013 I. Ruengeler Muenster Univ. of Appl. Sciences October 9, 2012 Stream Control Transmission Protocol (SCTP) Network Address Translation for Endpoints draft-ietf-tsvwg-natsupp-03.txt 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 a NAT) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation (NAPT). To date, specialized code for SCTP has not yet been added to most NATs 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 the protocol extensions required for the SCTP endpoints to help NAT's provide similar features of NAPT in the single-point and multi-point traversal scenario. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on April 12, 2013. Stewart, et al. Expires April 12, 2013 [Page 1] Internet-Draft SCTP NAT for Endpoints October 2012 Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Problem Space Overview . . . . . . . . . . . . . . . . . . . . 5 5. Handling of Internal Port Number and Verification Tag Collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Handling of Missing State . . . . . . . . . . . . . . . . . . 7 7. Multi Point Traversal Considerations . . . . . . . . . . . . . 8 8. Handling of Internal Port Number Collisions . . . . . . . . . 8 9. SCTP Socket API Considerations . . . . . . . . . . . . . . . . 10 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 11. Security Considerations . . . . . . . . . . . . . . . . . . . 10 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 13.1. Normative References . . . . . . . . . . . . . . . . . . 10 13.2. Informative References . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Stewart, et al. Expires April 12, 2013 [Page 2] Internet-Draft SCTP NAT for Endpoints October 2012 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 two hosts (behind a NAT) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation (NAPT). To date, specialized code for SCTP has not yet been added to most NATs 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 describes an SCTP specific chunks and procedures to help NAT's provide similar features of NAPT in the single point and multi-point traversal scenario. An SCTP implementation supporting this extension will follow these procedures to assure that in both single homed and multi-homed cases a NAT will maintain the proper state without needing to change port numbers. A NAT will need to follow these procedures for generating appropriate SCTP packet formats. NAT's should refer to [I-D.ietf-behave-sctpnat] for the BCP in using these formats. When considering this feature it is possible to have multiple levels of support. At each level, the Internal Host, External Host and NAT may or may not support the features described in this document. The following table illustrates the results of the various combinations of support and if communications can occur between two endpoints. +---------------+------------+---------------+---------------+ | Internal Host | NAT | External Host | Communication | +---------------+------------+---------------+---------------+ | Support | Support | Support | Yes | | Support | Support | No Support | Limited | | Support | No Support | Support | None | | Support | No Support | No Support | None | | No Support | Support | Support | Limited | | No Support | Support | No Support | Limited | | No Support | No Support | Support | None | | No Support | No Support | No Support | None | +---------------+------------+---------------+---------------+ Table 1: Communication possibilities From the table we can see that when a NAT does not support the Stewart, et al. Expires April 12, 2013 [Page 3] Internet-Draft SCTP NAT for Endpoints October 2012 extension no communication can occur. This is for the most part the current situation i.e. SCTP packets sent externally from behind a NAT are discarded by the NAT. In some cases, where the NAT supports the feature but one of the two external hosts does not support the feature communication may occur but in a limited way. For example only one host may be able to have a connection when a collision case occurs. 2. Conventions 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]. 3. Terminology This document uses the following terms, which are depicted in Figure 1. Private-Address (Priv-Addr): The private address that is known to the internal host. Internal-Port (Int-Port): The port number that is in use by the host holding the Private-Address. Internal-VTag (Int-VTag): The Verification Tag that the internal host has chosen for its communication. The VTag is a unique 32- bit tag that must accompany any incoming SCTP packet for this association to the Private-Address. External-Address (Ext-Addr): The address that an internal host is attempting to contact. External-Port (Ext-Port): The port number of the peer process at the External-Address. External-VTag (Ext-VTag): The Verification Tag that the host holding the External-Address has chosen for its communication. The VTag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the External-Address. Public-Address (Pub-Addr): The public address assigned to the NAT box which it uses as a source address when sending packets towards the External-Address. Stewart, et al. Expires April 12, 2013 [Page 4] Internet-Draft SCTP NAT for Endpoints October 2012 Internal Network | External Network | Private | Public External +---------+ Address | Address /--\/--\ Address +---------+ | SCTP | +-----+ / \ | SCTP | |end point|=========| NAT |=======| Internet |==========|end point| | A | +-----+ \ / | B | +---------+ Internal | \--/\--/ External+---------+ Internal Port | Port External VTag | VTag Figure 1: Basic network setup 4. Problem Space Overview When an SCTP endpoint is behind a NAT which supports [I-D.ietf-behave-sctpnat] a number of problems may arise as it tries to communicate with its peer: o More than one server behind a NAT may pick the same VTag and source port when talking to the same peer server. This creates a situation where the NAT will not be able to tell the two associations apart. This situation is discussed in Section 5. o When an SCTP endpoint is a server and talking with multiple peers and the peers are behind the same NAT, to the server the two endpoints cannot be distinguished. This case is discussed in Section 8. o A NAT could at one point during a conversation restart causing all of its state to be lost. This problem and its solution is discussed in Section 6. o An SCTP endpoint may be behind two NAT's giving it redundancy. The method to set up this scenario is discussed in Section 7. Each of these solutions requires additional chunks and parameters, defined in this document, and possibly modified handling procedures from those specified in [RFC4960]. 5. Handling of Internal Port Number and Verification Tag Collisions Consider the case where two hosts in the Private-Address space want to set up an SCTP association with the same server running on the same host in the Internet. This means that the External-Port and the External-Address are the same. If they both choose the same Stewart, et al. Expires April 12, 2013 [Page 5] Internet-Draft SCTP NAT for Endpoints October 2012 Internal-Port and Internal-VTag, the NAT box cannot distinguish incoming packets anymore. But this is very unlikely. The Internal- VTags are chosen at random and if the Internal-Ports are also chosen from the ephemeral port range at 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, in this unlikely event the NAT box MUST respond to the INIT chunk by sending an ABORT chunk with the M-bit set. The M-bit is a new bit defined by this document to express to SCTP that the source of this packet is a "middle" box, not the peer SCTP endpoint. The source address of the packet containing the ABORT chunk MUST be the destination address of the SCTP packet containing the INIT chunk. The sender of the packet containing the INIT chunk, upon reception of an ABORT with M-bit set SHOULD reinitiate the association setup procedure after choosing a new initiate tag. These procedures SHOULD be followed only if the appropriate error cause code for colliding NAT table state is included AND the association is in the COOKIE-WAIT state (i.e. it is awaiting a INIT-ACK). If the endpoint is in any other state an SCTP endpoint SHOULD NOT respond. 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 / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The following error cause with cause code 0x00B0 (Colliding NAT table entry) MUST be included in the ABORT chunk: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B0 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ INIT chunk / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Stewart, et al. Expires April 12, 2013 [Page 6] Internet-Draft SCTP NAT for Endpoints October 2012 6. Handling of Missing State If the NAT box receives a packet for which the lookup procedure does not find an entry in the NAT table, a packet containing an ERROR packet is sent back with the M-bit set. The source address of the packet containing the ERROR chunk MUST be the destination address of the incoming SCTP packet. The verification tag is reflected. The ERROR 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 = 9 | Reserved |M|T| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / zero or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The following error cause with cause code 0x00B1 (Missing NAT table entry) SHOULD be included in the ERROR chunk: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B1 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ Incoming Packet / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Upon reception by an SCTP end-point with this ERROR chunk the receiver SHOULD take the following actions: o Validate the verification tag is reflected by looking at the VTag that would have been included in the outgoing packet. o Validate that the peer of the SCTP association supports the dynamic address extension, if it does not discard the incoming ERROR chunk. o Generate a new ASCONF chunk as defined below including both sets of VTags so that the NAT may recover the appropriate state. The procedures for generating an ASCONF chunk can be found in [RFC5061]. Stewart, et al. Expires April 12, 2013 [Page 7] Internet-Draft SCTP NAT for Endpoints October 2012 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Parameter Type = 0xC008 | Parameter Length = 16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ASCONF-Request Correlation ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internal Verification Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | External Verification Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ If the NAT box receives a packet for which it has no NAT table entry and the packet contains an ASCONF chunk with a VTAG parameter, the NAT box MUST update its NAT table according to the verification tags in the VTAG parameter. The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either add the address and respond with an acknowledgment, if the address is new to the association (following all procedures defined in [RFC5061]). Or, if the address is already part of the association, the SCTP endpoint MUST NOT respond with an error, but instead should respond with an ASCONF-ACK chunk acknowledging the address but take no action (since the address is already in the association). 7. Multi Point Traversal Considerations If a multi-homed SCTP end-point behind a NAT connects to a peer, it SHOULD first set up the association single-homed with only one address causing the first NAT to populate its state. Then it SHOULD add each IP address using ASCONF chunks sent via their respective NATs. The address to add is the wildcard address and the lookup address SHOULD also contain the VTAG parameter pair illustrated above. 8. Handling of Internal Port Number Collisions When two SCTP hosts are behind a NAT and using the recommendations in [I-D.ietf-behave-sctpnat] it is possible that two SCTP hosts in the Private-Address space will want to set up an SCTP association with the same server running on the same host in the Internet. For the NAT appropriate tracking may be performed by assuring that the VTags are unique between the two hosts as defined in [I-D.ietf-behave-sctpnat]. But for the external SCTP server on the internet this means that the External-Port and the External-Address are the same. If they both have chosen the same Internal-Port the Stewart, et al. Expires April 12, 2013 [Page 8] Internet-Draft SCTP NAT for Endpoints October 2012 server cannot 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 DISABLE_RESTART 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 MUST do the following: o Include in the INIT-ACK a DISABLE_RESTART parameter to inform the client that it will support the feature. o Disable the restart procedures defined in [RFC4960] for this association. Servers that support this feature will need to be capable of maintaining multiple connections to what appears to be the same peer (behind the NAT) differentiated only by the VTags. The NAT, when processing the INIT-ACK, should note in its internal table that the external server supports the DISABLE_RESTART extension. This note is used when establishing future associations (i.e. when processing an INIT from an internal host) to decide if the connection should be allowed. The NAT MUST do the following when processing an INIT: o If the INIT is destined to an external address and port for which the NAT has no outbound connection, allow the INIT creating an internal mapping table. o If the INIT matches the external address and port of an already existing connection, validate that the external server supports the DISABLE_RESTART feature. If it does allow the INIT to be forwarded. o If the external server does not support the DISABLE_RESTART extension the NAT MUST send an ABORT with the M-bit set. The following error cause with cause code 0x00B2 (Duplicate Local Port with DISABLE_RESTART not Supported) MUST be included in the ABORT chunk: Stewart, et al. Expires April 12, 2013 [Page 9] Internet-Draft SCTP NAT for Endpoints October 2012 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B2 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ INIT chunk / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 9. SCTP Socket API Considerations TBD 10. IANA Considerations M-Bit for ABORT and ERROR chunk (0x02). Error cause Colliding NAT table entry, (0x00B1). Error cause Duplicate Local Port with DISABLE_RESTART not Supported, (0x00B2). Disable restart parameter (0xC007). ASCONF Parameter (0xC008). 11. Security Considerations TBD 12. Acknowledgments The authors wish to thank Jason But, Bryan Ford, David Hayes, Alfred Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for their invaluable comments. 13. References 13.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. Stewart, et al. Expires April 12, 2013 [Page 10] Internet-Draft SCTP NAT for Endpoints October 2012 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, "Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)", RFC 4895, August 2007. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, September 2007. [RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission Protocol (SCTP) Chunk Flags Registration", RFC 6096, January 2011. [I-D.ietf-tsvwg-sctp-udp-encaps] Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP Packets", draft-ietf-tsvwg-sctp-udp-encaps-04 (work in progress), July 2012. 13.2. Informative References [RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses", BCP 153, RFC 5735, January 2010. [RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)", RFC 6083, January 2011. [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. Yasevich, "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)", RFC 6458, December 2011. [I-D.ietf-behave-sctpnat] Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control Transmission Protocol (SCTP) Network Address Translation", draft-ietf-behave-sctpnat-06 (work in progress), March 2012. Stewart, et al. Expires April 12, 2013 [Page 11] Internet-Draft SCTP NAT for Endpoints October 2012 Authors' Addresses Randall R. Stewart Adara Networks Chapin, SC 29036 USA Email: randall@lakerest.net Michael Tuexen Muenster University of Applied Sciences Stegerwaldstrasse 39 48565 Steinfurt DE Email: tuexen@fh-muenster.de Irene Ruengeler Muenster University of Applied Sciences Stegerwaldstrasse 39 48565 Steinfurt DE Email: i.ruengeler@fh-muenster.de Stewart, et al. Expires April 12, 2013 [Page 12]