Network Working Group M. Tuexen Internet-Draft Muenster Univ. of Appl. Sciences Intended status: Standards Track R. Stewart Expires: August 23, 2013 Adara Networks February 19, 2013 UDP Encapsulation of SCTP Packets for End-Host to End-Host Communication draft-ietf-tsvwg-sctp-udp-encaps-11.txt Abstract This document describes a simple method of encapsulating SCTP Packets into UDP packets and its limitations. This allows the usage of SCTP in networks with legacy NAT not supporting SCTP. It can also be used to implement SCTP on hosts without directly accessing the IP-layer, for example implementing it as part of the application without requiring special privileges. Please note that this document does not provide all techniques necessary for building a complete NAT-capable application using SCTP. This document focuses on the functionality required within the SCTP stack and making this available via an API. It does not cover mechanism to determine whether UDP encapsulation is required to reach the peer and, if UDP encapsulation is used, which remote UDP port number can be used. This document covers only end-hosts and not tunneling (egress or ingress) end-points. 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 August 23, 2013. Copyright Notice Tuexen & Stewart Expires August 23, 2013 [Page 1] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Portable SCTP Implementations . . . . . . . . . . . . . . 3 3.2. Legacy NAT Traversal . . . . . . . . . . . . . . . . . . . 4 4. Unilateral Self-Address Fixing (UNSAF) Considerations . . . . 4 5. SCTP over UDP . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. Architectural Considerations . . . . . . . . . . . . . . . 4 5.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 5 5.3. Encapsulation Procedure . . . . . . . . . . . . . . . . . 6 5.4. Decapsulation Procedure . . . . . . . . . . . . . . . . . 6 5.5. ICMP Considerations . . . . . . . . . . . . . . . . . . . 7 5.6. Path MTU Considerations . . . . . . . . . . . . . . . . . 7 5.7. Handling of Embedded IP-addresses . . . . . . . . . . . . 8 5.8. ECN Considerations . . . . . . . . . . . . . . . . . . . . 8 6. Socket API Considerations . . . . . . . . . . . . . . . . . . 8 6.1. Get or Set the Remote UDP Encapsulation Port Number (SCTP_REMOTE_UDP_ENCAPS_PORT) . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10.1. Normative References . . . . . . . . . . . . . . . . . . . 10 10.2. Informative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Tuexen & Stewart Expires August 23, 2013 [Page 2] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 1. Introduction This document describes a simple method of encapsulating SCTP packets into UDP packets. SCTP as defined in [RFC4960] runs directly over IPv4 or IPv6. There are two main reasons for encapsulating SCTP packets: o Allow SCTP traffic to pass through legacy NATs, which do not provide native SCTP support as specified in [I-D.ietf-behave-sctpnat] and [I-D.ietf-tsvwg-natsupp]. o Allow SCTP to be implemented on hosts which do not provide direct access to the IP-layer. In particular, applications can use their own SCTP implementation if the operating system does not provide one. SCTP provides the necessary congestion control and reliability service that UDP does not perform. 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. Use Cases This section discusses two important use cases for encapsulating SCTP into UDP. 3.1. Portable SCTP Implementations Some operating systems support SCTP natively. For other operating systems implementations are available, but require special privileges to install and/or use them. In some cases no kernel implementation might be available at all. When providing an SCTP implementation as part of a user process, most operating systems require special privileges to access the IP layer directly. Using UDP encapsulation makes it possible to provide an SCTP implementation as part of a user process which does not require any special privileges. A crucial point for implementing SCTP in user space is that the source address of outgoing packets needs to be controlled. This is not an issue if the SCTP stack can use all addresses configured at Tuexen & Stewart Expires August 23, 2013 [Page 3] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 the IP-layer as source addresses. However, it is an issue when also using the address management required for NAT traversal, described in Section 5.7. 3.2. Legacy NAT Traversal Using UDP encapsulation allows SCTP communication when traversing legacy NATs (i.e those NATs not supporting SCTP as described in [I-D.ietf-behave-sctpnat] and [I-D.ietf-tsvwg-natsupp]). For single- homed associations IP addresses MUST NOT be listed in the INIT and INIT-ACK chunks. To use multiple addresses, the dynamic address reconfiguration extension described in [RFC5061] MUST be used only with wildcard addresses in the ASCONF chunks in combination with [RFC4895]. For multi-homed SCTP association the address management as described in Section 5.7 MUST be performed. SCTP sends periodic HEARTBEAT chunks on all idle paths. These can keep the NAT state alive. 4. Unilateral Self-Address Fixing (UNSAF) Considerations As [RFC3424] requires a limited scope, this document only covers SCTP end-points dealing with legacy constraints as described in Section 3. It doesn't cover generic tunneling end-points. Obviously, the exit strategy is to use hosts supporting SCTP natively and middleboxes supporting SCTP as specified in [I-D.ietf-behave-sctpnat] and [I-D.ietf-tsvwg-natsupp]). 5. SCTP over UDP 5.1. Architectural Considerations An SCTP implementation supporting UDP encapsulation MUST store a remote UDP encapsulation port number per destination address for each SCTP association. Each SCTP stack uses a single local UDP encapsulation port number as the destination port for all its incoming SCTP packets. UDP encapsulated SCTP is communicated over the IANA-assigned UDP port number 9899 (sctp-tunneling). However, implementations SHOULD allow other port numbers to be specified through APIs, as applications may have the need to communicate over different port numbers. If there is only a single SCTP implementation on a host (for example, a kernel Tuexen & Stewart Expires August 23, 2013 [Page 4] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 implementation being part of the operating system), using a single UDP encapsulation port number per host can be advantageous (e.g., this reduces the number of mappings in firewalls and NATs, among other things). Using a single UDP encapsulation port number per host is not possible if the SCTP stack is implemented as part of each application, there are multiple applications, and some of the applications want to use the same IP-address. 5.2. Packet Format To encapsulate an SCTP packet, a UDP header as defined in [RFC0768] is inserted between the IP header as defined in [RFC0791] and the SCTP common header as defined in [RFC4960]. Figure 1 shows the packet format of an encapsulated SCTP packet when IPv4 is used. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SCTP Common Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SCTP Chunk #1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SCTP Chunk #n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: An SCTP/UDP/IPv4 packet The packet format for an encapsulated SCTP packet when using IPv6 as defined in [RFC2460] is shown in Figure 2. Please note the the number m of IPv6 extension headers can be 0. Tuexen & Stewart Expires August 23, 2013 [Page 5] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv6 Base Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv6 Extension Header #1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv6 Extension Header #m | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SCTP Common Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SCTP Chunk #1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SCTP Chunk #n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: An SCTP/UDP/IPv6 packet 5.3. Encapsulation Procedure Within the UDP header, the source port MUST be the local UDP encapsulation port number of the SCTP stack, the destination port MUST be the remote UDP encapsulation port number stored for the association and the destination address to which the packet is sent (see Section 5.1). Because the SCTP packet is the UDP payload, the length of the UDP packet MUST be the length of the SCTP packet plus the size of the UDP header. For IPv4, the UDP checksum SHOULD be computed and the SCTP checksum MUST be computed, whereas for IPv6, the UDP checksum and the SCTP checksum MUST be computed. 5.4. Decapsulation Procedure When an encapsulated packet is received, the UDP header is removed. Then the generic lookup is performed, as done by an SCTP stack whenever a packet is received, to find the association for the received SCTP packet. After finding the SCTP association (which includes checking the verification tag), the UDP source port MUST be stored as the encapsulation port for the destination address the SCTP Tuexen & Stewart Expires August 23, 2013 [Page 6] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 packet is received from (see Section 5.1). When a non-encapsulated SCTP packet is received by the SCTP stack, the encapsulation of outgoing packets belonging to the same association and the corresponding destination address MUST be disabled. 5.5. ICMP Considerations When receiving ICMP or ICMPv6 response packets, there might not be enough bytes in the payload to identify the SCTP association which the SCTP packet triggering the ICMP or ICMPv6 packet belongs to. If a received ICMP or ICMPv6 packet can not be related to a specific SCTP association or the verification tag can't be verified, it MUST be discarded silently. This means in particular that the SCTP stack MUST NOT rely on receiving ICMP or ICMPv6 messages. Implementation constraints could prevent processing received ICMP or ICMPv6 messages. If received ICMP or ICMPv6 messages are processed, the following mapping SHOULD apply: 1. ICMP messages with type 'Destination Unreachable' and code 'Port Unreachable' SHOULD be treated as ICMP messages with type 'Protocol Unreachable' and code 'Destination Port unreachable. See [RFC0792] for more details. 2. ICMPv6 messages with type 'Destination Unreachable' and code 'Port unreachable' SHOULD be treated as ICMPv6 messages with type 'Parameter Problem' and code 'Unrecognized Next Header type encountered'. See [RFC4443] for more details. 5.6. Path MTU Considerations If an SCTP endpoint starts to encapsulate the packets of a path, it MUST decrease the Path MTU of that path by the size of the UDP header. If it stops encapsulating them, the Path MTU SHOULD be increased by the size of the UDP header. When performing Path MTU discovery as described in [RFC4820] and [RFC4821] it MUST be taken into account that one cannot rely on the feedback provided by ICMP or ICMPv6 due to the limitation laid out in Section 5.5. If the implementation does not allow control of the dont't fragment (DF)-bit contained in the IPv4 header, then Path MTU discovery can't be used. In this case, an implementation specific value should be used instead. Tuexen & Stewart Expires August 23, 2013 [Page 7] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 5.7. Handling of Embedded IP-addresses When using UDP encapsulation for legacy NAT traversal, IP addresses that might require translation MUST NOT be put into any SCTP packet. This means that a multi homed SCTP association is setup initially as a singled homed one and the protocol extension [RFC5061] in combination with [RFC4895] is used to add the other addresses. Only wildcard addresses are put into the SCTP packet. When addresses are changed during the lifetime of an association [RFC5061] MUST be used with wildcard addresses only. If an SCTP end- point receives an ABORT with the T-bit set, it MAY use this as an indication that the addresses seen by the peer might have changed. 5.8. ECN Considerations If the implementation supports the sending and receiving of the ECN bits for the IP protocols being used by an SCTP association, the ECN bits MUST NOT be changed during sending and receiving. 6. Socket API Considerations This section describes how the socket API defined in [RFC6458] needs to be extended to provide a way for the application to control the UDP encapsulation. Please note that this section is informational only. A socket API implementation based on [RFC6458] is extended by supporting one new read/write socket option. 6.1. Get or Set the Remote UDP Encapsulation Port Number (SCTP_REMOTE_UDP_ENCAPS_PORT) This socket option can be used to set and retrieve the UDP encapsulation port number. This allows an endpoint to encapsulate initial packets. struct sctp_udpencaps { sctp_assoc_t sue_assoc_id; struct sockaddr_storage sue_address; uint16_t sue_port; }; Tuexen & Stewart Expires August 23, 2013 [Page 8] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 sue_assoc_id: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets the application may fill in an association identifier or SCTP_FUTURE_ASSOC for this query. It is an error to use SCTP_{CURRENT|ALL}_ASSOC in sue_assoc_id. sue_address: This specifies which address is of interest. If a wildcard address is provided it applies only to future paths. sue_port: The UDP port number in network byte order used as the destination port number for UDP encapsulation. Providing a value of 0 disables UDP encapsulation. 7. IANA Considerations This document refers to the already assigned UDP port 9899 (sctp- tunneling). IANA is requested to update this assignment to refer to this document. As per [RFC6335] the Assignee should be [IESG] and the Contact should be [IETF_Chair]. Please note that this document does not cover TCP port 9899 (sctp- tunneling). 8. Security Considerations Encapsulating SCTP into UDP does not add any additional security considerations to the ones given in [RFC4960] and [RFC5061]. Firewalls inspecting SCTP packets must also be aware of the encapsulation and apply corresponding rules to the encapsulated packets. An attacker might send a malicious UDP packet towards an SCTP end- point to change the encapsulation port for a single remote address of a particular SCTP association. However, as specified in Section 5.4, this requires the usage of one the two negotiated verification tags. This protects against blind attackers the same way as described in [RFC4960] for SCTP over IPv4 or IPv6. Non-blind attackers can affect SCTP association using the UDP encapsulation described in this document in the same way as SCTP associations not using the UDP encapsulation of SCTP described here. 9. Acknowledgments The authors wish to thank Stewart Bryant, Dave Crocker, Gorry Fairhurst, Tero Kivinen, Barry Leiba, Pete Resnick, Martin Tuexen & Stewart Expires August 23, 2013 [Page 9] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 Stiemerling, Irene Ruengeler, and Dan Wing for their invaluable comments. 10. References 10.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. [RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)", RFC 4820, March 2007. [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [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. Tuexen & Stewart Expires August 23, 2013 [Page 10] Internet-Draft UDP Encapsulation of SCTP Packets February 2013 10.2. Informative References [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, August 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-07 (work in progress), October 2012. [I-D.ietf-tsvwg-natsupp] Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control Transmission Protocol (SCTP) Network Address Translation Support", draft-ietf-tsvwg-natsupp-04 (work in progress), October 2012. Authors' Addresses Michael Tuexen Muenster University of Applied Sciences Stegerwaldstrasse 39 48565 Steinfurt DE Email: tuexen@fh-muenster.de Randall R. Stewart Adara Networks Chapin, SC 29036 US Email: randall@lakerest.net Tuexen & Stewart Expires August 23, 2013 [Page 11]