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Internet Engineering Task Force (IETF) M. Tuexen Request for Comments: 6951 Muenster Univ. of Appl. Sciences Category: Standards Track R. Stewart ISSN: 2070-1721 Adara Networks

                                                              May 2013

UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets

               for End-Host to End-Host Communication


 This document describes a simple method of encapsulating Stream
 Control Transmission Protocol (SCTP) packets into UDP packets and its
 limitations.  This allows the usage of SCTP in networks with legacy
 NATs that do not support 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
 Please note that this document only describes the functionality
 required within an SCTP stack to add on UDP encapsulation, providing
 only those mechanisms for two end-hosts to communicate with each
 other over UDP ports.  In particular, it does not provide mechanisms
 to determine whether UDP encapsulation is being used by the peer, nor
 the mechanisms for determining which remote UDP port number can be
 used.  These functions are out of scope for this document.
 This document covers only end-hosts and not tunneling (egress or
 ingress) endpoints.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Tuexen & Stewart Standards Track [Page 1] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

Copyright Notice

 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
 ( 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 . . . . . . . . . . . . . . . . .   7
   5.5.  ICMP Considerations . . . . . . . . . . . . . . . . . . .   7
   5.6.  Path MTU Considerations . . . . . . . . . . . . . . . . .   7
   5.7.  Handling of Embedded IP Addresses . . . . . . . . . . . .   8
   5.8.  Explicit Congestion Notification (ECN) Considerations . .   8
 6.  Socket API Considerations . . . . . . . . . . . . . . . . . .   8
   6.1.  Get or Set the Remote UDP Encapsulation Port Number
         (SCTP_REMOTE_UDP_ENCAPS_PORT) . . . . . . . . . . . . . .   9
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
   10.2.  Informative References . . . . . . . . . . . . . . . . .  11

Tuexen & Stewart Standards Track [Page 2] RFC 6951 UDP Encapsulation of SCTP Packets May 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
 o  To allow SCTP traffic to pass through legacy NATs, which do not
    provide native SCTP support as specified in [BEHAVE] and
 o  To allow SCTP to be implemented on hosts that 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",
 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, a kernel implementation
 might not 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 that 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 Standards Track [Page 3] RFC 6951 UDP Encapsulation of SCTP Packets May 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
 [BEHAVE] and [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 (Address Configuration Change Chunks) in combination with
 For multihomed SCTP associations, 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
 endpoints dealing with legacy constraints as described in Section 3.
 It doesn't cover generic tunneling endpoints.
 Obviously, the exit strategy is to use hosts supporting SCTP natively
 and middleboxes supporting SCTP as specified in [BEHAVE] and

5. SCTP over UDP

5.1. Architectural Considerations

 UDP-encapsulated SCTP is normally communicated between SCTP stacks
 using the IANA-assigned UDP port number 9899 (sctp-tunneling) on both
 ends.  There are circumstances where other ports may be used on
 either end: As stated earlier, implementations in the application
 space might be required to use ports other than the registered port.
 Since NAT boxes might change UDP port numbers, the receiver might
 observe other UDP port numbers than were used by the sender.
 Discovery of alternate ports is outside of the scope of this
 document, but this section describes considerations for SCTP stack
 design in light of their potential use.
 Each SCTP stack uses a single local UDP encapsulation port number as
 the destination port for all its incoming SCTP packets.  While the

Tuexen & Stewart Standards Track [Page 4] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

 uniqueness of the local UDP encapsulation port number is not
 necessarily required for the protocol, this greatly simplifies
 implementation design, since different ports for each address would
 require a sender implementation to choose the appropriate port while
 doing source address selection.  Using a single local 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
 An SCTP implementation supporting UDP encapsulation MUST maintain a
 remote UDP encapsulation port number per destination address for each
 SCTP association.  Again, because the remote stack may be using ports
 other than the well-known port, each port may be different from each
 stack.  However, because of remapping of ports by NATs, the remote
 ports associated with different remote IP addresses may not be
 identical, even if they are associated with the same stack.
 Implementation note: Because the well-known port might not be used,
 implementations need to allow other port numbers to be specified as a
 local or remote UDP encapsulation port number through APIs.

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

Tuexen & Stewart Standards Track [Page 5] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

 The packet format for an encapsulated SCTP packet when using IPv6 as
 defined in [RFC2460] is shown in Figure 2.  Please note that the
 number m of IPv6 extension headers can be 0.
     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, and the destination port
 MUST be the remote UDP encapsulation port number maintained 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
 The SCTP checksum MUST be computed for IPv4 and IPv6, and the UDP
 checksum SHOULD be computed for IPv4 and IPv6.  (See [RFC0768]
 regarding IPv4; see [RFC2460] and [RFC6936] regarding IPv6.)
 Although UDP with a zero checksum over IPv6 is allowed under certain
 constraints [RFC6936], this document does not specify mechanisms for
 this mode.  Deployed support may be limited; also, at the time of
 writing, the use of a zero UDP checksum would be counter to the goal
 of legacy NAT traversal.

Tuexen & Stewart Standards Track [Page 6] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

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
 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

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 that the
 SCTP packet triggering the ICMP or ICMPv6 packet belongs to.  If a
 received ICMP or ICMPv6 packet cannot be related to a specific SCTP
 association or the verification tag cannot be verified, it MUST be
 discarded silently.  In particular, this means that the SCTP stack
 MUST NOT rely on receiving ICMP or ICMPv6 messages.  Implementation
 constraints could prevent processing received ICMP or ICMPv6
 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
     'Destination Unreachable' and code 'Protocol 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.

Tuexen & Stewart Standards Track [Page 7] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

 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 Don'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.

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 multihomed SCTP association is set up initially as
 a single-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, the
 protocol extension [RFC5061] MUST be used with wildcard addresses
 only.  If an SCTP endpoint 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. Explicit Congestion Notification (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.

Tuexen & Stewart Standards Track [Page 8] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

6.1. Get or Set the Remote UDP Encapsulation Port Number

 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;
 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 has updated this assignment to refer to this
 document.  As per [RFC6335], the Assignee is [IESG] and the Contact
 is [IETF_Chair].
 Please note that the TCP port 9899 (sctp-tunneling) assignment is not
 needed anymore, and IANA has removed this TCP port number assignment
 and marked TCP port 9899 as "Reserved".

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
 An attacker might send a malicious UDP packet towards an SCTP
 endpoint to change the encapsulation port for a single remote address
 of a particular SCTP association.  However, as specified in

Tuexen & Stewart Standards Track [Page 9] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

 Section 5.4, this requires the usage of one of 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
 Stiemerling, Irene Ruengeler, and Dan Wing for their invaluable

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
 [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.E. and R.M. 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.

Tuexen & Stewart Standards Track [Page 10] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

 [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

10.2. Informative References

 [BEHAVE]   Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
            Transmission Protocol (SCTP) Network Address Translation",
            Work in Progress, February 2013.
 [NATSUPP]  Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
            Transmission Protocol (SCTP) Network Address Translation
            Support", Work in Progress, February 2013.
 [RFC3424]  Daigle, L. 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.
 [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
            for the Use of IPv6 UDP Datagrams with Zero Checksums",
            RFC 6936, April 2013.

Tuexen & Stewart Standards Track [Page 11] RFC 6951 UDP Encapsulation of SCTP Packets May 2013

Authors' Addresses

 Michael Tuexen
 Muenster University of Applied Sciences
 Stegerwaldstrasse 39
 48565 Steinfurt
 Randall R. Stewart
 Adara Networks
 Chapin, SC  29036

Tuexen & Stewart Standards Track [Page 12]

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