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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 6951

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



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   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










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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



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   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



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   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.













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      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



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   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.



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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;
   };






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   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



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   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.






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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





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