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rfc:rfc6773

Internet Engineering Task Force (IETF) T. Phelan Request for Comments: 6773 Sonus Updates: 4340, 5762 G. Fairhurst Category: Standards Track University of Aberdeen ISSN: 2070-1721 C. Perkins

                                                 University of Glasgow
                                                         November 2012

DCCP-UDP: A Datagram Congestion Control Protocol UDP Encapsulation for

                           NAT Traversal

Abstract

 This document specifies an alternative encapsulation of the Datagram
 Congestion Control Protocol (DCCP), referred to as DCCP-UDP.  This
 encapsulation allows DCCP to be carried through the current
 generation of Network Address Translation (NAT) middleboxes without
 modification of those middleboxes.  This document also updates the
 Session Description Protocol (SDP) information for DCCP defined in
 RFC 5762.

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
 http://www.rfc-editor.org/info/rfc6773.

Phelan, et al. Standards Track [Page 1] RFC 6773 DCCP-UDP Encapsulation November 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.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
 3.  DCCP-UDP . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.1.  The UDP Header . . . . . . . . . . . . . . . . . . . . . .  5
   3.2.  The DCCP Generic Header  . . . . . . . . . . . . . . . . .  5
   3.3.  DCCP-UDP Checksum Procedures . . . . . . . . . . . . . . .  6
     3.3.1.  Partial Checksums and the Minimum Checksum
             Coverage Feature . . . . . . . . . . . . . . . . . . .  7
   3.4.  Network-Layer Options  . . . . . . . . . . . . . . . . . .  8
   3.5.  Explicit Congestion Notification . . . . . . . . . . . . .  8
   3.6.  ICMP Handling for Messages Relating to DCCP-UDP  . . . . .  8
   3.7.  Path Maximum Transmission Unit Discovery . . . . . . . . .  9
   3.8.  Usage of the UDP Port by DCCP-UDP  . . . . . . . . . . . .  9
   3.9.  Service Codes and the DCCP Port Registry . . . . . . . . . 11
 4.  DCCP-UDP and Higher-Layer Protocols  . . . . . . . . . . . . . 11
   5.1.  Protocol Identification  . . . . . . . . . . . . . . . . . 12
   5.2.  Signalling Encapsulated DCCP Ports . . . . . . . . . . . . 13
   5.3.  Connection Management  . . . . . . . . . . . . . . . . . . 14
   5.4.  Negotiating the DCCP-UDP Encapsulation versus Native
         DCCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.5.  Example of SDP Use . . . . . . . . . . . . . . . . . . . . 15
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   7.1.  UDP Port Allocation  . . . . . . . . . . . . . . . . . . . 17
   7.2.  DCCP Reset . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.3.  SDP Attribute Allocation . . . . . . . . . . . . . . . . . 17
 8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 18

Phelan, et al. Standards Track [Page 2] RFC 6773 DCCP-UDP Encapsulation November 2012

1. Introduction

 The Datagram Congestion Control Protocol (DCCP) [RFC4340] is a
 transport-layer protocol that provides upper layers with the ability
 to use non-reliable congestion-controlled flows.  The current
 specification for DCCP [RFC4340] specifies a direct native
 encapsulation in IPv4 or IPv6 packets.
 DCCP support has been specified for devices that use Network Address
 Translation (NAT) or Network Address and Port Translation (NAPT)
 [RFC5597].  However, there is a significant installed base of NAT/
 NAPT devices that do not support [RFC5597].  It is therefore useful
 to have an encapsulation for DCCP that is compatible with this
 installed base of NAT/NAPT devices that support [RFC4787] but do not
 support [RFC5597].  This document specifies that encapsulation, which
 is referred to as DCCP-UDP.  For convenience, the standard
 encapsulation for DCCP [RFC4340] (including [RFC5596] as required) is
 referred to as DCCP-STD.
 The encapsulation described in this document may also be used as a
 transition mechanism to enable support for DCCP in devices that
 support UDP but do not yet natively support DCCP.  This also allows
 the DCCP transport to be implemented within an application using
 DCCP-UDP.
 This document also updates the SDP specification for DCCP [RFC5762]
 to convey the encapsulation type.  In this respect only, it updates
 the method in [RFC5762].
 The DCCP-UDP encapsulation specified in this document supports all of
 the features contained in DCCP-STD, but with limited functionality
 for partial checksums.
 Network optimisations for DCCP-STP and UDP may need to be updated to
 allow these optimisations to take advantage of DCCP-UDP.
 Encapsulation with an additional UDP protocol header can complicate
 or prevent inspection of DCCP header fields by equipment along the
 network path in the case where multiple DCCP connections share the
 same UDP 4-tuple, for example, routers that wish to identify DCCP
 ports to perform Equal-Cost Multi-Path (ECMP) routing, network
 devices that wish to inspect DCCP ports to inform algorithms for
 sharing the network load across multiple links, firewalls that wish
 to inspect DCCP ports and service codes to inform algorithms that
 implement access rules, media gateways that inspect SDP information
 to derive characteristics of the transport and session, etc.

Phelan, et al. Standards Track [Page 3] RFC 6773 DCCP-UDP Encapsulation November 2012

2. Terminology

 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. DCCP-UDP

 The basic approach is to insert a UDP [RFC0768] header between the IP
 header and the DCCP packet.  Note that this is not a tunneling
 approach.  The IP addresses of the communicating end systems are
 carried in the IP header.  The method does not embed additional IP
 addresses.
 The method is designed to support use when these addresses are
 modified by a device that implements NAT/NAPT.  A NAT translates the
 IP addresses, which impacts the transport-layer checksum.  A NAPT
 device may also translate the port values (usually the source port).
 In both cases, the outer transport header that includes these values
 would need to be updated by the NAT/NAPT.
 A device offering or using DCCP services via DCCP-UDP encapsulation
 listens on a UDP port (default port, 6511) or may bind to a specified
 port utilising out-of-band signalling, such as the Session
 Description Protocol (SDP).  The DCCP-UDP server accepts incoming
 packets over the UDP transport and passes the received packets to the
 DCCP protocol module, after removing the UDP encapsulation.
 A DCCP implementation endpoint may simultaneously provide services
 over any or all combinations of DCCP-STD and/or DCCP-UDP
 encapsulations with IPv4 and/or IPv6.
 The basic format of a DCCP-UDP packet is:
  +-----------------------------------+
  |     IP Header (IPv4 or IPv6)      |  Variable length
  +-----------------------------------+
  |            UDP Header             |  8 bytes
  +-----------------------------------+
  |       DCCP Generic Header         |  12 or 16 bytes
  +-----------------------------------+
  | Additional (type-specific) Fields |  Variable length (could be 0)
  +-----------------------------------+
  |           DCCP Options            |  Variable length (could be 0)
  +-----------------------------------+
  |      Application Data Area        |  Variable length (could be 0)
  +-----------------------------------+

Phelan, et al. Standards Track [Page 4] RFC 6773 DCCP-UDP Encapsulation November 2012

 Section 3.8 describes usage of UDP ports.  This includes
 implementation of a DCCP-UDP encapsulation service as a daemon that
 listens on a well-known port, allowing multiplexing of different DCCP
 applications over the same port.

3.1. The UDP Header

 The format of the UDP header is specified in [RFC0768]:
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Source Port          |           Dest Port           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Length            |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 For DCCP-UDP, the fields are interpreted as follows:
 Source and Dest(ination) Ports: 16 bits each
    These fields identify the UDP ports on which the source and
    destination (respectively) of the packet are listening for
    incoming DCCP-UDP packets.  The UDP port values do not identify
    the DCCP source and destination ports.
 Length: 16 bits
    This field is the length of the UDP datagram, including the UDP
    header and the payload (for DCCP-UDP, the payload is a DCCP-UDP
    datagram).
 Checksum: 16 bits
    This field is the Internet checksum of a network-layer
    pseudoheader and Length bytes of the UDP packet [RFC0768].  The
    UDP checksum MUST NOT be zero for a UDP packet that carries DCCP-
    UDP.

3.2. The DCCP Generic Header

 The DCCP Generic Header [RFC4340] takes two forms, one with long
 sequence numbers (48 bits) and the other with short sequence numbers
 (24 bits).

Phelan, et al. Standards Track [Page 5] RFC 6773 DCCP-UDP Encapsulation November 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Source Port          |           Dest Port           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Data Offset  | CCVal | CsCov |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     |       |X|               |                               .
    | Res | Type  |=|   Reserved    |  Sequence Number (high bits)  .
    |     |       |1|               |                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Sequence Number (low bits)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     The Generic DCCP Header with Long Sequence Numbers [RFC4340]
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Source Port          |           Dest Port           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Data Offset  | CCVal | CsCov |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     |       |X|                                               |
    | Res | Type  |=|   Sequence Number (low bits)                  |
    |     |       |0|                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     The Generic DCCP Header with Short Sequence Numbers [RFC4340]
 All generic header fields, except for the Checksum field, have the
 meaning specified in [RFC4340], updated by [RFC5596].
 Section 3.8 describes how a DCCP-UDP implementation treats UDP and
 DCCP ports.

3.3. DCCP-UDP Checksum Procedures

 DCCP-UDP employs a checksum at the UDP level and eliminates the use
 of the DCCP checksum.  This approach was chosen to enable use of
 current NAT/NATP traversal methods developed for UDP.  Such methods
 will generally be unaware whether DCCP is being encapsulated and
 hence do not update the inner checksum in the DCCP header.  Standard
 DCCP requires protection of the DCCP header fields; this justifies
 any processing overhead incurred from calculating the UDP checksum.

Phelan, et al. Standards Track [Page 6] RFC 6773 DCCP-UDP Encapsulation November 2012

 In addition, UDP NAT traversal does not support partial checksums.
 Although this is still permitted end-to-end in the encapsulated DCCP
 datagram, links along the path will treat these as UDP packets and
 can not enable special partial checksum processing.
 DCCP-UDP does not update or modify the operation of UDP.  The UDP
 transport protocol is used in the following way:
 For DCCP-UDP, the function of the DCCP Checksum field is performed by
 the UDP Checksum field.  On transmission, the DCCP Checksum field
 SHOULD be set to zero.  On receipt, the DCCP Checksum field MUST be
 ignored.
 The UDP checksum MUST NOT be zero for a UDP packet that is sent using
 DCCP-UDP.  If the received UDP Checksum field is zero, the packet
 MUST be dropped.
 If the UDP Length field of a received packet is less than 20 (the UDP
 header length and minimum DCCP-UDP header length), the packet MUST be
 dropped.
 If the UDP Checksum field, computed using standard UDP methods, is
 invalid, the received packet MUST be dropped.
 If the UDP Length field in a received packet is less than the length
 of the UDP header plus the entire DCCP-UDP header (including the
 generic header and type-specific fields and options, if present) or
 if the UDP Length field is greater than the length of the packet from
 the beginning of the UDP header to the end of the packet, the packet
 MUST be dropped.

3.3.1. Partial Checksums and the Minimum Checksum Coverage Feature

 This document requires the UDP checksum to be enabled when using
 DCCP-UDP.  This checksum provides coverage of the entire encapsulated
 DCCP datagram.
 DCCP-UDP supports the syntax of partial checksums.  It also supports
 negotiation of the Minimum Checksum Coverage feature and settings of
 the CsCov field.  However, the UDP Checksum field in DCCP-UDP always
 covers the entire DCCP datagram, and the DCCP checksum is ignored on
 receipt.  An application that enables the partial checksums feature
 in the DCCP module will therefore experience a service that is
 functionally identical to using full DCCP checksum coverage.  This is
 also the service that the application would have received if it had
 used a network path that did not provide optimised processing for
 DCCP partial checksums.

Phelan, et al. Standards Track [Page 7] RFC 6773 DCCP-UDP Encapsulation November 2012

3.4. Network-Layer Options

 A DCCP-UDP implementation MAY transfer network-layer options intended
 for DCCP to the network-layer header of the encapsulating UDP packet.
 A DCCP-UDP endpoint that receives IP-options for the encapsulating
 UDP packet MAY forward these to the DCCP protocol module.  If the
 endpoint forwards a specific network-layer option to the DCCP module,
 it MUST also forward all subsequent packets with this option.
 Consistent forwarding is essential for correct operation of many end-
 to-end options.

3.5. Explicit Congestion Notification

 A DCCP-UDP endpoint SHOULD follow the procedures of DCCP-STD in
 [RFC4340], Section 12 by setting the Explicit Congestion Notification
 (ECN) in the IP headers of outgoing packets and examining the values
 received in the ECN fields of incoming IP packets, relaying any
 packet markings to the DCCP module.
 Implementations that do not support ECN MUST follow the procedures of
 DCCP-STD in [RFC4340], Section 12.1 with regard to implementations
 that are not ECN capable.

3.6. ICMP Handling for Messages Relating to DCCP-UDP

 To allow ICMP messages to be demultiplexed by the receiving endpoint,
 part of the original packet that resulted in the message is included
 in the payload of the ICMP error message.  The receiving endpoint can
 therefore use this information to associate the ICMP error with the
 transport protocol instance that resulted in the ICMP message.  When
 DCCP-UDP is used, the error message and the payload of the ICMP error
 message relate to the UDP transport.
 DCCP-UDP endpoints SHOULD forward ICMP messages relating to a UDP
 packet that carries a DCCP-UDP to the DCCP module.  This may imply
 translation of the payload of the ICMP message into a form that is
 recognised by the DCCP stack.  [RFC5927] describes precautions that
 are desirable before TCP acts on the receipt of an ICMP message.
 Similar precautions are desirable prior to forwarding by DCCP-UDP to
 the DCCP module.
 The minimal length ICMP error message generated in response to
 processing a UDP datagram only identifies the UDP source port and UDP
 destination port.  This ICMP message does not carry sufficient
 information to discover the encapsulated DCCP Port values.  A DCCP-

Phelan, et al. Standards Track [Page 8] RFC 6773 DCCP-UDP Encapsulation November 2012

 UDP endpoint that supports multiple DCCP connections over the same
 pair of UDP ports (see Section 3.8) may not therefore be able to
 associate an ICMP message with a unique DCCP-UDP connection.

3.7. Path Maximum Transmission Unit Discovery

 DCCP-UDP implementations MUST follow DCCP-STD [RFC4340], Section 14
 with regard to determining the maximum packet size and the use of
 Path Maximum Transmission Unit Discovery (PMTUD).  This requires the
 processing of ICMP Destination Unreachable messages with a code that
 indicates that an unfragmentable packet was too large to be forwarded
 (a "Datagram Too Big" message), as defined in RFC 4340.
 An effect of encapsulation is to incur additional datagram overhead.
 This will reduce the Maximum Packet Size (MPS) at the DCCP level.

3.8. Usage of the UDP Port by DCCP-UDP

 A DCCP-UDP server (that is, an initially passive endpoint that wishes
 to receive DCCP-Request packets [RFC4340] over DCCP-UDP) listens for
 connections on one or more UDP ports.  UDP port number 6511 has been
 allocated as the default listening UDP port for a DCCP-UDP server.
 Some NAT/NAPT topologies may require using a non-default listening
 port.
 The purpose of this IANA-assigned port is for the operating system or
 a framework to receive and process DCCP-UDP datagrams for delivery to
 the DCCP module (e.g., to support a system-wide DCCP-UDP daemon
 serving multiple DCCP applications or a DCCP-UDP server placed behind
 a firewall).
 An application-specific implementation SHOULD use an ephemeral port
 and advertise this port using outside means, e.g., SDP.  This method
 of implementation SHOULD NOT use the IANA-assigned port to listen for
 incoming DCCP-UDP packets.
 A DCCP-UDP client provides UDP source and destination ports as well
 as DCCP source and destination ports at connection initiation time.
 A client SHOULD ensure that each DCCP connection maps to a single
 DCCP-UDP connection by setting the UDP source port.  Choosing a
 distinct UDP source port for each distinct DCCP connection ensures
 that UDP-based flow identifiers differ whenever DCCP-based flow
 identifiers differ.  Specifically, two connections with different
 <source IP address, source DCCP port, destination IP address,
 destination DCCP port> DCCP 4-tuples will have different <source IP
 address, source UDP port, destination IP address, destination UDP
 port> UDP 4-tuples.

Phelan, et al. Standards Track [Page 9] RFC 6773 DCCP-UDP Encapsulation November 2012

 A DCCP-UDP server SHOULD accept datagrams from any UDP source port.
 There is a risk that the same DCCP source port number could be used
 by two endpoints, each behind a NAPT.  A DCCP-UDP server MUST
 therefore demultiplex a DCCP-UDP flow using both the UDP source and
 destination port numbers and the encapsulated DCCP ports.  This
 ensures than an active DCCP connection is uniquely identified by the
 6-tuple <source IP address, source UDP port, source DCCP port,
 destination IP address, destination UDP port, destination DCCP port>.
 (The active state of a DCCP connection is defined in Section 3.8: a
 DCCP connection becomes active following transmission of a DCCP-
 Request and becomes inactive after sending a DCCP-Close.)
 This demultiplexing at a DCCP-UDP endpoint occurs in two stages:
 1.  In the first stage, DCCP-UDP packets are demultiplexed using the
     UDP 4-tuple: <source IP address, source UDP port, destination IP
     address, destination UDP port>.
 2.  In the second stage, a receiving endpoint MUST ensure that two
     independent DCCP connections that were multiplexed to the same
     UDP 4-tuple are not associated with the same connection in the
     DCCP module.  The endpoint therefore needs to keep state for the
     set of active DCCP-UDP endpoints using each combination of a UDP
     4-tuple: <source IP address, source UDP port, destination IP
     address, destination UDP port>.  Two DCCP endpoint methods are
     specified.  A DCCP-UDP implementation MUST implement exactly one
     of these:
  • The DCCP server may accept only one active 6-tuple at any one

time for a given UDP 4-tuple. In this method, DCCP-UDP

        packets that do not match an active 6-tuple MUST NOT be passed
        to the DCCP module and the DCCP Server SHOULD send a DCCP-
        Reset with Reset Code 12, "Encapsulated Port Reuse".  An
        endpoint that receives a DCCP-Reset with this reset code will
        clear its connection state but MAY immediately try again using
        a different 4-tuple.  This provides protection should the same
        UDP 4-tuple be re-used by multiple DCCP connections, ensuring
        that only one DCCP connection is established at one time.
  • The DCCP server may support multiple DCCP connections over the

same UDP 4-tuple. In this method, the endpoint MUST then

        associate each 6-tuple with a single DCCP connection.  If an
        endpoint is unable to demultiplex the 6-tuple (e.g., due to
        internal resource limits), it MUST discard DCCP-UDP packets
        that do not match an active 6-tuple instead of forwarding them
        to the DCCP module.  The DCCP endpoint MAY send a DCCP-Reset

Phelan, et al. Standards Track [Page 10] RFC 6773 DCCP-UDP Encapsulation November 2012

        with Reset Code 12, "Encapsulated Port Reuse", indicating the
        connection has been closed but may be retried using a
        different UDP 4-tuple.

3.9. Service Codes and the DCCP Port Registry

 This section clarifies the usage of DCCP Service Codes and the
 registration of server ports by DCCP-UDP.  The section is not
 intended to update the procedures for allocating Service Codes or
 server ports.
 There is one Service Code registry and one DCCP port registration
 that apply to all combinations of encapsulation and IP version.  A
 DCCP Service Code specifies an application using DCCP regardless of
 the combination of DCCP encapsulation and IP version.  An application
 may choose not to support some combinations of encapsulation and IP
 version, but its Service Code will remain registered for those
 combinations, and the Service Code must not be used by other
 applications.  An application should not register different Service
 Codes for different combinations of encapsulation and IP version.
 [RFC5595] provides additional information about DCCP Service Codes.
 Similarly, a DCCP port registration is applicable to all combinations
 of encapsulation and IP version.  Again, an application may choose
 not to support some combinations of encapsulation and IP version on
 its registered DCCP port, although the port will remain registered
 for those combinations.  Applications should not register different
 DCCP ports just for the purpose of using different combinations of
 encapsulation.

4. DCCP-UDP and Higher-Layer Protocols

 The encapsulation of a higher-layer protocol within DCCP MUST be the
 same for both DCCP-STD and DCCP-UDP.  Encapsulation of Datagram
 Transport Layer Security (DTLS) over DCCP is defined in [RFC5238] and
 RTP over DCCP is defined in [RFC5762].  This document therefore does
 not update these encapsulations when using DCCP-UDP.

5. Signalling the Use of DCCP-UDP

 Applications often signal transport connection parameters through
 outside means, such as SDP.  Applications that define such methods
 for DCCP MUST define how the DCCP encapsulation is chosen and MUST
 allow either encapsulation to be signalled.  Where DCCP-STD and DCCP-
 UDP are both supported, DCCP-STD SHOULD be preferred.
 The Session Description Protocol (SDP) [RFC4566] and the offer/answer
 model [RFC3264] can be used to negotiate DCCP sessions, and [RFC5762]

Phelan, et al. Standards Track [Page 11] RFC 6773 DCCP-UDP Encapsulation November 2012

 defines SDP extensions for signalling the use of an RTP session
 running over DCCP connections.  However, since [RFC5762] predates
 this document, it does not define a mechanism for signalling that the
 DCCP-UDP encapsulation is to be used.  This section updates [RFC5762]
 to describe how SDP can be used to signal RTP sessions running over
 the DCCP-UDP encapsulation.
 The new SDP support specified in this section is expected to be
 useful when the offering party is on the public Internet or in the
 same private addressing realm as the answering party.  In this case,
 the DCCP-UDP server has a public address.  The client may either have
 a public address or be behind a NAT/NAPT.  This scenario has the
 potential to be an important use case.  Some other NAT/NAPT
 topologies may result in the advertised port being unreachable via
 the NAT/NAPT.

5.1. Protocol Identification

 SDP uses a media ("m=") line to convey details of the media format
 and transport protocol used.  The ABNF syntax [RFC5234] of a media
 line for DCCP is as follows (from [RFC4566]):
    media-field =         %x6d "=" media SP port ["/" integer]
                          SP proto 1*(SP fmt) CRLF
 The proto field denotes the transport protocol used for the media,
 while the port indicates the transport port to which the media is
 sent, following [RFC5762].  This document defines the following five
 values of the proto field to indicate media transported using DCCP-
 UDP encapsulation:
    UDP/DCCP
    UDP/DCCP/RTP/AVP
    UDP/DCCP/RTP/SAVP
    UDP/DCCP/RTP/AVPF
    UDP/DCCP/RTP/SAVPF
 The "UDP/DCCP" protocol identifier is similar to the "DCCP" protocol
 identifier defined in [RFC5762] and denotes the DCCP transport
 protocol encapsulated in UDP, but not its upper-layer protocol.
 The "UDP/DCCP/RTP/AVP" protocol identifier refers to RTP using the
 RTP Profile for Audio and Video Conferences with Minimal Control
 [RFC3551] running over the DCCP-UDP encapsulation.

Phelan, et al. Standards Track [Page 12] RFC 6773 DCCP-UDP Encapsulation November 2012

 The "UDP/DCCP/RTP/SAVP" protocol identifier refers to RTP using the
 Secure Real-time Transport Protocol [RFC3711] running over the DCCP-
 UDP encapsulation.
 The "UDP/DCCP/RTP/AVPF" protocol identifier refers to RTP using the
 Extended RTP Profile for RTCP-based Feedback [RFC4585] running over
 the DCCP-UDP encapsulation.
 The "UDP/DCCP/RTP/SAVPF" protocol identifier refers to RTP using the
 Extended Secure RTP Profile for RTCP-based Feedback [RFC5124] running
 over the DCCP-UDP encapsulation.
 The fmt value in the "m=" line is used as described in [RFC5762].
 The port number specified in the "m=" line indicates the UDP port
 that is used for the DCCP-UDP encapsulation service.  The DCCP port
 number MUST be sent using an associated "a=dccp-port:" attribute, as
 described in Section 5.2.
 The use of ports with DCCP-UDP encapsulation is described further in
 Section 3.8.

5.2. Signalling Encapsulated DCCP Ports

 When using DCCP-UDP, the UDP port used for the encapsulation is
 signalled using the SDP "m=" line.  The DCCP ports MUST NOT be
 included in the "m=" line but are instead signalled using a new SDP
 attribute ("dccp-port") defined according to the following ABNF:
        dccp-port-attr = %x61 "=dccp-port:" dccp-port
        dccp-port = 1*DIGIT
 where DIGIT is as defined in [RFC5234].  This is a media-level
 attribute that is not subject to the charset attribute.  The
 "a=dccp-port:" attribute MUST be included when the protocol
 identifiers described in Section 5.1 are used.
 The use of ports with DCCP-UDP encapsulation is described further in
 Section 3.8.
 o  If the "a=rtcp:" attribute [RFC3605] is used, then the signalled
    port is the DCCP port used for RTCP.
 o  If the "a=rtcp-mux" attribute [RFC5761] is negotiated, then RTP
    and RTCP are multiplexed onto a single DCCP port; otherwise,
    separate DCCP ports are used for RTP and RTCP [RFC5762].

Phelan, et al. Standards Track [Page 13] RFC 6773 DCCP-UDP Encapsulation November 2012

    NOTE: In each case, only a single UDP port is used for the DCCP-
    UDP encapsulation.
 o  If the "a=rtcp-mux" attribute is not present, then the second of
    the two demultiplexing methods described in Section 3.8 MUST be
    implemented; otherwise, the second DCCP connection for the RTCP
    flow will be rejected.  For this reason, using "a=rtcp-mux" is
    RECOMMENDED when using RTP over DCCP-UDP.

5.3. Connection Management

 The "a=setup:" attribute is used in a manner compatible with
 [RFC5762], Section 5.3 to indicate which of the DCCP-UDP endpoints
 should initiate the DCCP-UDP connection establishment.

5.4. Negotiating the DCCP-UDP Encapsulation versus Native DCCP

 An endpoint that supports both native DCCP and the DCCP-UDP
 encapsulation may wish to signal support for both options in an SDP
 offer, allowing the answering party the option of using native DCCP
 where possible, while falling back to the DCCP-UDP encapsulation
 otherwise.
 An approach to doing this might be to include candidates for the
 DCCP-UDP encapsulation and native DCCP into an Interactive
 Connectivity Establishment (ICE) [RFC5245] exchange.  Since DCCP is
 connection-oriented, these candidates would need to be encoded into
 ICE in a manner analogous to TCP candidates defined in [RFC6544].
 Both active and passive candidates could be supported for native DCCP
 and DCCP-UDP encapsulation, as may DCCP simultaneous-open candidates
 [RFC5596].  In choosing local preference values, it may make sense to
 prefer DCCP-UDP over native DCCP in cases where low connection setup
 time is important and to prioritise native DCCP in cases where low
 overhead is preferred (on the assumption that DCCP-UDP is more likely
 to work through legacy NAT but has higher overhead).  The details of
 this encoding into ICE are left for future study.
 While ICE is appropriate for selecting basic use of DCCP-UDP versus
 DCCP-STD, it may not be appropriate for negotiating different RTP
 profiles with each transport encapsulation.  The SDP Capability
 Negotiation framework [RFC5939] may be more suitable.  Section 3.7 of
 RFC 5939 specifies how to provide attributes and transport protocols
 as capabilities and negotiate them using the framework.  The details
 of the use of SDP Capability Negotiation with DCCP are left for
 future study.

Phelan, et al. Standards Track [Page 14] RFC 6773 DCCP-UDP Encapsulation November 2012

5.5. Example of SDP Use

 The example below shows an SDP offer, where an application signals
 support for DCCP-UDP:
        v=0
        o=alice 1129377363 1 IN IP4 192.0.2.47
        s=-
        c=IN IP4 192.0.2.47
        t=0 0
        m=video 50234 UDP/DCCP/RTP/AVP 99
        a=rtpmap:99 h261/90000
        a=dccp-service-code:SC=x52545056
        a=dccp-port:5004
        a=rtcp:5005
        a=setup:passive
        a=connection:new
 The answering party at 192.0.2.128 receives this offer and responds
 with the following answer:
        v=0
        o=bob 1129377364 1 IN IP4 192.0.2.128
        s=-
        c=IN IP4 192.0.2.128
        t=0 0
        m=video 40123 UDP/DCCP/RTP/AVP 99
        a=rtpmap:99 h261/90000
        a=dccp-service-code:SC:RTPV
        a=dccp-port:9
        a=setup:active
        a=connection:new
 Note that the "m=" line in the answer includes the UDP port number of
 the encapsulation service.  The DCCP service code is set to "RTPV",
 signalled using the "a=dccp-service-code" attribute [RFC5762].  The
 "a=dccp-port:" attribute in the answer is set to 9 (the discard port)
 in the usual manner for an active connection-oriented endpoint.
 The answering party will then attempt to establish a DCCP-UDP
 connection to the offering party.  The connection request will use an
 ephemeral DCCP source port and DCCP destination port 5004.  The UDP
 packet encapsulating that request will have UDP source port 40123 and
 UDP destination port 50234.

Phelan, et al. Standards Track [Page 15] RFC 6773 DCCP-UDP Encapsulation November 2012

6. Security Considerations

 DCCP-UDP provides all of the security risk-mitigation measures
 present in DCCP-STD and also all of the security risks.  It does not
 maintain additional state at the encapsulation layer.
 The tunnel encapsulation recommends processing of ICMP messages
 received for packets sent using DCCP-UDP and translation to allow use
 by DCCP.  [RFC5927] describes precautions that are desirable before
 TCP acts on receipt of ICMP messages.  Similar precautions are
 desirable for endpoints processing ICMP for DCCP-UDP.  The purpose of
 DCCP-UDP is to allow DCCP to pass through NAT/NAPT devices;
 therefore, it exposes DCCP to the risks associated with passing
 through NAT devices.  It does not create any new risks with regard to
 NAT/NAPT devices.
 DCCP-UDP may also allow DCCP applications to pass through existing
 firewall devices using rules for UDP, if the administrators of the
 devices so choose.  A simple use may either allow all DCCP
 applications or allow none.
 A firewall that interprets this specification could inspect the
 encapsulated DCCP header to filter based on the inner DCCP header
 information.  Full control of DCCP connections by applications will
 require enhancements to firewalls, as discussed in [RFC4340] and
 related RFCs (e.g., [RFC5595]).
 Datagram Transport Layer Security (DTLS) provides mechanisms that can
 be used to provide security protection for the encapsulated DCCP
 packets.  DTLS may be used in two ways:
 o  Individual DCCP connections may be protected in the same way that
    DTLS is used with native DCCP [RFC5595].  This does not encrypt
    the UDP transport header added by DCCP-UDP.
 o  This specification also permits the use of DTLS with the UDP
    transport that encapsulates DCCP packets.  When DTLS is used at
    the encapsulation layer, this protects the DCCP headers.  This
    prevents the headers from being inspected or updated by network
    middleboxes (such as firewalls and NAPT).  It also eliminates the
    need for a separate DTLS handshake for each DCCP connection.

Phelan, et al. Standards Track [Page 16] RFC 6773 DCCP-UDP Encapsulation November 2012

7. IANA Considerations

 IANA has made the allocations described in the following sections.

7.1. UDP Port Allocation

 IANA has allocated a UDP port (6511) for the DCCP-UDP service.  This
 port is allocated for use by a transport service rather than an
 application.  In this case, the name of the transport should
 explicitly appear in the registry.  Use of this port is defined in
 Section 3.8

7.2. DCCP Reset

 IANA has assigned a new DCCP reset code (12) in the DCCP Reset Codes
 Registry, with the short description "Encapsulated Port Reuse".  This
 code applies to all DCCP congestion control IDs.  Use of this reset
 code is defined in Section 3.8.  Section 5.6 of [RFC4340] defines
 three "Data" bytes that are carried by a DCCP Reset.  For this reset
 code, these are defined as follows:
 o  Data byte 1: The DCCP Packet Type of the DCCP datagram that
    resulted in the error message.
 o  Data bytes 2 & 3: The encapsulated UDP source port from the DCCP-
    UDP datagram that triggered the ICMP message, in network order.

7.3. SDP Attribute Allocation

 IANA has allocated the following new SDP attribute ("att-field"):
    Contact name: DCCP Working Group
    Attribute name: dccp-port
    Long-form attribute name in English: Encapsulated DCCP Port
    Type of attribute: Media level only
    Subject to charset attribute?  No
    Purpose of the attribute: See this document, Section 5.1
    Allowed attribute values: See this document, Section 5.1

Phelan, et al. Standards Track [Page 17] RFC 6773 DCCP-UDP Encapsulation November 2012

8. Acknowledgments

 This document was produced by the DCCP WG.  The following individuals
 contributed during the working group last call: Andrew Lentvorski,
 Lloyd Wood, Pasi Sarolahti, Gerrit Renker, Eddie Kohler, and Dan
 Wing.

9. References

9.1. Normative References

 [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            August 1980.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
            in Session Description Protocol (SDP)", RFC 3605,
            October 2003.
 [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
            Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
            Protocol (DCCP)", RFC 5762, April 2010.

9.2. Informative References

 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            June 2002.
 [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
            Video Conferences with Minimal Control", STD 65, RFC 3551,
            July 2003.
 [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
            Norrman, "The Secure Real-time Transport Protocol (SRTP)",
            RFC 3711, March 2004.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.

Phelan, et al. Standards Track [Page 18] RFC 6773 DCCP-UDP Encapsulation November 2012

 [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
            "Extended RTP Profile for Real-time Transport Control
            Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
            July 2006.
 [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
            (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
            RFC 4787, January 2007.
 [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
            Real-time Transport Control Protocol (RTCP)-Based Feedback
            (RTP/SAVPF)", RFC 5124, February 2008.
 [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
            the Datagram Congestion Control Protocol (DCCP)",
            RFC 5238, May 2008.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245,
            April 2010.
 [RFC5595]  Fairhurst, G., "The Datagram Congestion Control Protocol
            (DCCP) Service Codes", RFC 5595, September 2009.
 [RFC5596]  Fairhurst, G., "Datagram Congestion Control Protocol
            (DCCP) Simultaneous-Open Technique to Facilitate NAT/
            Middlebox Traversal", RFC 5596, September 2009.
 [RFC5597]  Denis-Courmont, R., "Network Address Translation (NAT)
            Behavioral Requirements for the Datagram Congestion
            Control Protocol", BCP 150, RFC 5597, September 2009.
 [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
            Control Packets on a Single Port", RFC 5761, April 2010.
 [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
 [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
            Capability Negotiation", RFC 5939, September 2010.
 [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. B. Roach,
            "TCP Candidates with Interactive Connectivity
            Establishment (ICE)", RFC 6544, March 2012.

Phelan, et al. Standards Track [Page 19] RFC 6773 DCCP-UDP Encapsulation November 2012

Authors' Addresses

 Tom Phelan
 Sonus Networks
 7 Technology Dr.
 Westford, MA  01886
 US
 Phone: +1 978 614 8456
 EMail: tphelan@sonusnet.com
 Godred Fairhurst
 University of Aberdeen
 School of Engineering
 Fraser Noble Building
 Aberdeen, Scotland  AB24 3UE
 UK
 EMail: gorry@erg.abdn.ac.uk
 URI:   http://www.erg.abdn.ac.uk
 Colin Perkins
 University of Glasgow
 School of Computing Science
 Glasgow, Scotland  G12 8QQ
 UK
 EMail: csp@csperkins.org
 URI:   http://csperkins.org/

Phelan, et al. Standards Track [Page 20]

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