path: root/doc/rfc/rfc6773.txt

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.
















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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




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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.






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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)
    +-----------------------------------+




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   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).







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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          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.





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   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.




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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-





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   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.




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   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





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          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]



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   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.



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   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].




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      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.






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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.







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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.










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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







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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.





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   [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.







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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/




















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