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Internet Engineering Task Force (IETF)                         T. PhelanRequest for Comments: 6773                                         SonusUpdates:4340,5762                                         G. FairhurstCategory: Standards Track                         University of AberdeenISSN: 2070-1721                                               C. Perkins                                                   University of Glasgow                                                           November 2012DCCP-UDP: A Datagram Congestion Control Protocol UDP Encapsulation forNAT TraversalAbstract   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 inRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6773.Phelan, et al.               Standards Track                    [Page 1]

RFC 6773                 DCCP-UDP Encapsulation            November 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .43.  DCCP-UDP . . . . . . . . . . . . . . . . . . . . . . . . . . .43.1.  The UDP Header . . . . . . . . . . . . . . . . . . . . . .53.2.  The DCCP Generic Header  . . . . . . . . . . . . . . . . .53.3.  DCCP-UDP Checksum Procedures . . . . . . . . . . . . . . .6       3.3.1.  Partial Checksums and the Minimum Checksum               Coverage Feature . . . . . . . . . . . . . . . . . . .73.4.  Network-Layer Options  . . . . . . . . . . . . . . . . . .83.5.  Explicit Congestion Notification . . . . . . . . . . . . .83.6.  ICMP Handling for Messages Relating to DCCP-UDP  . . . . .83.7.  Path Maximum Transmission Unit Discovery . . . . . . . . .93.8.  Usage of the UDP Port by DCCP-UDP  . . . . . . . . . . . .93.9.  Service Codes and the DCCP Port Registry . . . . . . . . .114.  DCCP-UDP and Higher-Layer Protocols  . . . . . . . . . . . . .115.1.  Protocol Identification  . . . . . . . . . . . . . . . . .125.2.  Signalling Encapsulated DCCP Ports . . . . . . . . . . . .135.3.  Connection Management  . . . . . . . . . . . . . . . . . .14     5.4.  Negotiating the DCCP-UDP Encapsulation versus Native           DCCP . . . . . . . . . . . . . . . . . . . . . . . . . . .145.5.  Example of SDP Use . . . . . . . . . . . . . . . . . . . .156.  Security Considerations  . . . . . . . . . . . . . . . . . . .167.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .177.1.  UDP Port Allocation  . . . . . . . . . . . . . . . . . . .177.2.  DCCP Reset . . . . . . . . . . . . . . . . . . . . . . . .177.3.  SDP Attribute Allocation . . . . . . . . . . . . . . . . .178.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .189.  References . . . . . . . . . . . . . . . . . . . . . . . . . .189.1.  Normative References . . . . . . . . . . . . . . . . . . .189.2.  Informative References . . . . . . . . . . . . . . . . . .18Phelan, et al.               Standards Track                    [Page 2]

RFC 6773                 DCCP-UDP Encapsulation            November 20121.  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 20122.  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 2012Section 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 20123.4.  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 (seeSection 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 inRFC 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 inSection 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-ResetPhelan, 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 inSection 5.2.   The use of ports with DCCP-UDP encapsulation is described further inSection 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 inSection 5.1 are used.   The use of ports with DCCP-UDP encapsulation is described further inSection 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 inSection 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 20125.5.  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 20126.  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 20127.  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 inSection 3.87.2.  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 inSection 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 5.1Phelan, et al.               Standards Track                   [Page 17]

RFC 6773                 DCCP-UDP Encapsulation            November 20128.  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.  References9.1.  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 2012Authors' 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]