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IETF RFC 6773
Last modified on Wednesday, November 14th, 2012
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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/RFC 6773.
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) [RFC 4340] is a
transport-layer protocol that provides upper layers with the ability
to use non-reliable congestion-controlled flows. The current
specification for DCCP [RFC 4340] 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)
[RFC 5597]. However, there is a significant installed base of NAT/
NAPT devices that do not support [RFC 5597]. It is therefore useful
to have an encapsulation for DCCP that is compatible with this
installed base of NAT/NAPT devices that support [RFC 4787] but do not
support [RFC 5597]. This document specifies that encapsulation, which
is referred to as DCCP-UDP. For convenience, the standard
encapsulation for DCCP [RFC 4340] (including [RFC 5596] 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 [RFC 5762]
to convey the encapsulation type. In this respect only, it updates
the method in [RFC 5762].
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 [RFC 2119].
3. DCCP-UDP
The basic approach is to insert a UDP [RFC 768] 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 [RFC 768]:
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 [RFC 768]. 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 [RFC 4340] 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 [RFC 4340]
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 [RFC 4340]
All generic header fields, except for the Checksum field, have the
meaning specified in [RFC 4340], updated by [RFC 5596].
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
[RFC 4340], 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 [RFC 4340], 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. [RFC 5927] 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 [RFC 4340], 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 [RFC 4340] 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.
[RFC 5595] 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 [RFC 5238] and
RTP over DCCP is defined in [RFC 5762]. 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) [RFC 4566] and the offer/answer
model [RFC 3264] can be used to negotiate DCCP sessions, and [RFC 5762]
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 [RFC 5762] predates
this document, it does not define a mechanism for signalling that the
DCCP-UDP encapsulation is to be used. This section updates [RFC 5762]
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 [RFC 5234] of a media
line for DCCP is as follows (from [RFC 4566]):
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 [RFC 5762]. 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 [RFC 5762] 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
[RFC 3551] 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 [RFC 3711] 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 [RFC 4585] 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 [RFC 5124] running
over the DCCP-UDP encapsulation.
The fmt value in the "m=" line is used as described in [RFC 5762].
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 [RFC 5234]. 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 [RFC 3605] is used, then the signalled
port is the DCCP port used for RTCP.
o If the "a=rtcp-mux" attribute [RFC 5761] is negotiated, then RTP
and RTCP are multiplexed onto a single DCCP port; otherwise,
separate DCCP ports are used for RTP and RTCP [RFC 5762].
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
[RFC 5762], 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) [RFC 5245] exchange. Since DCCP is
connection-oriented, these candidates would need to be encoded into
ICE in a manner analogous to TCP candidates defined in [RFC 6544].
Both active and passive candidates could be supported for native DCCP
and DCCP-UDP encapsulation, as may DCCP simultaneous-open candidates
[RFC 5596]. 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 [RFC 5939] 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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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 [RFC 5762]. 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. [RFC 5927] 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 [RFC 4340] and
related RFCs (e.g., [RFC 5595]).
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 [RFC 5595]. 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 [RFC 4340] 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
[RFC 768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
October 2003.
[RFC 4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 5762] Perkins, C., "RTP and the Datagram Congestion Control
Protocol (DCCP)", RFC 5762, April 2010.
9.2. Informative References
[RFC 3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC 3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC 4566] 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
[RFC 4585] 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.
[RFC 4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC 5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC 5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, May 2008.
[RFC 5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC 5595] Fairhurst, G., "The Datagram Congestion Control Protocol
(DCCP) Service Codes", RFC 5595, September 2009.
[RFC 5596] Fairhurst, G., "Datagram Congestion Control Protocol
(DCCP) Simultaneous-Open Technique to Facilitate NAT/
Middlebox Traversal", RFC 5596, September 2009.
[RFC 5597] Denis-Courmont, R., "Network Address Translation (NAT)
Behavioral Requirements for the Datagram Congestion
Control Protocol", BCP 150, RFC 5597, September 2009.
[RFC 5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC 5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC 5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.
[RFC 6544] 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
RFC TOTAL SIZE: 45073 bytes
PUBLICATION DATE: Wednesday, November 14th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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