In
computer networking, the Datagram Congestion Control Protocol (DCCP) is a message-oriented
transport layer protocol. DCCP implements reliable connection setup, teardown,
Explicit Congestion Notification (ECN),
congestion control, and feature negotiation. The
IETF published DCCP as , a
proposed standard, in March 2006. provides an introduction.
Operation
DCCP provides a way to gain access to congestion-control mechanisms without having to implement them at the
application layer
An application layer is an abstraction layer that specifies the shared communications protocols and Interface (computing), interface methods used by Host (network), hosts in a communications network. An ''application layer'' abstraction is speci ...
. It allows for flow-based semantics like in
Transmission Control Protocol (TCP), but does not provide reliable in-order delivery. Sequenced delivery within multiple streams as in the
Stream Control Transmission Protocol
The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the p ...
(SCTP) is not available in DCCP. A DCCP connection contains
acknowledgment traffic as well as data traffic. Acknowledgments inform a sender whether its packets have arrived, and whether they were marked by
Explicit Congestion Notification (ECN). Acknowledgements are transmitted as reliably as the congestion control mechanism in use requires, possibly completely reliably.
DCCP has the option for very long (48-bit) sequence numbers corresponding to a packet ID, rather than a byte ID as in TCP. The long length of the sequence numbers aims to guard against "some blind attacks, such as the injection of DCCP-Resets into the connection".
Applications
DCCP is useful for applications with timing constraints on the delivery of data. Such applications include
streaming media
Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content it ...
,
multiplayer online games and
Internet telephony. In such applications, old messages quickly become useless, so that getting new messages is preferred to resending lost messages. such applications have often either settled for TCP or used
User Datagram Protocol (UDP) and implemented their own congestion-control mechanisms, or have no congestion control at all. While being useful for these applications, DCCP can also serve as a general congestion-control mechanism for UDP-based applications, by adding, as needed, mechanisms for reliable or in-order delivery on top of UDP/DCCP. In this context, DCCP allows the use of different, but generally
TCP-friendly congestion-control mechanisms.
Implementations
The following operating systems implement DCCP:
*
FreeBSD
FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
, version 5.1 as patch
*
Linux since version 2.6.14
Userspace library:
DCCP-TP implementation is optimized for portability, but has had no changes since June 2008.
GoDCCPpurpose of this implementation is to provide a standardized, portable NAT-friendly framework for peer-to-peer communications with flexible congestion control, depending on application.
Packet Structure
The DCCP generic header takes different forms depending on the value of X, the Extended Sequence Numbers bit. If X is one, the Sequence Number field is 48 bits long, and the generic header takes 16 bytes, as follows.
If X is zero, only the low 24 bits of the Sequence Number are transmitted, and the generic header is 12 bytes long.
;Source port (16 bits):Identifies the sending port
;Destination port (16 bits):Identifies the receiving port
;Data Offset: (8 bits): The offset from the start of the packet's DCCP header to the start of its application data area, in 32-bit words.
;CCVal (4 bits):Used by the HC-Sender CCID
;Checksum Coverage (CsCov) (4 bits): Checksum Coverage determines the parts of the packet that are covered by the Checksum field.
;Checksum (16 bits): The Internet checksum of the packet's DCCP header (including options), a network-layer pseudoheader, and, depending on Checksum Coverage, all, some, or none of the application data
;Reserved (Res) (3 bits): Senders MUST set this field to all zeroes on generated packets, and receivers MUST ignore its value
;Type (4 bits): The Type field specifies the type of the packet
;Extended Sequence Numbers (X) (1 bit): Set to one to indicate the use of an extended generic header with 48-bit Sequence and Acknowledgement Numbers
;Sequence Number (48 or 24 bits): Identifies the packet uniquely in the sequence of all packets the source sent on this connection
Current development
Similarly to the extension of
TCP
TCP may refer to:
Science and technology
* Transformer coupled plasma
* Tool Center Point, see Robot end effector
Computing
* Transmission Control Protocol, a fundamental Internet standard
* Telephony control protocol, a Bluetooth communication s ...
protocol by multipath capability (
MPTCP) also for DCCP the multipath feature is under discussion at IETF correspondingly denoted as
MP-DCCP. First implementations have already been developed, tested, and presented in a collaborative approach between operators and academia
and are available as an open source solution.
See also
*
Stream Control Transmission Protocol
The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the p ...
(SCTP)
*
References
External links
IETF Datagram Congestion Control Protocol (dccp) Charter
Protocol Specifications
* — Datagram Congestion Control Protocol
* — The Datagram Congestion Control Protocol (DCCP) Service Codes
* — DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal
* — RTP and the DCCP
* — Datagram Transport Layer Security (DTLS) over DCCP
* — Quick-Start for DCCP
* — A Datagram Congestion Control Protocol UDP Encapsulation for NAT Traversal
Congestion Control IDs
* — Profile for DCCP Congestion Control ID 2: TCP-like Congestion Control
* — Profile for DCCP Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
* — Profile for DCCP Congestion Control ID 4: TCP-Friendly Rate Control for Small Packets (TFRC-SP)
Other Information
* {{IETF RFC, 4336, link=no — Problem Statement for the Datagram Congestion Control Protocol (DCCP)
DCCP page from one of DCCP authorsDCCP support in LinuxDatagram Congestion Control Protocol (DCCP)
Transport layer protocols