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+DCCP protocol
+============
+
+
+Contents
+========
+
+- Introduction
+- Missing features
+- Socket options
+- Notes
+
+Introduction
+============
+
+Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
+oriented protocol designed to solve issues present in UDP and TCP, particularly
+for real-time and multimedia (streaming) traffic.
+It divides into a base protocol (RFC 4340) and plugable congestion control
+modules called CCIDs. Like plugable TCP congestion control, at least one CCID
+needs to be enabled in order for the protocol to function properly. In the Linux
+implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
+the TCP-friendly CCID3 (RFC 4342), are optional.
+For a brief introduction to CCIDs and suggestions for choosing a CCID to match
+given applications, see section 10 of RFC 4340.
+
+It has a base protocol and pluggable congestion control IDs (CCIDs).
+
+DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
+is at http://www.ietf.org/html.charters/dccp-charter.html
+
+Missing features
+================
+
+The Linux DCCP implementation does not currently support all the features that are
+specified in RFCs 4340...42.
+
+The known bugs are at:
+ http://linux-net.osdl.org/index.php/TODO#DCCP
+
+For more up-to-date versions of the DCCP implementation, please consider using
+the experimental DCCP test tree; instructions for checking this out are on:
+http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
+
+
+Socket options
+==============
+
+DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
+service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
+the socket will fall back to 0 (which means that no meaningful service code
+is present). On active sockets this is set before connect(); specifying more
+than one code has no effect (all subsequent service codes are ignored). The
+case is different for passive sockets, where multiple service codes (up to 32)
+can be set before calling bind().
+
+DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
+size (application payload size) in bytes, see RFC 4340, section 14.
+
+DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
+timewait state when closing the connection (RFC 4340, 8.3). The usual case is
+that the closing server sends a CloseReq, whereupon the client holds timewait
+state. When this boolean socket option is on, the server sends a Close instead
+and will enter TIMEWAIT. This option must be set after accept() returns.
+
+DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
+partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
+always cover the entire packet and that only fully covered application data is
+accepted by the receiver. Hence, when using this feature on the sender, it must
+be enabled at the receiver, too with suitable choice of CsCov.
+
+DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
+ range 0..15 are acceptable. The default setting is 0 (full coverage),
+ values between 1..15 indicate partial coverage.
+DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
+ sets a threshold, where again values 0..15 are acceptable. The default
+ of 0 means that all packets with a partial coverage will be discarded.
+ Values in the range 1..15 indicate that packets with minimally such a
+ coverage value are also acceptable. The higher the number, the more
+ restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
+ settings are inherited to the child socket after accept().
+
+The following two options apply to CCID 3 exclusively and are getsockopt()-only.
+In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
+DCCP_SOCKOPT_CCID_RX_INFO
+ Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
+ optlen must be set to at least sizeof(struct tfrc_rx_info).
+DCCP_SOCKOPT_CCID_TX_INFO
+ Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
+ optlen must be set to at least sizeof(struct tfrc_tx_info).
+
+On unidirectional connections it is useful to close the unused half-connection
+via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
+
+Sysctl variables
+================
+Several DCCP default parameters can be managed by the following sysctls
+(sysctl net.dccp.default or /proc/sys/net/dccp/default):
+
+request_retries
+ The number of active connection initiation retries (the number of
+ Requests minus one) before timing out. In addition, it also governs
+ the behaviour of the other, passive side: this variable also sets
+ the number of times DCCP repeats sending a Response when the initial
+ handshake does not progress from RESPOND to OPEN (i.e. when no Ack
+ is received after the initial Request). This value should be greater
+ than 0, suggested is less than 10. Analogue of tcp_syn_retries.
+
+retries1
+ How often a DCCP Response is retransmitted until the listening DCCP
+ side considers its connecting peer dead. Analogue of tcp_retries1.
+
+retries2
+ The number of times a general DCCP packet is retransmitted. This has
+ importance for retransmitted acknowledgments and feature negotiation,
+ data packets are never retransmitted. Analogue of tcp_retries2.
+
+send_ndp = 1
+ Whether or not to send NDP count options (sec. 7.7.2).
+
+send_ackvec = 1
+ Whether or not to send Ack Vector options (sec. 11.5).
+
+ack_ratio = 2
+ The default Ack Ratio (sec. 11.3) to use.
+
+tx_ccid = 2
+ Default CCID for the sender-receiver half-connection.
+
+rx_ccid = 2
+ Default CCID for the receiver-sender half-connection.
+
+seq_window = 100
+ The initial sequence window (sec. 7.5.2).
+
+tx_qlen = 5
+ The size of the transmit buffer in packets. A value of 0 corresponds
+ to an unbounded transmit buffer.
+
+sync_ratelimit = 125 ms
+ The timeout between subsequent DCCP-Sync packets sent in response to
+ sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
+ of this parameter is milliseconds; a value of 0 disables rate-limiting.
+
+IOCTLS
+======
+FIONREAD
+ Works as in udp(7): returns in the `int' argument pointer the size of
+ the next pending datagram in bytes, or 0 when no datagram is pending.
+
+Notes
+=====
+
+DCCP does not travel through NAT successfully at present on many boxes. This is
+because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
+support for DCCP has been added.
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