16 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
17 based protocol designed to solve issues present in UDP and TCP particularly
18 for real time and multimedia traffic.
20 It has a base protocol and pluggable congestion control IDs (CCIDs).
22 It is at proposed standard RFC status and the homepage for DCCP as a protocol
24 http://www.read.cs.ucla.edu/dccp/
29 The DCCP implementation does not currently have all the features that are in
32 The known bugs are at:
33 http://linux-net.osdl.org/index.php/TODO#DCCP
38 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
39 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
40 the socket will fall back to 0 (which means that no meaningful service code
41 is present). On active sockets this is set before connect(); specifying more
42 than one code has no effect (all subsequent service codes are ignored). The
43 case is different for passive sockets, where multiple service codes (up to 32)
44 can be set before calling bind().
46 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
47 size (application payload size) in bytes, see RFC 4340, section 14.
49 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
50 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
51 always cover the entire packet and that only fully covered application data is
52 accepted by the receiver. Hence, when using this feature on the sender, it must
53 be enabled at the receiver, too with suitable choice of CsCov.
55 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
56 range 0..15 are acceptable. The default setting is 0 (full coverage),
57 values between 1..15 indicate partial coverage.
58 DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it
59 sets a threshold, where again values 0..15 are acceptable. The default
60 of 0 means that all packets with a partial coverage will be discarded.
61 Values in the range 1..15 indicate that packets with minimally such a
62 coverage value are also acceptable. The higher the number, the more
63 restrictive this setting (see [RFC 4340, sec. 9.2.1]).
65 The following two options apply to CCID 3 exclusively and are getsockopt()-only.
66 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
67 DCCP_SOCKOPT_CCID_RX_INFO
68 Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
69 optlen must be set to at least sizeof(struct tfrc_rx_info).
70 DCCP_SOCKOPT_CCID_TX_INFO
71 Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
72 optlen must be set to at least sizeof(struct tfrc_tx_info).
77 Several DCCP default parameters can be managed by the following sysctls
78 (sysctl net.dccp.default or /proc/sys/net/dccp/default):
81 The number of active connection initiation retries (the number of
82 Requests minus one) before timing out. In addition, it also governs
83 the behaviour of the other, passive side: this variable also sets
84 the number of times DCCP repeats sending a Response when the initial
85 handshake does not progress from RESPOND to OPEN (i.e. when no Ack
86 is received after the initial Request). This value should be greater
87 than 0, suggested is less than 10. Analogue of tcp_syn_retries.
90 How often a DCCP Response is retransmitted until the listening DCCP
91 side considers its connecting peer dead. Analogue of tcp_retries1.
94 The number of times a general DCCP packet is retransmitted. This has
95 importance for retransmitted acknowledgments and feature negotiation,
96 data packets are never retransmitted. Analogue of tcp_retries2.
99 Whether or not to send NDP count options (sec. 7.7.2).
102 Whether or not to send Ack Vector options (sec. 11.5).
105 The default Ack Ratio (sec. 11.3) to use.
108 Default CCID for the sender-receiver half-connection.
111 Default CCID for the receiver-sender half-connection.
114 The initial sequence window (sec. 7.5.2).
117 The size of the transmit buffer in packets. A value of 0 corresponds
118 to an unbounded transmit buffer.
120 sync_ratelimit = 125 ms
121 The timeout between subsequent DCCP-Sync packets sent in response to
122 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
123 of this parameter is milliseconds; a value of 0 disables rate-limiting.
128 DCCP does not travel through NAT successfully at present on many boxes. This is
129 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
130 support for DCCP has been added.