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 DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
23 is at http://www.ietf.org/html.charters/dccp-charter.html
28 The Linux DCCP implementation does not currently support all the features that are
29 specified in RFCs 4340...42.
31 The known bugs are at:
32 http://linux-net.osdl.org/index.php/TODO#DCCP
34 For more up-to-date versions of the DCCP implementation, please consider using
35 the experimental DCCP test tree; instructions for checking this out are on:
36 http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
42 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
43 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
44 the socket will fall back to 0 (which means that no meaningful service code
45 is present). On active sockets this is set before connect(); specifying more
46 than one code has no effect (all subsequent service codes are ignored). The
47 case is different for passive sockets, where multiple service codes (up to 32)
48 can be set before calling bind().
50 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
51 size (application payload size) in bytes, see RFC 4340, section 14.
53 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
54 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
55 always cover the entire packet and that only fully covered application data is
56 accepted by the receiver. Hence, when using this feature on the sender, it must
57 be enabled at the receiver, too with suitable choice of CsCov.
59 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
60 range 0..15 are acceptable. The default setting is 0 (full coverage),
61 values between 1..15 indicate partial coverage.
62 DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
63 sets a threshold, where again values 0..15 are acceptable. The default
64 of 0 means that all packets with a partial coverage will be discarded.
65 Values in the range 1..15 indicate that packets with minimally such a
66 coverage value are also acceptable. The higher the number, the more
67 restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
68 settings are inherited to the child socket after accept().
70 The following two options apply to CCID 3 exclusively and are getsockopt()-only.
71 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
72 DCCP_SOCKOPT_CCID_RX_INFO
73 Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
74 optlen must be set to at least sizeof(struct tfrc_rx_info).
75 DCCP_SOCKOPT_CCID_TX_INFO
76 Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
77 optlen must be set to at least sizeof(struct tfrc_tx_info).
79 On unidirectional connections it is useful to close the unused half-connection
80 via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
84 Several DCCP default parameters can be managed by the following sysctls
85 (sysctl net.dccp.default or /proc/sys/net/dccp/default):
88 The number of active connection initiation retries (the number of
89 Requests minus one) before timing out. In addition, it also governs
90 the behaviour of the other, passive side: this variable also sets
91 the number of times DCCP repeats sending a Response when the initial
92 handshake does not progress from RESPOND to OPEN (i.e. when no Ack
93 is received after the initial Request). This value should be greater
94 than 0, suggested is less than 10. Analogue of tcp_syn_retries.
97 How often a DCCP Response is retransmitted until the listening DCCP
98 side considers its connecting peer dead. Analogue of tcp_retries1.
101 The number of times a general DCCP packet is retransmitted. This has
102 importance for retransmitted acknowledgments and feature negotiation,
103 data packets are never retransmitted. Analogue of tcp_retries2.
106 Whether or not to send NDP count options (sec. 7.7.2).
109 Whether or not to send Ack Vector options (sec. 11.5).
112 The default Ack Ratio (sec. 11.3) to use.
115 Default CCID for the sender-receiver half-connection.
118 Default CCID for the receiver-sender half-connection.
121 The initial sequence window (sec. 7.5.2).
124 The size of the transmit buffer in packets. A value of 0 corresponds
125 to an unbounded transmit buffer.
127 sync_ratelimit = 125 ms
128 The timeout between subsequent DCCP-Sync packets sent in response to
129 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
130 of this parameter is milliseconds; a value of 0 disables rate-limiting.
135 DCCP does not travel through NAT successfully at present on many boxes. This is
136 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
137 support for DCCP has been added.