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1DCCP protocol
2============
3
4
5Contents
6========
7
8- Introduction
9- Missing features
10- Socket options
11- Notes
12
13Introduction
14============
15
16Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
17based protocol designed to solve issues present in UDP and TCP particularly
18for real time and multimedia traffic.
19
20It has a base protocol and pluggable congestion control IDs (CCIDs).
21
22It is at proposed standard RFC status and the homepage for DCCP as a protocol
23is at:
24 http://www.read.cs.ucla.edu/dccp/
25
26Missing features
27================
28
29The DCCP implementation does not currently have all the features that are in
30the RFC.
31
32The known bugs are at:
33 http://linux-net.osdl.org/index.php/TODO#DCCP
34
35Socket options
36==============
37
38DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
39service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
40the socket will fall back to 0 (which means that no meaningful service code
41is present). Connecting sockets set at most one service option; for
42listening sockets, multiple service codes can be specified.
43
44DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
45partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
46always cover the entire packet and that only fully covered application data is
47accepted by the receiver. Hence, when using this feature on the sender, it must
48be enabled at the receiver, too with suitable choice of CsCov.
49
50DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
51 range 0..15 are acceptable. The default setting is 0 (full coverage),
52 values between 1..15 indicate partial coverage.
53DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it
54 sets a threshold, where again values 0..15 are acceptable. The default
55 of 0 means that all packets with a partial coverage will be discarded.
56 Values in the range 1..15 indicate that packets with minimally such a
57 coverage value are also acceptable. The higher the number, the more
58 restrictive this setting (see [RFC 4340, sec. 9.2.1]).
59
60The following two options apply to CCID 3 exclusively and are getsockopt()-only.
61In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
62DCCP_SOCKOPT_CCID_RX_INFO
63 Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
64 optlen must be set to at least sizeof(struct tfrc_rx_info).
65DCCP_SOCKOPT_CCID_TX_INFO
66 Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
67 optlen must be set to at least sizeof(struct tfrc_tx_info).
68
69
70Sysctl variables
71================
72Several DCCP default parameters can be managed by the following sysctls
73(sysctl net.dccp.default or /proc/sys/net/dccp/default):
74
75request_retries
76 The number of active connection initiation retries (the number of
77 Requests minus one) before timing out. In addition, it also governs
78 the behaviour of the other, passive side: this variable also sets
79 the number of times DCCP repeats sending a Response when the initial
80 handshake does not progress from RESPOND to OPEN (i.e. when no Ack
81 is received after the initial Request). This value should be greater
82 than 0, suggested is less than 10. Analogue of tcp_syn_retries.
83
84retries1
85 How often a DCCP Response is retransmitted until the listening DCCP
86 side considers its connecting peer dead. Analogue of tcp_retries1.
87
88retries2
89 The number of times a general DCCP packet is retransmitted. This has
90 importance for retransmitted acknowledgments and feature negotiation,
91 data packets are never retransmitted. Analogue of tcp_retries2.
92
93send_ndp = 1
94 Whether or not to send NDP count options (sec. 7.7.2).
95
96send_ackvec = 1
97 Whether or not to send Ack Vector options (sec. 11.5).
98
99ack_ratio = 2
100 The default Ack Ratio (sec. 11.3) to use.
101
102tx_ccid = 2
103 Default CCID for the sender-receiver half-connection.
104
105rx_ccid = 2
106 Default CCID for the receiver-sender half-connection.
107
108seq_window = 100
109 The initial sequence window (sec. 7.5.2).
110
111tx_qlen = 5
112 The size of the transmit buffer in packets. A value of 0 corresponds
113 to an unbounded transmit buffer.
114
115Notes
116=====
117
118DCCP does not travel through NAT successfully at present on many boxes. This is
119because the checksum covers the psuedo-header as per TCP and UDP. Linux NAT
120support for DCCP has been added.