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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
17oriented protocol designed to solve issues present in UDP and TCP, particularly
18for real-time and multimedia (streaming) traffic.
19It divides into a base protocol (RFC 4340) and plugable congestion control
20modules called CCIDs. Like plugable TCP congestion control, at least one CCID
21needs to be enabled in order for the protocol to function properly. In the Linux
22implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
23the TCP-friendly CCID3 (RFC 4342), are optional.
24For a brief introduction to CCIDs and suggestions for choosing a CCID to match
25given applications, see section 10 of RFC 4340.
26
27It has a base protocol and pluggable congestion control IDs (CCIDs).
28
29DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
30is at http://www.ietf.org/html.charters/dccp-charter.html
31
32Missing features
33================
34
35The Linux DCCP implementation does not currently support all the features that are
36specified in RFCs 4340...42.
37
38The known bugs are at:
39 http://linux-net.osdl.org/index.php/TODO#DCCP
40
41For more up-to-date versions of the DCCP implementation, please consider using
42the experimental DCCP test tree; instructions for checking this out are on:
43http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
44
45
46Socket options
47==============
48
49DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
50service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
51the socket will fall back to 0 (which means that no meaningful service code
52is present). On active sockets this is set before connect(); specifying more
53than one code has no effect (all subsequent service codes are ignored). The
54case is different for passive sockets, where multiple service codes (up to 32)
55can be set before calling bind().
56
57DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
58size (application payload size) in bytes, see RFC 4340, section 14.
59
60DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
61timewait state when closing the connection (RFC 4340, 8.3). The usual case is
62that the closing server sends a CloseReq, whereupon the client holds timewait
63state. When this boolean socket option is on, the server sends a Close instead
64and will enter TIMEWAIT. This option must be set after accept() returns.
65
66DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
67partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
68always cover the entire packet and that only fully covered application data is
69accepted by the receiver. Hence, when using this feature on the sender, it must
70be enabled at the receiver, too with suitable choice of CsCov.
71
72DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
73 range 0..15 are acceptable. The default setting is 0 (full coverage),
74 values between 1..15 indicate partial coverage.
75DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
76 sets a threshold, where again values 0..15 are acceptable. The default
77 of 0 means that all packets with a partial coverage will be discarded.
78 Values in the range 1..15 indicate that packets with minimally such a
79 coverage value are also acceptable. The higher the number, the more
80 restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
81 settings are inherited to the child socket after accept().
82
83The following two options apply to CCID 3 exclusively and are getsockopt()-only.
84In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
85DCCP_SOCKOPT_CCID_RX_INFO
86 Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
87 optlen must be set to at least sizeof(struct tfrc_rx_info).
88DCCP_SOCKOPT_CCID_TX_INFO
89 Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
90 optlen must be set to at least sizeof(struct tfrc_tx_info).
91
92On unidirectional connections it is useful to close the unused half-connection
93via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
94
95Sysctl variables
96================
97Several DCCP default parameters can be managed by the following sysctls
98(sysctl net.dccp.default or /proc/sys/net/dccp/default):
99
100request_retries
101 The number of active connection initiation retries (the number of
102 Requests minus one) before timing out. In addition, it also governs
103 the behaviour of the other, passive side: this variable also sets
104 the number of times DCCP repeats sending a Response when the initial
105 handshake does not progress from RESPOND to OPEN (i.e. when no Ack
106 is received after the initial Request). This value should be greater
107 than 0, suggested is less than 10. Analogue of tcp_syn_retries.
108
109retries1
110 How often a DCCP Response is retransmitted until the listening DCCP
111 side considers its connecting peer dead. Analogue of tcp_retries1.
112
113retries2
114 The number of times a general DCCP packet is retransmitted. This has
115 importance for retransmitted acknowledgments and feature negotiation,
116 data packets are never retransmitted. Analogue of tcp_retries2.
117
118send_ndp = 1
119 Whether or not to send NDP count options (sec. 7.7.2).
120
121send_ackvec = 1
122 Whether or not to send Ack Vector options (sec. 11.5).
123
124ack_ratio = 2
125 The default Ack Ratio (sec. 11.3) to use.
126
127tx_ccid = 2
128 Default CCID for the sender-receiver half-connection.
129
130rx_ccid = 2
131 Default CCID for the receiver-sender half-connection.
132
133seq_window = 100
134 The initial sequence window (sec. 7.5.2).
135
136tx_qlen = 5
137 The size of the transmit buffer in packets. A value of 0 corresponds
138 to an unbounded transmit buffer.
139
140sync_ratelimit = 125 ms
141 The timeout between subsequent DCCP-Sync packets sent in response to
142 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
143 of this parameter is milliseconds; a value of 0 disables rate-limiting.
144
145IOCTLS
146======
147FIONREAD
148 Works as in udp(7): returns in the `int' argument pointer the size of
149 the next pending datagram in bytes, or 0 when no datagram is pending.
150
151Notes
152=====
153
154DCCP does not travel through NAT successfully at present on many boxes. This is
155because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
156support for DCCP has been added.