1 .. SPDX-License-Identifier: GPL-2.0
2
3 =============
4 DCCP protocol
5 =============
6
7
8 .. Contents
9 - Introduction
10 - Missing features
11 - Socket options
12 - Sysctl variables
13 - IOCTLs
14 - Other tunables
15 - Notes
16
17
18 Introduction
19 ============
20 Datagram Congestion Control Protocol (DCCP) is
21 oriented protocol designed to solve issues pre
22 for real-time and multimedia (streaming) traff
23 It divides into a base protocol (RFC 4340) and
24 modules called CCIDs. Like pluggable TCP conge
25 needs to be enabled in order for the protocol
26 implementation, this is the TCP-like CCID2 (RF
27 the TCP-friendly CCID3 (RFC 4342), are optiona
28 For a brief introduction to CCIDs and suggesti
29 given applications, see section 10 of RFC 4340
30
31 It has a base protocol and pluggable congestio
32
33 DCCP is a Proposed Standard (RFC 2026), and th
34 is at http://www.ietf.org/html.charters/dccp-c
35
36
37 Missing features
38 ================
39 The Linux DCCP implementation does not current
40 specified in RFCs 4340...42.
41
42 The known bugs are at:
43
44 http://www.linuxfoundation.org/collabo
45
46 For more up-to-date versions of the DCCP imple
47 the experimental DCCP test tree; instructions
48 http://www.linuxfoundation.org/collaborate/wor
49
50
51 Socket options
52 ==============
53 DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing pol
54 a policy ID as argument and can only be set be
55 during an established connection are not suppo
56 defined: the "simple" policy (DCCPQ_POLICY_SIM
57 and a priority-based variant (DCCPQ_POLICY_PRI
58 u32 priority value as ancillary data to sendms
59 a higher packet priority (similar to SO_PRIORI
60 be formatted using a cmsg(3) message header fi
61
62 cmsg->cmsg_level = SOL_DCCP;
63 cmsg->cmsg_type = DCCP_SCM_PRIORITY;
64 cmsg->cmsg_len = CMSG_LEN(sizeof(uin
65
66 DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum l
67 value is always interpreted as unbounded queue
68 the interpretation of this parameter depends o
69 (see above): the "simple" policy will enforce
70 EAGAIN, whereas the "prio" policy enforces a f
71 lowest-priority packet first. The default valu
72 initialised from /proc/sys/net/dccp/default/tx
73
74 DCCP_SOCKOPT_SERVICE sets the service. The spe
75 service codes (RFC 4340, sec. 8.1.2); if this
76 the socket will fall back to 0 (which means th
77 is present). On active sockets this is set bef
78 than one code has no effect (all subsequent se
79 case is different for passive sockets, where m
80 can be set before calling bind().
81
82 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retr
83 size (application payload size) in bytes, see
84
85 DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only
86 supported by the endpoint. The option value is
87 size is passed as option length. The minimum a
88 value returned in the optlen argument always r
89 built-in CCIDs.
90
91 DCCP_SOCKOPT_CCID is write-only and sets both
92 time, combining the operation of the next two
93 preferable over the latter two, since often ap
94 type of CCID for both directions; and mixed us
95 understood. This socket option takes as argume
96 an array of uint8_t values, which must match a
97 must be registered on the socket before callin
98
99 DCCP_SOCKOPT_TX_CCID is read/write. It returns
100 the preference list for the TX CCID, using the
101 Please note that the getsockopt argument type
102
103 DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCK
104
105 DCCP_SOCKOPT_SERVER_TIMEWAIT enables the serve
106 timewait state when closing the connection (RF
107 that the closing server sends a CloseReq, wher
108 state. When this boolean socket option is on,
109 and will enter TIMEWAIT. This option must be s
110
111 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_
112 partial checksum coverage (RFC 4340, sec. 9.2)
113 always cover the entire packet and that only f
114 accepted by the receiver. Hence, when using th
115 be enabled at the receiver, too with suitable
116
117 DCCP_SOCKOPT_SEND_CSCOV sets the sender checks
118 range 0..15 are acceptable. The defaul
119 values between 1..15 indicate partial
120
121 DCCP_SOCKOPT_RECV_CSCOV is for the receiver an
122 sets a threshold, where again values 0
123 of 0 means that all packets with a par
124 Values in the range 1..15 indicate tha
125 coverage value are also acceptable. Th
126 restrictive this setting (see [RFC 434
127 settings are inherited to the child so
128
129 The following two options apply to CCID 3 excl
130 In either case, a TFRC info struct (defined in
131
132 DCCP_SOCKOPT_CCID_RX_INFO
133 Returns a ``struct tfrc_rx_info`` in o
134 optlen must be set to at least sizeof(
135
136 DCCP_SOCKOPT_CCID_TX_INFO
137 Returns a ``struct tfrc_tx_info`` in o
138 optlen must be set to at least sizeof(
139
140 On unidirectional connections it is useful to
141 via shutdown (SHUT_WR or SHUT_RD): this will r
142
143
144 Sysctl variables
145 ================
146 Several DCCP default parameters can be managed
147 (sysctl net.dccp.default or /proc/sys/net/dccp
148
149 request_retries
150 The number of active connection initia
151 Requests minus one) before timing out.
152 the behaviour of the other, passive si
153 the number of times DCCP repeats sendi
154 handshake does not progress from RESPO
155 is received after the initial Request)
156 than 0, suggested is less than 10. Ana
157
158 retries1
159 How often a DCCP Response is retransmi
160 side considers its connecting peer dea
161
162 retries2
163 The number of times a general DCCP pac
164 importance for retransmitted acknowled
165 data packets are never retransmitted.
166
167 tx_ccid = 2
168 Default CCID for the sender-receiver h
169 choice of CCID, the Send Ack Vector fe
170
171 rx_ccid = 2
172 Default CCID for the receiver-sender h
173
174 seq_window = 100
175 The initial sequence window (sec. 7.5.
176 the local ackno validity and the remot
177 Values in the range Wmin = 32 (RFC 434
178
179 tx_qlen = 5
180 The size of the transmit buffer in pac
181 to an unbounded transmit buffer.
182
183 sync_ratelimit = 125 ms
184 The timeout between subsequent DCCP-Sy
185 sequence-invalid packets on the same s
186 of this parameter is milliseconds; a v
187
188
189 IOCTLS
190 ======
191 FIONREAD
192 Works as in udp(7): returns in the ``i
193 the next pending datagram in bytes, or
194
195 SIOCOUTQ
196 Returns the number of unsent data byte
197 into the buffer specified by the argum
198
199 Other tunables
200 ==============
201 Per-route rto_min support
202 CCID-2 supports the RTAX_RTO_MIN per-r
203 of the RTO timer. This setting can be
204 of iproute2; for example::
205
206 > ip route change 10.0.0.0/24
207 > ip route add 10.0.0.254/3
208 > ip route show dev wlan0
209
210 CCID-3 also supports the rto_min setti
211 bound for the expiry of the nofeedback
212 with very low RTTs (e.g., loopback, Gb
213
214
215 Notes
216 =====
217 DCCP does not travel through NAT successfully
218 because the checksum covers the pseudo-header
219 support for DCCP has been added.
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