1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ============= 3 =============
4 DCCP protocol 4 DCCP protocol
5 ============= 5 =============
6 6
7 7
8 .. Contents 8 .. Contents
9 - Introduction 9 - Introduction
10 - Missing features 10 - Missing features
11 - Socket options 11 - Socket options
12 - Sysctl variables 12 - Sysctl variables
13 - IOCTLs 13 - IOCTLs
14 - Other tunables 14 - Other tunables
15 - Notes 15 - Notes
16 16
17 17
18 Introduction 18 Introduction
19 ============ 19 ============
20 Datagram Congestion Control Protocol (DCCP) is 20 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
21 oriented protocol designed to solve issues pre 21 oriented protocol designed to solve issues present in UDP and TCP, particularly
22 for real-time and multimedia (streaming) traff 22 for real-time and multimedia (streaming) traffic.
23 It divides into a base protocol (RFC 4340) and 23 It divides into a base protocol (RFC 4340) and pluggable congestion control
24 modules called CCIDs. Like pluggable TCP conge 24 modules called CCIDs. Like pluggable TCP congestion control, at least one CCID
25 needs to be enabled in order for the protocol 25 needs to be enabled in order for the protocol to function properly. In the Linux
26 implementation, this is the TCP-like CCID2 (RF 26 implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
27 the TCP-friendly CCID3 (RFC 4342), are optiona 27 the TCP-friendly CCID3 (RFC 4342), are optional.
28 For a brief introduction to CCIDs and suggesti 28 For a brief introduction to CCIDs and suggestions for choosing a CCID to match
29 given applications, see section 10 of RFC 4340 29 given applications, see section 10 of RFC 4340.
30 30
31 It has a base protocol and pluggable congestio 31 It has a base protocol and pluggable congestion control IDs (CCIDs).
32 32
33 DCCP is a Proposed Standard (RFC 2026), and th 33 DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
34 is at http://www.ietf.org/html.charters/dccp-c 34 is at http://www.ietf.org/html.charters/dccp-charter.html
35 35
36 36
37 Missing features 37 Missing features
38 ================ 38 ================
39 The Linux DCCP implementation does not current 39 The Linux DCCP implementation does not currently support all the features that are
40 specified in RFCs 4340...42. 40 specified in RFCs 4340...42.
41 41
42 The known bugs are at: 42 The known bugs are at:
43 43
44 http://www.linuxfoundation.org/collabo 44 http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP
45 45
46 For more up-to-date versions of the DCCP imple 46 For more up-to-date versions of the DCCP implementation, please consider using
47 the experimental DCCP test tree; instructions 47 the experimental DCCP test tree; instructions for checking this out are on:
48 http://www.linuxfoundation.org/collaborate/wor 48 http://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree
49 49
50 50
51 Socket options 51 Socket options
52 ============== 52 ==============
53 DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing pol 53 DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
54 a policy ID as argument and can only be set be 54 a policy ID as argument and can only be set before the connection (i.e. changes
55 during an established connection are not suppo 55 during an established connection are not supported). Currently, two policies are
56 defined: the "simple" policy (DCCPQ_POLICY_SIM 56 defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
57 and a priority-based variant (DCCPQ_POLICY_PRI 57 and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
58 u32 priority value as ancillary data to sendms 58 u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
59 a higher packet priority (similar to SO_PRIORI 59 a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
60 be formatted using a cmsg(3) message header fi 60 be formatted using a cmsg(3) message header filled in as follows::
61 61
62 cmsg->cmsg_level = SOL_DCCP; 62 cmsg->cmsg_level = SOL_DCCP;
63 cmsg->cmsg_type = DCCP_SCM_PRIORITY; 63 cmsg->cmsg_type = DCCP_SCM_PRIORITY;
64 cmsg->cmsg_len = CMSG_LEN(sizeof(uin 64 cmsg->cmsg_len = CMSG_LEN(sizeof(uint32_t)); /* or CMSG_LEN(4) */
65 65
66 DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum l 66 DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
67 value is always interpreted as unbounded queue 67 value is always interpreted as unbounded queue length. If different from zero,
68 the interpretation of this parameter depends o 68 the interpretation of this parameter depends on the current dequeuing policy
69 (see above): the "simple" policy will enforce 69 (see above): the "simple" policy will enforce a fixed queue size by returning
70 EAGAIN, whereas the "prio" policy enforces a f 70 EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
71 lowest-priority packet first. The default valu 71 lowest-priority packet first. The default value for this parameter is
72 initialised from /proc/sys/net/dccp/default/tx 72 initialised from /proc/sys/net/dccp/default/tx_qlen.
73 73
74 DCCP_SOCKOPT_SERVICE sets the service. The spe 74 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
75 service codes (RFC 4340, sec. 8.1.2); if this 75 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
76 the socket will fall back to 0 (which means th 76 the socket will fall back to 0 (which means that no meaningful service code
77 is present). On active sockets this is set bef 77 is present). On active sockets this is set before connect(); specifying more
78 than one code has no effect (all subsequent se 78 than one code has no effect (all subsequent service codes are ignored). The
79 case is different for passive sockets, where m 79 case is different for passive sockets, where multiple service codes (up to 32)
80 can be set before calling bind(). 80 can be set before calling bind().
81 81
82 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retr 82 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
83 size (application payload size) in bytes, see 83 size (application payload size) in bytes, see RFC 4340, section 14.
84 84
85 DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only 85 DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
86 supported by the endpoint. The option value is 86 supported by the endpoint. The option value is an array of type uint8_t whose
87 size is passed as option length. The minimum a 87 size is passed as option length. The minimum array size is 4 elements, the
88 value returned in the optlen argument always r 88 value returned in the optlen argument always reflects the true number of
89 built-in CCIDs. 89 built-in CCIDs.
90 90
91 DCCP_SOCKOPT_CCID is write-only and sets both 91 DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
92 time, combining the operation of the next two 92 time, combining the operation of the next two socket options. This option is
93 preferable over the latter two, since often ap 93 preferable over the latter two, since often applications will use the same
94 type of CCID for both directions; and mixed us 94 type of CCID for both directions; and mixed use of CCIDs is not currently well
95 understood. This socket option takes as argume 95 understood. This socket option takes as argument at least one uint8_t value, or
96 an array of uint8_t values, which must match a 96 an array of uint8_t values, which must match available CCIDS (see above). CCIDs
97 must be registered on the socket before callin 97 must be registered on the socket before calling connect() or listen().
98 98
99 DCCP_SOCKOPT_TX_CCID is read/write. It returns 99 DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
100 the preference list for the TX CCID, using the 100 the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
101 Please note that the getsockopt argument type 101 Please note that the getsockopt argument type here is ``int``, not uint8_t.
102 102
103 DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCK 103 DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
104 104
105 DCCP_SOCKOPT_SERVER_TIMEWAIT enables the serve 105 DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
106 timewait state when closing the connection (RF 106 timewait state when closing the connection (RFC 4340, 8.3). The usual case is
107 that the closing server sends a CloseReq, wher 107 that the closing server sends a CloseReq, whereupon the client holds timewait
108 state. When this boolean socket option is on, 108 state. When this boolean socket option is on, the server sends a Close instead
109 and will enter TIMEWAIT. This option must be s 109 and will enter TIMEWAIT. This option must be set after accept() returns.
110 110
111 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_ 111 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
112 partial checksum coverage (RFC 4340, sec. 9.2) 112 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
113 always cover the entire packet and that only f 113 always cover the entire packet and that only fully covered application data is
114 accepted by the receiver. Hence, when using th 114 accepted by the receiver. Hence, when using this feature on the sender, it must
115 be enabled at the receiver, too with suitable 115 be enabled at the receiver, too with suitable choice of CsCov.
116 116
117 DCCP_SOCKOPT_SEND_CSCOV sets the sender checks 117 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
118 range 0..15 are acceptable. The defaul 118 range 0..15 are acceptable. The default setting is 0 (full coverage),
119 values between 1..15 indicate partial 119 values between 1..15 indicate partial coverage.
120 120
121 DCCP_SOCKOPT_RECV_CSCOV is for the receiver an 121 DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
122 sets a threshold, where again values 0 122 sets a threshold, where again values 0..15 are acceptable. The default
123 of 0 means that all packets with a par 123 of 0 means that all packets with a partial coverage will be discarded.
124 Values in the range 1..15 indicate tha 124 Values in the range 1..15 indicate that packets with minimally such a
125 coverage value are also acceptable. Th 125 coverage value are also acceptable. The higher the number, the more
126 restrictive this setting (see [RFC 434 126 restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
127 settings are inherited to the child so 127 settings are inherited to the child socket after accept().
128 128
129 The following two options apply to CCID 3 excl 129 The following two options apply to CCID 3 exclusively and are getsockopt()-only.
130 In either case, a TFRC info struct (defined in 130 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
131 131
132 DCCP_SOCKOPT_CCID_RX_INFO 132 DCCP_SOCKOPT_CCID_RX_INFO
133 Returns a ``struct tfrc_rx_info`` in o 133 Returns a ``struct tfrc_rx_info`` in optval; the buffer for optval and
134 optlen must be set to at least sizeof( 134 optlen must be set to at least sizeof(struct tfrc_rx_info).
135 135
136 DCCP_SOCKOPT_CCID_TX_INFO 136 DCCP_SOCKOPT_CCID_TX_INFO
137 Returns a ``struct tfrc_tx_info`` in o 137 Returns a ``struct tfrc_tx_info`` in optval; the buffer for optval and
138 optlen must be set to at least sizeof( 138 optlen must be set to at least sizeof(struct tfrc_tx_info).
139 139
140 On unidirectional connections it is useful to 140 On unidirectional connections it is useful to close the unused half-connection
141 via shutdown (SHUT_WR or SHUT_RD): this will r 141 via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
142 142
143 143
144 Sysctl variables 144 Sysctl variables
145 ================ 145 ================
146 Several DCCP default parameters can be managed 146 Several DCCP default parameters can be managed by the following sysctls
147 (sysctl net.dccp.default or /proc/sys/net/dccp 147 (sysctl net.dccp.default or /proc/sys/net/dccp/default):
148 148
149 request_retries 149 request_retries
150 The number of active connection initia 150 The number of active connection initiation retries (the number of
151 Requests minus one) before timing out. 151 Requests minus one) before timing out. In addition, it also governs
152 the behaviour of the other, passive si 152 the behaviour of the other, passive side: this variable also sets
153 the number of times DCCP repeats sendi 153 the number of times DCCP repeats sending a Response when the initial
154 handshake does not progress from RESPO 154 handshake does not progress from RESPOND to OPEN (i.e. when no Ack
155 is received after the initial Request) 155 is received after the initial Request). This value should be greater
156 than 0, suggested is less than 10. Ana 156 than 0, suggested is less than 10. Analogue of tcp_syn_retries.
157 157
158 retries1 158 retries1
159 How often a DCCP Response is retransmi 159 How often a DCCP Response is retransmitted until the listening DCCP
160 side considers its connecting peer dea 160 side considers its connecting peer dead. Analogue of tcp_retries1.
161 161
162 retries2 162 retries2
163 The number of times a general DCCP pac 163 The number of times a general DCCP packet is retransmitted. This has
164 importance for retransmitted acknowled 164 importance for retransmitted acknowledgments and feature negotiation,
165 data packets are never retransmitted. 165 data packets are never retransmitted. Analogue of tcp_retries2.
166 166
167 tx_ccid = 2 167 tx_ccid = 2
168 Default CCID for the sender-receiver h 168 Default CCID for the sender-receiver half-connection. Depending on the
169 choice of CCID, the Send Ack Vector fe 169 choice of CCID, the Send Ack Vector feature is enabled automatically.
170 170
171 rx_ccid = 2 171 rx_ccid = 2
172 Default CCID for the receiver-sender h 172 Default CCID for the receiver-sender half-connection; see tx_ccid.
173 173
174 seq_window = 100 174 seq_window = 100
175 The initial sequence window (sec. 7.5. 175 The initial sequence window (sec. 7.5.2) of the sender. This influences
176 the local ackno validity and the remot 176 the local ackno validity and the remote seqno validity windows (7.5.1).
177 Values in the range Wmin = 32 (RFC 434 177 Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
178 178
179 tx_qlen = 5 179 tx_qlen = 5
180 The size of the transmit buffer in pac 180 The size of the transmit buffer in packets. A value of 0 corresponds
181 to an unbounded transmit buffer. 181 to an unbounded transmit buffer.
182 182
183 sync_ratelimit = 125 ms 183 sync_ratelimit = 125 ms
184 The timeout between subsequent DCCP-Sy 184 The timeout between subsequent DCCP-Sync packets sent in response to
185 sequence-invalid packets on the same s 185 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
186 of this parameter is milliseconds; a v 186 of this parameter is milliseconds; a value of 0 disables rate-limiting.
187 187
188 188
189 IOCTLS 189 IOCTLS
190 ====== 190 ======
191 FIONREAD 191 FIONREAD
192 Works as in udp(7): returns in the ``i 192 Works as in udp(7): returns in the ``int`` argument pointer the size of
193 the next pending datagram in bytes, or 193 the next pending datagram in bytes, or 0 when no datagram is pending.
194 194
195 SIOCOUTQ 195 SIOCOUTQ
196 Returns the number of unsent data byte 196 Returns the number of unsent data bytes in the socket send queue as ``int``
197 into the buffer specified by the argum 197 into the buffer specified by the argument pointer.
198 198
199 Other tunables 199 Other tunables
200 ============== 200 ==============
201 Per-route rto_min support 201 Per-route rto_min support
202 CCID-2 supports the RTAX_RTO_MIN per-r 202 CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
203 of the RTO timer. This setting can be 203 of the RTO timer. This setting can be modified via the 'rto_min' option
204 of iproute2; for example:: 204 of iproute2; for example::
205 205
206 > ip route change 10.0.0.0/24 206 > ip route change 10.0.0.0/24 rto_min 250j dev wlan0
207 > ip route add 10.0.0.254/3 207 > ip route add 10.0.0.254/32 rto_min 800j dev wlan0
208 > ip route show dev wlan0 208 > ip route show dev wlan0
209 209
210 CCID-3 also supports the rto_min setti 210 CCID-3 also supports the rto_min setting: it is used to define the lower
211 bound for the expiry of the nofeedback 211 bound for the expiry of the nofeedback timer. This can be useful on LANs
212 with very low RTTs (e.g., loopback, Gb 212 with very low RTTs (e.g., loopback, Gbit ethernet).
213 213
214 214
215 Notes 215 Notes
216 ===== 216 =====
217 DCCP does not travel through NAT successfully 217 DCCP does not travel through NAT successfully at present on many boxes. This is
218 because the checksum covers the pseudo-header 218 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
219 support for DCCP has been added. 219 support for DCCP has been added.
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