1.. SPDX-License-Identifier: GPL-2.0 2 3============= 4DCCP 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 18Introduction 19============ 20Datagram Congestion Control Protocol (DCCP) is an unreliable, connection 21oriented protocol designed to solve issues present in UDP and TCP, particularly 22for real-time and multimedia (streaming) traffic. 23It divides into a base protocol (RFC 4340) and pluggable congestion control 24modules called CCIDs. Like pluggable TCP congestion control, at least one CCID 25needs to be enabled in order for the protocol to function properly. In the Linux 26implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as 27the TCP-friendly CCID3 (RFC 4342), are optional. 28For a brief introduction to CCIDs and suggestions for choosing a CCID to match 29given applications, see section 10 of RFC 4340. 30 31It has a base protocol and pluggable congestion control IDs (CCIDs). 32 33DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol 34is at http://www.ietf.org/html.charters/dccp-charter.html 35 36 37Missing features 38================ 39The Linux DCCP implementation does not currently support all the features that are 40specified in RFCs 4340...42. 41 42The known bugs are at: 43 44 http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP 45 46For more up-to-date versions of the DCCP implementation, please consider using 47the experimental DCCP test tree; instructions for checking this out are on: 48http://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree 49 50 51Socket options 52============== 53DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes 54a policy ID as argument and can only be set before the connection (i.e. changes 55during an established connection are not supported). Currently, two policies are 56defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special, 57and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an 58u32 priority value as ancillary data to sendmsg(), where higher numbers indicate 59a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to 60be formatted using a cmsg(3) message header filled in as follows:: 61 62 cmsg->cmsg_level = SOL_DCCP; 63 cmsg->cmsg_type = DCCP_SCM_PRIORITY; 64 cmsg->cmsg_len = CMSG_LEN(sizeof(uint32_t)); /* or CMSG_LEN(4) */ 65 66DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero 67value is always interpreted as unbounded queue length. If different from zero, 68the interpretation of this parameter depends on the current dequeuing policy 69(see above): the "simple" policy will enforce a fixed queue size by returning 70EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the 71lowest-priority packet first. The default value for this parameter is 72initialised from /proc/sys/net/dccp/default/tx_qlen. 73 74DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of 75service codes (RFC 4340, sec. 8.1.2); if this socket option is not set, 76the socket will fall back to 0 (which means that no meaningful service code 77is present). On active sockets this is set before connect(); specifying more 78than one code has no effect (all subsequent service codes are ignored). The 79case is different for passive sockets, where multiple service codes (up to 32) 80can be set before calling bind(). 81 82DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet 83size (application payload size) in bytes, see RFC 4340, section 14. 84 85DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs 86supported by the endpoint. The option value is an array of type uint8_t whose 87size is passed as option length. The minimum array size is 4 elements, the 88value returned in the optlen argument always reflects the true number of 89built-in CCIDs. 90 91DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same 92time, combining the operation of the next two socket options. This option is 93preferable over the latter two, since often applications will use the same 94type of CCID for both directions; and mixed use of CCIDs is not currently well 95understood. This socket option takes as argument at least one uint8_t value, or 96an array of uint8_t values, which must match available CCIDS (see above). CCIDs 97must be registered on the socket before calling connect() or listen(). 98 99DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets 100the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID. 101Please note that the getsockopt argument type here is ``int``, not uint8_t. 102 103DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID. 104 105DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold 106timewait state when closing the connection (RFC 4340, 8.3). The usual case is 107that the closing server sends a CloseReq, whereupon the client holds timewait 108state. When this boolean socket option is on, the server sends a Close instead 109and will enter TIMEWAIT. This option must be set after accept() returns. 110 111DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the 112partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums 113always cover the entire packet and that only fully covered application data is 114accepted by the receiver. Hence, when using this feature on the sender, it must 115be enabled at the receiver, too with suitable choice of CsCov. 116 117DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the 118 range 0..15 are acceptable. The default setting is 0 (full coverage), 119 values between 1..15 indicate partial coverage. 120 121DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it 122 sets a threshold, where again values 0..15 are acceptable. The default 123 of 0 means that all packets with a partial coverage will be discarded. 124 Values in the range 1..15 indicate that packets with minimally such a 125 coverage value are also acceptable. The higher the number, the more 126 restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage 127 settings are inherited to the child socket after accept(). 128 129The following two options apply to CCID 3 exclusively and are getsockopt()-only. 130In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned. 131 132DCCP_SOCKOPT_CCID_RX_INFO 133 Returns a ``struct tfrc_rx_info`` in optval; the buffer for optval and 134 optlen must be set to at least sizeof(struct tfrc_rx_info). 135 136DCCP_SOCKOPT_CCID_TX_INFO 137 Returns a ``struct tfrc_tx_info`` in optval; the buffer for optval and 138 optlen must be set to at least sizeof(struct tfrc_tx_info). 139 140On unidirectional connections it is useful to close the unused half-connection 141via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs. 142 143 144Sysctl variables 145================ 146Several DCCP default parameters can be managed by the following sysctls 147(sysctl net.dccp.default or /proc/sys/net/dccp/default): 148 149request_retries 150 The number of active connection initiation retries (the number of 151 Requests minus one) before timing out. In addition, it also governs 152 the behaviour of the other, passive side: this variable also sets 153 the number of times DCCP repeats sending a Response when the initial 154 handshake does not progress from RESPOND to OPEN (i.e. when no Ack 155 is received after the initial Request). This value should be greater 156 than 0, suggested is less than 10. Analogue of tcp_syn_retries. 157 158retries1 159 How often a DCCP Response is retransmitted until the listening DCCP 160 side considers its connecting peer dead. Analogue of tcp_retries1. 161 162retries2 163 The number of times a general DCCP packet is retransmitted. This has 164 importance for retransmitted acknowledgments and feature negotiation, 165 data packets are never retransmitted. Analogue of tcp_retries2. 166 167tx_ccid = 2 168 Default CCID for the sender-receiver half-connection. Depending on the 169 choice of CCID, the Send Ack Vector feature is enabled automatically. 170 171rx_ccid = 2 172 Default CCID for the receiver-sender half-connection; see tx_ccid. 173 174seq_window = 100 175 The initial sequence window (sec. 7.5.2) of the sender. This influences 176 the local ackno validity and the remote seqno validity windows (7.5.1). 177 Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set. 178 179tx_qlen = 5 180 The size of the transmit buffer in packets. A value of 0 corresponds 181 to an unbounded transmit buffer. 182 183sync_ratelimit = 125 ms 184 The timeout between subsequent DCCP-Sync packets sent in response to 185 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit 186 of this parameter is milliseconds; a value of 0 disables rate-limiting. 187 188 189IOCTLS 190====== 191FIONREAD 192 Works as in udp(7): returns in the ``int`` argument pointer the size of 193 the next pending datagram in bytes, or 0 when no datagram is pending. 194 195SIOCOUTQ 196 Returns the number of unsent data bytes in the socket send queue as ``int`` 197 into the buffer specified by the argument pointer. 198 199Other tunables 200============== 201Per-route rto_min support 202 CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value 203 of the RTO timer. This setting can be modified via the 'rto_min' option 204 of iproute2; for example:: 205 206 > ip route change 10.0.0.0/24 rto_min 250j dev wlan0 207 > ip route add 10.0.0.254/32 rto_min 800j dev wlan0 208 > ip route show dev wlan0 209 210 CCID-3 also supports the rto_min setting: it is used to define the lower 211 bound for the expiry of the nofeedback timer. This can be useful on LANs 212 with very low RTTs (e.g., loopback, Gbit ethernet). 213 214 215Notes 216===== 217DCCP does not travel through NAT successfully at present on many boxes. This is 218because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT 219support for DCCP has been added. 220