Based on kernel version 4.1. Page generated on 2015-06-28 12:13 EST.
1 DCCP protocol 2 ============= 3 4 5 Contents 6 ======== 7 - Introduction 8 - Missing features 9 - Socket options 10 - Sysctl variables 11 - IOCTLs 12 - Other tunables 13 - Notes 14 15 16 Introduction 17 ============ 18 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection 19 oriented protocol designed to solve issues present in UDP and TCP, particularly 20 for real-time and multimedia (streaming) traffic. 21 It divides into a base protocol (RFC 4340) and pluggable congestion control 22 modules called CCIDs. Like pluggable TCP congestion control, at least one CCID 23 needs to be enabled in order for the protocol to function properly. In the Linux 24 implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as 25 the TCP-friendly CCID3 (RFC 4342), are optional. 26 For a brief introduction to CCIDs and suggestions for choosing a CCID to match 27 given applications, see section 10 of RFC 4340. 28 29 It has a base protocol and pluggable congestion control IDs (CCIDs). 30 31 DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol 32 is at http://www.ietf.org/html.charters/dccp-charter.html 33 34 35 Missing features 36 ================ 37 The Linux DCCP implementation does not currently support all the features that are 38 specified in RFCs 4340...42. 39 40 The known bugs are at: 41 http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP 42 43 For more up-to-date versions of the DCCP implementation, please consider using 44 the experimental DCCP test tree; instructions for checking this out are on: 45 http://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree 46 47 48 Socket options 49 ============== 50 DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes 51 a policy ID as argument and can only be set before the connection (i.e. changes 52 during an established connection are not supported). Currently, two policies are 53 defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special, 54 and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an 55 u32 priority value as ancillary data to sendmsg(), where higher numbers indicate 56 a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to 57 be formatted using a cmsg(3) message header filled in as follows: 58 cmsg->cmsg_level = SOL_DCCP; 59 cmsg->cmsg_type = DCCP_SCM_PRIORITY; 60 cmsg->cmsg_len = CMSG_LEN(sizeof(uint32_t)); /* or CMSG_LEN(4) */ 61 62 DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero 63 value is always interpreted as unbounded queue length. If different from zero, 64 the interpretation of this parameter depends on the current dequeuing policy 65 (see above): the "simple" policy will enforce a fixed queue size by returning 66 EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the 67 lowest-priority packet first. The default value for this parameter is 68 initialised from /proc/sys/net/dccp/default/tx_qlen. 69 70 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of 71 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set, 72 the socket will fall back to 0 (which means that no meaningful service code 73 is present). On active sockets this is set before connect(); specifying more 74 than one code has no effect (all subsequent service codes are ignored). The 75 case is different for passive sockets, where multiple service codes (up to 32) 76 can be set before calling bind(). 77 78 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet 79 size (application payload size) in bytes, see RFC 4340, section 14. 80 81 DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs 82 supported by the endpoint. The option value is an array of type uint8_t whose 83 size is passed as option length. The minimum array size is 4 elements, the 84 value returned in the optlen argument always reflects the true number of 85 built-in CCIDs. 86 87 DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same 88 time, combining the operation of the next two socket options. This option is 89 preferable over the latter two, since often applications will use the same 90 type of CCID for both directions; and mixed use of CCIDs is not currently well 91 understood. This socket option takes as argument at least one uint8_t value, or 92 an array of uint8_t values, which must match available CCIDS (see above). CCIDs 93 must be registered on the socket before calling connect() or listen(). 94 95 DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets 96 the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID. 97 Please note that the getsockopt argument type here is `int', not uint8_t. 98 99 DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID. 100 101 DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold 102 timewait state when closing the connection (RFC 4340, 8.3). The usual case is 103 that the closing server sends a CloseReq, whereupon the client holds timewait 104 state. When this boolean socket option is on, the server sends a Close instead 105 and will enter TIMEWAIT. This option must be set after accept() returns. 106 107 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the 108 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums 109 always cover the entire packet and that only fully covered application data is 110 accepted by the receiver. Hence, when using this feature on the sender, it must 111 be enabled at the receiver, too with suitable choice of CsCov. 112 113 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the 114 range 0..15 are acceptable. The default setting is 0 (full coverage), 115 values between 1..15 indicate partial coverage. 116 DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it 117 sets a threshold, where again values 0..15 are acceptable. The default 118 of 0 means that all packets with a partial coverage will be discarded. 119 Values in the range 1..15 indicate that packets with minimally such a 120 coverage value are also acceptable. The higher the number, the more 121 restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage 122 settings are inherited to the child socket after accept(). 123 124 The following two options apply to CCID 3 exclusively and are getsockopt()-only. 125 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned. 126 DCCP_SOCKOPT_CCID_RX_INFO 127 Returns a `struct tfrc_rx_info' in optval; the buffer for optval and 128 optlen must be set to at least sizeof(struct tfrc_rx_info). 129 DCCP_SOCKOPT_CCID_TX_INFO 130 Returns a `struct tfrc_tx_info' in optval; the buffer for optval and 131 optlen must be set to at least sizeof(struct tfrc_tx_info). 132 133 On unidirectional connections it is useful to close the unused half-connection 134 via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs. 135 136 137 Sysctl variables 138 ================ 139 Several DCCP default parameters can be managed by the following sysctls 140 (sysctl net.dccp.default or /proc/sys/net/dccp/default): 141 142 request_retries 143 The number of active connection initiation retries (the number of 144 Requests minus one) before timing out. In addition, it also governs 145 the behaviour of the other, passive side: this variable also sets 146 the number of times DCCP repeats sending a Response when the initial 147 handshake does not progress from RESPOND to OPEN (i.e. when no Ack 148 is received after the initial Request). This value should be greater 149 than 0, suggested is less than 10. Analogue of tcp_syn_retries. 150 151 retries1 152 How often a DCCP Response is retransmitted until the listening DCCP 153 side considers its connecting peer dead. Analogue of tcp_retries1. 154 155 retries2 156 The number of times a general DCCP packet is retransmitted. This has 157 importance for retransmitted acknowledgments and feature negotiation, 158 data packets are never retransmitted. Analogue of tcp_retries2. 159 160 tx_ccid = 2 161 Default CCID for the sender-receiver half-connection. Depending on the 162 choice of CCID, the Send Ack Vector feature is enabled automatically. 163 164 rx_ccid = 2 165 Default CCID for the receiver-sender half-connection; see tx_ccid. 166 167 seq_window = 100 168 The initial sequence window (sec. 7.5.2) of the sender. This influences 169 the local ackno validity and the remote seqno validity windows (7.5.1). 170 Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set. 171 172 tx_qlen = 5 173 The size of the transmit buffer in packets. A value of 0 corresponds 174 to an unbounded transmit buffer. 175 176 sync_ratelimit = 125 ms 177 The timeout between subsequent DCCP-Sync packets sent in response to 178 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit 179 of this parameter is milliseconds; a value of 0 disables rate-limiting. 180 181 182 IOCTLS 183 ====== 184 FIONREAD 185 Works as in udp(7): returns in the `int' argument pointer the size of 186 the next pending datagram in bytes, or 0 when no datagram is pending. 187 188 189 Other tunables 190 ============== 191 Per-route rto_min support 192 CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value 193 of the RTO timer. This setting can be modified via the 'rto_min' option 194 of iproute2; for example: 195 > ip route change 10.0.0.0/24 rto_min 250j dev wlan0 196 > ip route add 10.0.0.254/32 rto_min 800j dev wlan0 197 > ip route show dev wlan0 198 CCID-3 also supports the rto_min setting: it is used to define the lower 199 bound for the expiry of the nofeedback timer. This can be useful on LANs 200 with very low RTTs (e.g., loopback, Gbit ethernet). 201 202 203 Notes 204 ===== 205 DCCP does not travel through NAT successfully at present on many boxes. This is 206 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT 207 support for DCCP has been added.