1DCCP protocol
   7- Introduction
   8- Missing features
   9- Socket options
  10- Sysctl variables
  11- IOCTLs
  12- Other tunables
  13- Notes
  18Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
  19oriented protocol designed to solve issues present in UDP and TCP, particularly
  20for real-time and multimedia (streaming) traffic.
  21It divides into a base protocol (RFC 4340) and plugable congestion control
  22modules called CCIDs. Like plugable TCP congestion control, at least one CCID
  23needs to be enabled in order for the protocol to function properly. In the Linux
  24implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
  25the TCP-friendly CCID3 (RFC 4342), are optional.
  26For a brief introduction to CCIDs and suggestions for choosing a CCID to match
  27given applications, see section 10 of RFC 4340.
  29It has a base protocol and pluggable congestion control IDs (CCIDs).
  31DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
  32is at
  35Missing features
  37The Linux DCCP implementation does not currently support all the features that are
  38specified in RFCs 4340...42.
  40The known bugs are at:
  43For more up-to-date versions of the DCCP implementation, please consider using
  44the experimental DCCP test tree; instructions for checking this out are on:
  48Socket options
  50DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
  51a policy ID as argument and can only be set before the connection (i.e. changes
  52during an established connection are not supported). Currently, two policies are
  53defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
  54and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
  55u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
  56a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
  57be 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) */
  62DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
  63value is always interpreted as unbounded queue length. If different from zero,
  64the interpretation of this parameter depends on the current dequeuing policy
  65(see above): the "simple" policy will enforce a fixed queue size by returning
  66EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
  67lowest-priority packet first. The default value for this parameter is
  68initialised from /proc/sys/net/dccp/default/tx_qlen.
  70DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
  71service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
  72the socket will fall back to 0 (which means that no meaningful service code
  73is present). On active sockets this is set before connect(); specifying more
  74than one code has no effect (all subsequent service codes are ignored). The
  75case is different for passive sockets, where multiple service codes (up to 32)
  76can be set before calling bind().
  78DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
  79size (application payload size) in bytes, see RFC 4340, section 14.
  81DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
  82supported by the endpoint. The option value is an array of type uint8_t whose
  83size is passed as option length. The minimum array size is 4 elements, the
  84value returned in the optlen argument always reflects the true number of
  85built-in CCIDs.
  87DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
  88time, combining the operation of the next two socket options. This option is
  89preferrable over the latter two, since often applications will use the same
  90type of CCID for both directions; and mixed use of CCIDs is not currently well
  91understood. This socket option takes as argument at least one uint8_t value, or
  92an array of uint8_t values, which must match available CCIDS (see above). CCIDs
  93must be registered on the socket before calling connect() or listen().
  95DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
  96the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
  97Please note that the getsockopt argument type here is `int', not uint8_t.
  99DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
 101DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
 102timewait state when closing the connection (RFC 4340, 8.3). The usual case is
 103that the closing server sends a CloseReq, whereupon the client holds timewait
 104state. When this boolean socket option is on, the server sends a Close instead
 105and will enter TIMEWAIT. This option must be set after accept() returns.
 107DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
 108partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
 109always cover the entire packet and that only fully covered application data is
 110accepted by the receiver. Hence, when using this feature on the sender, it must
 111be enabled at the receiver, too with suitable choice of CsCov.
 113DCCP_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.
 116DCCP_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().
 124The following two options apply to CCID 3 exclusively and are getsockopt()-only.
 125In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
 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).
 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).
 133On unidirectional connections it is useful to close the unused half-connection
 134via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
 137Sysctl variables
 139Several DCCP default parameters can be managed by the following sysctls
 140(sysctl net.dccp.default or /proc/sys/net/dccp/default):
 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.
 152        How often a DCCP Response is retransmitted until the listening DCCP
 153        side considers its connecting peer dead. Analogue of tcp_retries1.
 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.
 160tx_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.
 164rx_ccid = 2
 165        Default CCID for the receiver-sender half-connection; see tx_ccid.
 167seq_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.
 172tx_qlen = 5
 173        The size of the transmit buffer in packets. A value of 0 corresponds
 174        to an unbounded transmit buffer.
 176sync_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.
 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.
 189Other tunables
 191Per-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   rto_min 250j dev wlan0
 196                > ip route add 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).
 205DCCP does not travel through NAT successfully at present on many boxes. This is
 206because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
 207support for DCCP has been added.
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