DCCP protocol ============ Contents ======== - Introduction - Missing features - Socket options - Notes Introduction ============ Datagram Congestion Control Protocol (DCCP) is an unreliable, connection oriented protocol designed to solve issues present in UDP and TCP, particularly for real-time and multimedia (streaming) traffic. It divides into a base protocol (RFC 4340) and plugable congestion control modules called CCIDs. Like plugable TCP congestion control, at least one CCID needs to be enabled in order for the protocol to function properly. In the Linux implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as the TCP-friendly CCID3 (RFC 4342), are optional. For a brief introduction to CCIDs and suggestions for choosing a CCID to match given applications, see section 10 of RFC 4340. It has a base protocol and pluggable congestion control IDs (CCIDs). DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol is at http://www.ietf.org/html.charters/dccp-charter.html Missing features ================ The Linux DCCP implementation does not currently support all the features that are specified in RFCs 4340...42. The known bugs are at: http://linux-net.osdl.org/index.php/TODO#DCCP For more up-to-date versions of the DCCP implementation, please consider using the experimental DCCP test tree; instructions for checking this out are on: http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree Socket options ============== DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of service codes (RFC 4340, sec. 8.1.2); if this socket option is not set, the socket will fall back to 0 (which means that no meaningful service code is present). On active sockets this is set before connect(); specifying more than one code has no effect (all subsequent service codes are ignored). The case is different for passive sockets, where multiple service codes (up to 32) can be set before calling bind(). DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet size (application payload size) in bytes, see RFC 4340, section 14. DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs supported by the endpoint (see include/linux/dccp.h for symbolic constants). The caller needs to provide a sufficiently large (> 2) array of type uint8_t. DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold timewait state when closing the connection (RFC 4340, 8.3). The usual case is that the closing server sends a CloseReq, whereupon the client holds timewait state. When this boolean socket option is on, the server sends a Close instead and will enter TIMEWAIT. This option must be set after accept() returns. DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums always cover the entire packet and that only fully covered application data is accepted by the receiver. Hence, when using this feature on the sender, it must be enabled at the receiver, too with suitable choice of CsCov. DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the range 0..15 are acceptable. The default setting is 0 (full coverage), values between 1..15 indicate partial coverage. DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it sets a threshold, where again values 0..15 are acceptable. The default of 0 means that all packets with a partial coverage will be discarded. Values in the range 1..15 indicate that packets with minimally such a coverage value are also acceptable. The higher the number, the more restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage settings are inherited to the child socket after accept(). The following two options apply to CCID 3 exclusively and are getsockopt()-only. In either case, a TFRC info struct (defined in ) is returned. DCCP_SOCKOPT_CCID_RX_INFO Returns a `struct tfrc_rx_info' in optval; the buffer for optval and optlen must be set to at least sizeof(struct tfrc_rx_info). DCCP_SOCKOPT_CCID_TX_INFO Returns a `struct tfrc_tx_info' in optval; the buffer for optval and optlen must be set to at least sizeof(struct tfrc_tx_info). On unidirectional connections it is useful to close the unused half-connection via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs. Sysctl variables ================ Several DCCP default parameters can be managed by the following sysctls (sysctl net.dccp.default or /proc/sys/net/dccp/default): request_retries The number of active connection initiation retries (the number of Requests minus one) before timing out. In addition, it also governs the behaviour of the other, passive side: this variable also sets the number of times DCCP repeats sending a Response when the initial handshake does not progress from RESPOND to OPEN (i.e. when no Ack is received after the initial Request). This value should be greater than 0, suggested is less than 10. Analogue of tcp_syn_retries. retries1 How often a DCCP Response is retransmitted until the listening DCCP side considers its connecting peer dead. Analogue of tcp_retries1. retries2 The number of times a general DCCP packet is retransmitted. This has importance for retransmitted acknowledgments and feature negotiation, data packets are never retransmitted. Analogue of tcp_retries2. send_ndp = 1 Whether or not to send NDP count options (sec. 7.7.2). send_ackvec = 1 Whether or not to send Ack Vector options (sec. 11.5). ack_ratio = 2 The default Ack Ratio (sec. 11.3) to use. tx_ccid = 2 Default CCID for the sender-receiver half-connection. rx_ccid = 2 Default CCID for the receiver-sender half-connection. seq_window = 100 The initial sequence window (sec. 7.5.2). tx_qlen = 5 The size of the transmit buffer in packets. A value of 0 corresponds to an unbounded transmit buffer. sync_ratelimit = 125 ms The timeout between subsequent DCCP-Sync packets sent in response to sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit of this parameter is milliseconds; a value of 0 disables rate-limiting. IOCTLS ====== FIONREAD Works as in udp(7): returns in the `int' argument pointer the size of the next pending datagram in bytes, or 0 when no datagram is pending. Notes ===== DCCP does not travel through NAT successfully at present on many boxes. This is because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT support for DCCP has been added.