| DCCP protocol |
| ============ |
| |
| |
| Contents |
| ======== |
| |
| - Introduction |
| - Missing features |
| - Socket options |
| - Notes |
| |
| Introduction |
| ============ |
| |
| Datagram Congestion Control Protocol (DCCP) is an unreliable, connection |
| based protocol designed to solve issues present in UDP and TCP particularly |
| for real time and multimedia traffic. |
| |
| It has a base protocol and pluggable congestion control IDs (CCIDs). |
| |
| It is at proposed standard RFC status and the homepage for DCCP as a protocol |
| is at: |
| http://www.read.cs.ucla.edu/dccp/ |
| |
| Missing features |
| ================ |
| |
| The DCCP implementation does not currently have all the features that are in |
| the RFC. |
| |
| The known bugs are at: |
| http://linux-net.osdl.org/index.php/TODO#DCCP |
| |
| 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). Connecting sockets set at most one service option; for |
| listening sockets, multiple service codes can be specified. |
| |
| 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_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_SEND_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]). |
| |
| The following two options apply to CCID 3 exclusively and are getsockopt()-only. |
| In either case, a TFRC info struct (defined in <linux/tfrc.h>) 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). |
| |
| |
| 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. |
| |
| Notes |
| ===== |
| |
| DCCP does not travel through NAT successfully at present on many boxes. This is |
| because the checksum covers the psuedo-header as per TCP and UDP. Linux NAT |
| support for DCCP has been added. |