Documentation/networking/dccp.txt

   1 DCCP protocol
   2 ============
   3
   4
   5 Contents
   6 ========
   7
   8 - Introduction
   9 - Missing features
  10 - Socket options
  11 - Notes
  12
  13 Introduction
  14 ============
  15
  16 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
  17 based protocol designed to solve issues present in UDP and TCP particularly
  18 for real time and multimedia traffic.
  19
  20 It has a base protocol and pluggable congestion control IDs (CCIDs).
  21
  22 It is at proposed standard RFC status and the homepage for DCCP as a protocol
  23 is at:
  24         http://www.read.cs.ucla.edu/dccp/
  25
  26 Missing features
  27 ================
  28
  29 The DCCP implementation does not currently have all the features that are in
  30 the RFC.
  31
  32 The known bugs are at:
  33         http://linux-net.osdl.org/index.php/TODO#DCCP
  34
  35 Socket options
  36 ==============
  37
  38 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
  39 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
  40 the socket will fall back to 0 (which means that no meaningful service code
  41 is present). On active sockets this is set before connect(); specifying more
  42 than one code has no effect (all subsequent service codes are ignored). The
  43 case is different for passive sockets, where multiple service codes (up to 32)
  44 can be set before calling bind().
  45
  46 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
  47 size (application payload size) in bytes, see RFC 4340, section 14.
  48
  49 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
  50 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
  51 always cover the entire packet and that only fully covered application data is
  52 accepted by the receiver. Hence, when using this feature on the sender, it must
  53 be enabled at the receiver, too with suitable choice of CsCov.
  54
  55 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
  56         range 0..15 are acceptable. The default setting is 0 (full coverage),
  57         values between 1..15 indicate partial coverage.
  58 DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
  59         sets a threshold, where again values 0..15 are acceptable. The default
  60         of 0 means that all packets with a partial coverage will be discarded.
  61         Values in the range 1..15 indicate that packets with minimally such a
  62         coverage value are also acceptable. The higher the number, the more
  63         restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
  64         settings are inherited to the child socket after accept().
  65
  66 The following two options apply to CCID 3 exclusively and are getsockopt()-only.
  67 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
  68 DCCP_SOCKOPT_CCID_RX_INFO
  69         Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
  70         optlen must be set to at least sizeof(struct tfrc_rx_info).
  71 DCCP_SOCKOPT_CCID_TX_INFO
  72         Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
  73         optlen must be set to at least sizeof(struct tfrc_tx_info).
  74
  75
  76 Sysctl variables
  77 ================
  78 Several DCCP default parameters can be managed by the following sysctls
  79 (sysctl net.dccp.default or /proc/sys/net/dccp/default):
  80
  81 request_retries
  82         The number of active connection initiation retries (the number of
  83         Requests minus one) before timing out. In addition, it also governs
  84         the behaviour of the other, passive side: this variable also sets
  85         the number of times DCCP repeats sending a Response when the initial
  86         handshake does not progress from RESPOND to OPEN (i.e. when no Ack
  87         is received after the initial Request).  This value should be greater
  88         than 0, suggested is less than 10. Analogue of tcp_syn_retries.
  89
  90 retries1
  91         How often a DCCP Response is retransmitted until the listening DCCP
  92         side considers its connecting peer dead. Analogue of tcp_retries1.
  93
  94 retries2
  95         The number of times a general DCCP packet is retransmitted. This has
  96         importance for retransmitted acknowledgments and feature negotiation,
  97         data packets are never retransmitted. Analogue of tcp_retries2.
  98
  99 send_ndp = 1
 100         Whether or not to send NDP count options (sec. 7.7.2).
 101
 102 send_ackvec = 1
 103         Whether or not to send Ack Vector options (sec. 11.5).
 104
 105 ack_ratio = 2
 106         The default Ack Ratio (sec. 11.3) to use.
 107
 108 tx_ccid = 2
 109         Default CCID for the sender-receiver half-connection.
 110
 111 rx_ccid = 2
 112         Default CCID for the receiver-sender half-connection.
 113
 114 seq_window = 100
 115         The initial sequence window (sec. 7.5.2).
 116
 117 tx_qlen = 5
 118         The size of the transmit buffer in packets. A value of 0 corresponds
 119         to an unbounded transmit buffer.
 120
 121 sync_ratelimit = 125 ms
 122         The timeout between subsequent DCCP-Sync packets sent in response to
 123         sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
 124         of this parameter is milliseconds; a value of 0 disables rate-limiting.
 125
 126 Notes
 127 =====
 128
 129 DCCP does not travel through NAT successfully at present on many boxes. This is
 130 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
 131 support for DCCP has been added.