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). Connecting sockets set at most one service option; for
  42 listening sockets, multiple service codes can be specified.
  43
  44 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
  45 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
  46 always cover the entire packet and that only fully covered application data is
  47 accepted by the receiver. Hence, when using this feature on the sender, it must
  48 be enabled at the receiver, too with suitable choice of CsCov.
  49
  50 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
  51         range 0..15 are acceptable. The default setting is 0 (full coverage),
  52         values between 1..15 indicate partial coverage.
  53 DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it
  54         sets a threshold, where again values 0..15 are acceptable. The default
  55         of 0 means that all packets with a partial coverage will be discarded.
  56         Values in the range 1..15 indicate that packets with minimally such a
  57         coverage value are also acceptable. The higher the number, the more
  58         restrictive this setting (see [RFC 4340, sec. 9.2.1]).
  59
  60 The following two options apply to CCID 3 exclusively and are getsockopt()-only.
  61 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
  62 DCCP_SOCKOPT_CCID_RX_INFO
  63         Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
  64         optlen must be set to at least sizeof(struct tfrc_rx_info).
  65 DCCP_SOCKOPT_CCID_TX_INFO
  66         Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
  67         optlen must be set to at least sizeof(struct tfrc_tx_info).
  68
  69
  70 Sysctl variables
  71 ================
  72 Several DCCP default parameters can be managed by the following sysctls
  73 (sysctl net.dccp.default or /proc/sys/net/dccp/default):
  74
  75 request_retries
  76         The number of active connection initiation retries (the number of
  77         Requests minus one) before timing out. In addition, it also governs
  78         the behaviour of the other, passive side: this variable also sets
  79         the number of times DCCP repeats sending a Response when the initial
  80         handshake does not progress from RESPOND to OPEN (i.e. when no Ack
  81         is received after the initial Request).  This value should be greater
  82         than 0, suggested is less than 10. Analogue of tcp_syn_retries.
  83
  84 retries1
  85         How often a DCCP Response is retransmitted until the listening DCCP
  86         side considers its connecting peer dead. Analogue of tcp_retries1.
  87
  88 retries2
  89         The number of times a general DCCP packet is retransmitted. This has
  90         importance for retransmitted acknowledgments and feature negotiation,
  91         data packets are never retransmitted. Analogue of tcp_retries2.
  92
  93 send_ndp = 1
  94         Whether or not to send NDP count options (sec. 7.7.2).
  95
  96 send_ackvec = 1
  97         Whether or not to send Ack Vector options (sec. 11.5).
  98
  99 ack_ratio = 2
 100         The default Ack Ratio (sec. 11.3) to use.
 101
 102 tx_ccid = 2
 103         Default CCID for the sender-receiver half-connection.
 104
 105 rx_ccid = 2
 106         Default CCID for the receiver-sender half-connection.
 107
 108 seq_window = 100
 109         The initial sequence window (sec. 7.5.2).
 110
 111 tx_qlen = 5
 112         The size of the transmit buffer in packets. A value of 0 corresponds
 113         to an unbounded transmit buffer.
 114
 115 Notes
 116 =====
 117
 118 DCCP does not travel through NAT successfully at present on many boxes. This is
 119 because the checksum covers the psuedo-header as per TCP and UDP. Linux NAT
 120 support for DCCP has been added.