linux/Documentation/networking/dccp.txt
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   1DCCP protocol
   2============
   3
   4
   5Contents
   6========
   7
   8- Introduction
   9- Missing features
  10- Socket options
  11- Notes
  12
  13Introduction
  14============
  15
  16Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
  17oriented protocol designed to solve issues present in UDP and TCP, particularly
  18for real-time and multimedia (streaming) traffic.
  19It divides into a base protocol (RFC 4340) and plugable congestion control
  20modules called CCIDs. Like plugable TCP congestion control, at least one CCID
  21needs to be enabled in order for the protocol to function properly. In the Linux
  22implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
  23the TCP-friendly CCID3 (RFC 4342), are optional.
  24For a brief introduction to CCIDs and suggestions for choosing a CCID to match
  25given applications, see section 10 of RFC 4340.
  26
  27It has a base protocol and pluggable congestion control IDs (CCIDs).
  28
  29DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
  30is at http://www.ietf.org/html.charters/dccp-charter.html
  31
  32Missing features
  33================
  34
  35The Linux DCCP implementation does not currently support all the features that are
  36specified in RFCs 4340...42.
  37
  38The known bugs are at:
  39        http://linux-net.osdl.org/index.php/TODO#DCCP
  40
  41For more up-to-date versions of the DCCP implementation, please consider using
  42the experimental DCCP test tree; instructions for checking this out are on:
  43http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
  44
  45
  46Socket options
  47==============
  48
  49DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
  50service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
  51the socket will fall back to 0 (which means that no meaningful service code
  52is present). On active sockets this is set before connect(); specifying more
  53than one code has no effect (all subsequent service codes are ignored). The
  54case is different for passive sockets, where multiple service codes (up to 32)
  55can be set before calling bind().
  56
  57DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
  58size (application payload size) in bytes, see RFC 4340, section 14.
  59
  60DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
  61supported by the endpoint (see include/linux/dccp.h for symbolic constants).
  62The caller needs to provide a sufficiently large (> 2) array of type uint8_t.
  63
  64DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
  65time, combining the operation of the next two socket options. This option is
  66preferrable over the latter two, since often applications will use the same
  67type of CCID for both directions; and mixed use of CCIDs is not currently well
  68understood. This socket option takes as argument at least one uint8_t value, or
  69an array of uint8_t values, which must match available CCIDS (see above). CCIDs
  70must be registered on the socket before calling connect() or listen().
  71
  72DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
  73the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
  74Please note that the getsockopt argument type here is `int', not uint8_t.
  75
  76DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
  77
  78DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
  79timewait state when closing the connection (RFC 4340, 8.3). The usual case is
  80that the closing server sends a CloseReq, whereupon the client holds timewait
  81state. When this boolean socket option is on, the server sends a Close instead
  82and will enter TIMEWAIT. This option must be set after accept() returns.
  83
  84DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
  85partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
  86always cover the entire packet and that only fully covered application data is
  87accepted by the receiver. Hence, when using this feature on the sender, it must
  88be enabled at the receiver, too with suitable choice of CsCov.
  89
  90DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
  91        range 0..15 are acceptable. The default setting is 0 (full coverage),
  92        values between 1..15 indicate partial coverage.
  93DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
  94        sets a threshold, where again values 0..15 are acceptable. The default
  95        of 0 means that all packets with a partial coverage will be discarded.
  96        Values in the range 1..15 indicate that packets with minimally such a
  97        coverage value are also acceptable. The higher the number, the more
  98        restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
  99        settings are inherited to the child socket after accept().
 100
 101The following two options apply to CCID 3 exclusively and are getsockopt()-only.
 102In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
 103DCCP_SOCKOPT_CCID_RX_INFO
 104        Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
 105        optlen must be set to at least sizeof(struct tfrc_rx_info).
 106DCCP_SOCKOPT_CCID_TX_INFO
 107        Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
 108        optlen must be set to at least sizeof(struct tfrc_tx_info).
 109
 110On unidirectional connections it is useful to close the unused half-connection
 111via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
 112
 113Sysctl variables
 114================
 115Several DCCP default parameters can be managed by the following sysctls
 116(sysctl net.dccp.default or /proc/sys/net/dccp/default):
 117
 118request_retries
 119        The number of active connection initiation retries (the number of
 120        Requests minus one) before timing out. In addition, it also governs
 121        the behaviour of the other, passive side: this variable also sets
 122        the number of times DCCP repeats sending a Response when the initial
 123        handshake does not progress from RESPOND to OPEN (i.e. when no Ack
 124        is received after the initial Request).  This value should be greater
 125        than 0, suggested is less than 10. Analogue of tcp_syn_retries.
 126
 127retries1
 128        How often a DCCP Response is retransmitted until the listening DCCP
 129        side considers its connecting peer dead. Analogue of tcp_retries1.
 130
 131retries2
 132        The number of times a general DCCP packet is retransmitted. This has
 133        importance for retransmitted acknowledgments and feature negotiation,
 134        data packets are never retransmitted. Analogue of tcp_retries2.
 135
 136tx_ccid = 2
 137        Default CCID for the sender-receiver half-connection. Depending on the
 138        choice of CCID, the Send Ack Vector feature is enabled automatically.
 139
 140rx_ccid = 2
 141        Default CCID for the receiver-sender half-connection; see tx_ccid.
 142
 143seq_window = 100
 144        The initial sequence window (sec. 7.5.2) of the sender. This influences
 145        the local ackno validity and the remote seqno validity windows (7.5.1).
 146
 147tx_qlen = 5
 148        The size of the transmit buffer in packets. A value of 0 corresponds
 149        to an unbounded transmit buffer.
 150
 151sync_ratelimit = 125 ms
 152        The timeout between subsequent DCCP-Sync packets sent in response to
 153        sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
 154        of this parameter is milliseconds; a value of 0 disables rate-limiting.
 155
 156IOCTLS
 157======
 158FIONREAD
 159        Works as in udp(7): returns in the `int' argument pointer the size of
 160        the next pending datagram in bytes, or 0 when no datagram is pending.
 161
 162Notes
 163=====
 164
 165DCCP does not travel through NAT successfully at present on many boxes. This is
 166because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
 167support for DCCP has been added.
 168
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