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131 lines
4.8 KiB
131 lines
4.8 KiB
DCCP protocol
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============
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Contents
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========
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- Introduction
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- Missing features
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- Socket options
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- Notes
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Introduction
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============
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Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
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based protocol designed to solve issues present in UDP and TCP particularly
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for real time and multimedia traffic.
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It has a base protocol and pluggable congestion control IDs (CCIDs).
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It is at proposed standard RFC status and the homepage for DCCP as a protocol
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is at:
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http://www.read.cs.ucla.edu/dccp/
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Missing features
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================
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The DCCP implementation does not currently have all the features that are in
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the RFC.
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The known bugs are at:
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http://linux-net.osdl.org/index.php/TODO#DCCP
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Socket options
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==============
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DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
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service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
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the socket will fall back to 0 (which means that no meaningful service code
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is present). On active sockets this is set before connect(); specifying more
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than one code has no effect (all subsequent service codes are ignored). The
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case is different for passive sockets, where multiple service codes (up to 32)
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can be set before calling bind().
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DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
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size (application payload size) in bytes, see RFC 4340, section 14.
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DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
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partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
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always cover the entire packet and that only fully covered application data is
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accepted by the receiver. Hence, when using this feature on the sender, it must
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be enabled at the receiver, too with suitable choice of CsCov.
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DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
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range 0..15 are acceptable. The default setting is 0 (full coverage),
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values between 1..15 indicate partial coverage.
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DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
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sets a threshold, where again values 0..15 are acceptable. The default
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of 0 means that all packets with a partial coverage will be discarded.
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Values in the range 1..15 indicate that packets with minimally such a
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coverage value are also acceptable. The higher the number, the more
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restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
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settings are inherited to the child socket after accept().
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The following two options apply to CCID 3 exclusively and are getsockopt()-only.
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In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
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DCCP_SOCKOPT_CCID_RX_INFO
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Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
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optlen must be set to at least sizeof(struct tfrc_rx_info).
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DCCP_SOCKOPT_CCID_TX_INFO
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Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
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optlen must be set to at least sizeof(struct tfrc_tx_info).
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Sysctl variables
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================
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Several DCCP default parameters can be managed by the following sysctls
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(sysctl net.dccp.default or /proc/sys/net/dccp/default):
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request_retries
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The number of active connection initiation retries (the number of
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Requests minus one) before timing out. In addition, it also governs
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the behaviour of the other, passive side: this variable also sets
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the number of times DCCP repeats sending a Response when the initial
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handshake does not progress from RESPOND to OPEN (i.e. when no Ack
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is received after the initial Request). This value should be greater
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than 0, suggested is less than 10. Analogue of tcp_syn_retries.
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retries1
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How often a DCCP Response is retransmitted until the listening DCCP
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side considers its connecting peer dead. Analogue of tcp_retries1.
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retries2
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The number of times a general DCCP packet is retransmitted. This has
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importance for retransmitted acknowledgments and feature negotiation,
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data packets are never retransmitted. Analogue of tcp_retries2.
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send_ndp = 1
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Whether or not to send NDP count options (sec. 7.7.2).
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send_ackvec = 1
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Whether or not to send Ack Vector options (sec. 11.5).
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ack_ratio = 2
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The default Ack Ratio (sec. 11.3) to use.
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tx_ccid = 2
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Default CCID for the sender-receiver half-connection.
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rx_ccid = 2
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Default CCID for the receiver-sender half-connection.
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seq_window = 100
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The initial sequence window (sec. 7.5.2).
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tx_qlen = 5
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The size of the transmit buffer in packets. A value of 0 corresponds
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to an unbounded transmit buffer.
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sync_ratelimit = 125 ms
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The timeout between subsequent DCCP-Sync packets sent in response to
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sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
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of this parameter is milliseconds; a value of 0 disables rate-limiting.
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Notes
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=====
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DCCP does not travel through NAT successfully at present on many boxes. This is
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because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
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support for DCCP has been added.
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