============
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.
+oriented protocol designed to solve issues present in UDP and TCP, particularly
+for real-time and multimedia (streaming) traffic.
+It divides into a base protocol (RFC 4340) and plugable congestion control
+modules called CCIDs. Like plugable TCP congestion control, at least one CCID
+needs to be enabled in order for the protocol to function properly. In the Linux
+implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
+the TCP-friendly CCID3 (RFC 4342), are optional.
+For a brief introduction to CCIDs and suggestions for choosing a CCID to match
+given applications, see section 10 of RFC 4340.
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/
+DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
+is at http://www.ietf.org/html.charters/dccp-charter.html
Missing features
================
-The DCCP implementation does not currently have all the features that are in
-the RFC.
+The Linux DCCP implementation does not currently support all the features that are
+specified in RFCs 4340...42.
The known bugs are at:
http://linux-net.osdl.org/index.php/TODO#DCCP
+For more up-to-date versions of the DCCP implementation, please consider using
+the experimental DCCP test tree; instructions for checking this out are on:
+http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
+
+
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.
+is present). On active sockets this is set before connect(); specifying more
+than one code has no effect (all subsequent service codes are ignored). The
+case is different for passive sockets, where multiple service codes (up to 32)
+can be set before calling bind().
+
+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_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
+supported by the endpoint (see include/linux/dccp.h for symbolic constants).
+The caller needs to provide a sufficiently large (> 2) array of type uint8_t.
+
+DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
+time, combining the operation of the next two socket options. This option is
+preferrable over the latter two, since often applications will use the same
+type of CCID for both directions; and mixed use of CCIDs is not currently well
+understood. This socket option takes as argument at least one uint8_t value, or
+an array of uint8_t values, which must match available CCIDS (see above). CCIDs
+must be registered on the socket before calling connect() or listen().
+
+DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
+the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
+Please note that the getsockopt argument type here is `int', not uint8_t.
+
+DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
+
+DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
+timewait state when closing the connection (RFC 4340, 8.3). The usual case is
+that the closing server sends a CloseReq, whereupon the client holds timewait
+state. When this boolean socket option is on, the server sends a Close instead
+and will enter TIMEWAIT. This option must be set after accept() returns.
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
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
+DCCP_SOCKOPT_RECV_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]).
+ restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
+ settings are inherited to the child socket after accept().
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.
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).
+On unidirectional connections it is useful to close the unused half-connection
+via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
Sysctl variables
================
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.
+ Default CCID for the sender-receiver half-connection. Depending on the
+ choice of CCID, the Send Ack Vector feature is enabled automatically.
rx_ccid = 2
- Default CCID for the receiver-sender half-connection.
+ Default CCID for the receiver-sender half-connection; see tx_ccid.
seq_window = 100
The initial sequence window (sec. 7.5.2).
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.
+
+IOCTLS
+======
+FIONREAD
+ Works as in udp(7): returns in the `int' argument pointer the size of
+ the next pending datagram in bytes, or 0 when no datagram is pending.
+
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
+because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
support for DCCP has been added.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff