Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+DCCP protocol
+=============
+
+
+.. Contents
+   - Introduction
+   - Missing features
+   - Socket options
+   - Sysctl variables
+   - IOCTLs
+   - Other tunables
+   - Notes
+
+
+Introduction
+============
+Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
+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 pluggable congestion control
+modules called CCIDs. Like pluggable 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).
+
+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 Linux DCCP implementation does not currently support all the features that are
+specified in RFCs 4340...42.
+
+The known bugs are at:
+
+	http://www.linuxfoundation.org/collaborate/workgroups/networking/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://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree
+
+
+Socket options
+==============
+DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
+a policy ID as argument and can only be set before the connection (i.e. changes
+during an established connection are not supported). Currently, two policies are
+defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
+and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
+u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
+a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
+be formatted using a cmsg(3) message header filled in as follows::
+
+	cmsg->cmsg_level = SOL_DCCP;
+	cmsg->cmsg_type	 = DCCP_SCM_PRIORITY;
+	cmsg->cmsg_len	 = CMSG_LEN(sizeof(uint32_t));	/* or CMSG_LEN(4) */
+
+DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
+value is always interpreted as unbounded queue length. If different from zero,
+the interpretation of this parameter depends on the current dequeuing policy
+(see above): the "simple" policy will enforce a fixed queue size by returning
+EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
+lowest-priority packet first. The default value for this parameter is
+initialised from /proc/sys/net/dccp/default/tx_qlen.
+
+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). 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. The option value is an array of type uint8_t whose
+size is passed as option length. The minimum array size is 4 elements, the
+value returned in the optlen argument always reflects the true number of
+built-in CCIDs.
+
+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
+preferable 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
+always cover the entire packet and that only fully covered application data is
+accepted by the receiver. Hence, when using this feature on the sender, it must
+be enabled at the receiver, too with suitable choice of CsCov.
+
+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_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]). 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.
+
+DCCP_SOCKOPT_CCID_RX_INFO
+	Returns a ``struct tfrc_rx_info`` in optval; the buffer for optval and
+	optlen must be set to at least sizeof(struct tfrc_rx_info).
+
+DCCP_SOCKOPT_CCID_TX_INFO
+	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
+================
+Several DCCP default parameters can be managed by the following sysctls
+(sysctl net.dccp.default or /proc/sys/net/dccp/default):
+
+request_retries
+	The number of active connection initiation retries (the number of
+	Requests minus one) before timing out. In addition, it also governs
+	the behaviour of the other, passive side: this variable also sets
+	the number of times DCCP repeats sending a Response when the initial
+	handshake does not progress from RESPOND to OPEN (i.e. when no Ack
+	is received after the initial Request).  This value should be greater
+	than 0, suggested is less than 10. Analogue of tcp_syn_retries.
+
+retries1
+	How often a DCCP Response is retransmitted until the listening DCCP
+	side considers its connecting peer dead. Analogue of tcp_retries1.
+
+retries2
+	The number of times a general DCCP packet is retransmitted. This has
+	importance for retransmitted acknowledgments and feature negotiation,
+	data packets are never retransmitted. Analogue of tcp_retries2.
+
+tx_ccid = 2
+	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; see tx_ccid.
+
+seq_window = 100
+	The initial sequence window (sec. 7.5.2) of the sender. This influences
+	the local ackno validity and the remote seqno validity windows (7.5.1).
+	Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
+
+tx_qlen = 5
+	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.
+
+SIOCOUTQ
+	Returns the number of unsent data bytes in the socket send queue as ``int``
+	into the buffer specified by the argument pointer.
+
+Other tunables
+==============
+Per-route rto_min support
+	CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
+	of the RTO timer. This setting can be modified via the 'rto_min' option
+	of iproute2; for example::
+
+		> ip route change 10.0.0.0/24   rto_min 250j dev wlan0
+		> ip route add    10.0.0.254/32 rto_min 800j dev wlan0
+		> ip route show dev wlan0
+
+	CCID-3 also supports the rto_min setting: it is used to define the lower
+	bound for the expiry of the nofeedback timer. This can be useful on LANs
+	with very low RTTs (e.g., loopback, Gbit ethernet).
+
+
+Notes
+=====
+DCCP does not travel through NAT successfully at present on many boxes. This is
+because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
+support for DCCP has been added.