docs: networking: convert dccp.txt to ReST

.. SPDX-License-Identifier: GPL-2.0

=============

DCCP protocol

=============

Contents

========

.. Contents

   - Introduction

   - Missing features

   - Socket options

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

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:

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_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.

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_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.

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

	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

	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

IOCTLS

======

FIONREAD

	Works as in udp(7): returns in the `int' argument pointer the size of

	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.

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:

	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).

   cdc_mbim

   cops

   cxacru

   dccp

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