[mirror_ubuntu-bionic-kernel.git] / Documentation / networking / dccp.txt

DCCP protocol
============


Contents
========

- Introduction
- Missing features
- Socket options
- Notes

Introduction
============

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.

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/

Missing features
================

The DCCP implementation does not currently have all the features that are in
the RFC.

The known bugs are at:
	http://linux-net.osdl.org/index.php/TODO#DCCP

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.

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_SEND_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]).

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).


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.

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.

rx_ccid = 2
	Default CCID for the receiver-sender half-connection.

seq_window = 100
	The initial sequence window (sec. 7.5.2).

tx_qlen = 5
	The size of the transmit buffer in packets. A value of 0 corresponds
	to an unbounded transmit buffer.

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
support for DCCP has been added.
Commit	Line	Data
98069ff4 IM	1	DCCP protocol
	2	============
	3
98069ff4 IM	4
	5	Contents
	6	========
	7
	8	- Introduction
	9	- Missing features
	10	- Socket options
	11	- Notes
	12
	13	Introduction
	14	============
	15
	16	Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
	17	based protocol designed to solve issues present in UDP and TCP particularly
	18	for real time and multimedia traffic.
	19
	20	It has a base protocol and pluggable congestion control IDs (CCIDs).
	21
5fce9a2d IM	22	It is at proposed standard RFC status and the homepage for DCCP as a protocol
5fce9a2d IM	23	is at:
ddfe10b8	24	http://www.read.cs.ucla.edu/dccp/
98069ff4 IM	25
	26	Missing features
	27	================
	28
	29	The DCCP implementation does not currently have all the features that are in
ddfe10b8	30	the RFC.
98069ff4	31
ddfe10b8 IM	32	The known bugs are at:
ddfe10b8 IM	33	http://linux-net.osdl.org/index.php/TODO#DCCP
98069ff4 IM	34
	35	Socket options
	36	==============
	37
00e4d116 GR	38	DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
	39	service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
	40	the socket will fall back to 0 (which means that no meaningful service code
	41	is present). Connecting sockets set at most one service option; for
	42	listening sockets, multiple service codes can be specified.
98069ff4	43
6f4e5fff GR	44	DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
	45	partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
	46	always cover the entire packet and that only fully covered application data is
	47	accepted by the receiver. Hence, when using this feature on the sender, it must
	48	be enabled at the receiver, too with suitable choice of CsCov.
	49
	50	DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
	51	range 0..15 are acceptable. The default setting is 0 (full coverage),
	52	values between 1..15 indicate partial coverage.
	53	DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it
	54	sets a threshold, where again values 0..15 are acceptable. The default
	55	of 0 means that all packets with a partial coverage will be discarded.
	56	Values in the range 1..15 indicate that packets with minimally such a
	57	coverage value are also acceptable. The higher the number, the more
	58	restrictive this setting (see [RFC 4340, sec. 9.2.1]).
	59
f2645101 GR	60	The following two options apply to CCID 3 exclusively and are getsockopt()-only.
	61	In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
	62	DCCP_SOCKOPT_CCID_RX_INFO
	63	Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
	64	optlen must be set to at least sizeof(struct tfrc_rx_info).
	65	DCCP_SOCKOPT_CCID_TX_INFO
	66	Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
	67	optlen must be set to at least sizeof(struct tfrc_tx_info).
	68
	69
2e2e9e92 GR	70	Sysctl variables
	71	================
	72	Several DCCP default parameters can be managed by the following sysctls
	73	(sysctl net.dccp.default or /proc/sys/net/dccp/default):
	74
	75	request_retries
	76	The number of active connection initiation retries (the number of
	77	Requests minus one) before timing out. In addition, it also governs
	78	the behaviour of the other, passive side: this variable also sets
	79	the number of times DCCP repeats sending a Response when the initial
	80	handshake does not progress from RESPOND to OPEN (i.e. when no Ack
	81	is received after the initial Request). This value should be greater
	82	than 0, suggested is less than 10. Analogue of tcp_syn_retries.
	83
	84	retries1
	85	How often a DCCP Response is retransmitted until the listening DCCP
	86	side considers its connecting peer dead. Analogue of tcp_retries1.
	87
	88	retries2
	89	The number of times a general DCCP packet is retransmitted. This has
	90	importance for retransmitted acknowledgments and feature negotiation,
	91	data packets are never retransmitted. Analogue of tcp_retries2.
	92
	93	send_ndp = 1
	94	Whether or not to send NDP count options (sec. 7.7.2).
	95
	96	send_ackvec = 1
	97	Whether or not to send Ack Vector options (sec. 11.5).
	98
	99	ack_ratio = 2
	100	The default Ack Ratio (sec. 11.3) to use.
	101
	102	tx_ccid = 2
	103	Default CCID for the sender-receiver half-connection.
	104
	105	rx_ccid = 2
	106	Default CCID for the receiver-sender half-connection.
	107
	108	seq_window = 100
	109	The initial sequence window (sec. 7.5.2).
	110
82e3ab9d IM	111	tx_qlen = 5
	112	The size of the transmit buffer in packets. A value of 0 corresponds
	113	to an unbounded transmit buffer.
	114
98069ff4 IM	115	Notes
	116	=====
	117
ddfe10b8 IM	118	DCCP does not travel through NAT successfully at present on many boxes. This is
	119	because the checksum covers the psuedo-header as per TCP and UDP. Linux NAT
	120	support for DCCP has been added.