[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). 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_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.

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.

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.

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.
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
126acd5b GR	41	is present). On active sockets this is set before connect(); specifying more
	42	than one code has no effect (all subsequent service codes are ignored). The
	43	case is different for passive sockets, where multiple service codes (up to 32)
	44	can be set before calling bind().
98069ff4	45
7c559a9e GR	46	DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
	47	size (application payload size) in bytes, see RFC 4340, section 14.
	48
6f4e5fff GR	49	DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
	50	partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
	51	always cover the entire packet and that only fully covered application data is
	52	accepted by the receiver. Hence, when using this feature on the sender, it must
	53	be enabled at the receiver, too with suitable choice of CsCov.
	54
	55	DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
	56	range 0..15 are acceptable. The default setting is 0 (full coverage),
	57	values between 1..15 indicate partial coverage.
2bfd754d	58	DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
6f4e5fff GR	59	sets a threshold, where again values 0..15 are acceptable. The default
	60	of 0 means that all packets with a partial coverage will be discarded.
	61	Values in the range 1..15 indicate that packets with minimally such a
	62	coverage value are also acceptable. The higher the number, the more
2bfd754d GR	63	restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
2bfd754d GR	64	settings are inherited to the child socket after accept().
6f4e5fff	65
f2645101 GR	66	The following two options apply to CCID 3 exclusively and are getsockopt()-only.
	67	In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
	68	DCCP_SOCKOPT_CCID_RX_INFO
	69	Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
	70	optlen must be set to at least sizeof(struct tfrc_rx_info).
	71	DCCP_SOCKOPT_CCID_TX_INFO
	72	Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
	73	optlen must be set to at least sizeof(struct tfrc_tx_info).
	74
8e8c71f1 GR	75	On unidirectional connections it is useful to close the unused half-connection
8e8c71f1 GR	76	via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
f2645101	77
2e2e9e92 GR	78	Sysctl variables
	79	================
	80	Several DCCP default parameters can be managed by the following sysctls
	81	(sysctl net.dccp.default or /proc/sys/net/dccp/default):
	82
	83	request_retries
	84	The number of active connection initiation retries (the number of
	85	Requests minus one) before timing out. In addition, it also governs
	86	the behaviour of the other, passive side: this variable also sets
	87	the number of times DCCP repeats sending a Response when the initial
	88	handshake does not progress from RESPOND to OPEN (i.e. when no Ack
	89	is received after the initial Request). This value should be greater
	90	than 0, suggested is less than 10. Analogue of tcp_syn_retries.
	91
	92	retries1
	93	How often a DCCP Response is retransmitted until the listening DCCP
	94	side considers its connecting peer dead. Analogue of tcp_retries1.
	95
	96	retries2
	97	The number of times a general DCCP packet is retransmitted. This has
	98	importance for retransmitted acknowledgments and feature negotiation,
	99	data packets are never retransmitted. Analogue of tcp_retries2.
	100
	101	send_ndp = 1
	102	Whether or not to send NDP count options (sec. 7.7.2).
	103
	104	send_ackvec = 1
	105	Whether or not to send Ack Vector options (sec. 11.5).
	106
	107	ack_ratio = 2
	108	The default Ack Ratio (sec. 11.3) to use.
	109
	110	tx_ccid = 2
	111	Default CCID for the sender-receiver half-connection.
	112
	113	rx_ccid = 2
	114	Default CCID for the receiver-sender half-connection.
	115
	116	seq_window = 100
	117	The initial sequence window (sec. 7.5.2).
	118
82e3ab9d IM	119	tx_qlen = 5
	120	The size of the transmit buffer in packets. A value of 0 corresponds
	121	to an unbounded transmit buffer.
	122
a94f0f97 GR	123	sync_ratelimit = 125 ms
	124	The timeout between subsequent DCCP-Sync packets sent in response to
	125	sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
	126	of this parameter is milliseconds; a value of 0 disables rate-limiting.
	127
98069ff4 IM	128	Notes
	129	=====
	130
ddfe10b8	131	DCCP does not travel through NAT successfully at present on many boxes. This is
126acd5b	132	because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
ddfe10b8	133	support for DCCP has been added.