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Based on kernel version 2.6.34. Page generated on 2010-05-31 16:03 EST.

1	DCCP protocol
2	============
3	
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	oriented protocol designed to solve issues present in UDP and TCP, particularly
18	for real-time and multimedia (streaming) traffic.
19	It divides into a base protocol (RFC 4340) and plugable congestion control
20	modules called CCIDs. Like plugable TCP congestion control, at least one CCID
21	needs to be enabled in order for the protocol to function properly. In the Linux
22	implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
23	the TCP-friendly CCID3 (RFC 4342), are optional.
24	For a brief introduction to CCIDs and suggestions for choosing a CCID to match
25	given applications, see section 10 of RFC 4340.
26	
27	It has a base protocol and pluggable congestion control IDs (CCIDs).
28	
29	DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
30	is at http://www.ietf.org/html.charters/dccp-charter.html
31	
32	Missing features
33	================
34	
35	The Linux DCCP implementation does not currently support all the features that are
36	specified in RFCs 4340...42.
37	
38	The known bugs are at:
39		http://linux-net.osdl.org/index.php/TODO#DCCP
40	
41	For more up-to-date versions of the DCCP implementation, please consider using
42	the experimental DCCP test tree; instructions for checking this out are on:
43	http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
44	
45	
46	Socket options
47	==============
48	
49	DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
50	service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
51	the socket will fall back to 0 (which means that no meaningful service code
52	is present). On active sockets this is set before connect(); specifying more
53	than one code has no effect (all subsequent service codes are ignored). The
54	case is different for passive sockets, where multiple service codes (up to 32)
55	can be set before calling bind().
56	
57	DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
58	size (application payload size) in bytes, see RFC 4340, section 14.
59	
60	DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
61	supported by the endpoint. The option value is an array of type uint8_t whose
62	size is passed as option length. The minimum array size is 4 elements, the
63	value returned in the optlen argument always reflects the true number of
64	built-in CCIDs.
65	
66	DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
67	time, combining the operation of the next two socket options. This option is
68	preferrable over the latter two, since often applications will use the same
69	type of CCID for both directions; and mixed use of CCIDs is not currently well
70	understood. This socket option takes as argument at least one uint8_t value, or
71	an array of uint8_t values, which must match available CCIDS (see above). CCIDs
72	must be registered on the socket before calling connect() or listen().
73	
74	DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
75	the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
76	Please note that the getsockopt argument type here is `int', not uint8_t.
77	
78	DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
79	
80	DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
81	timewait state when closing the connection (RFC 4340, 8.3). The usual case is
82	that the closing server sends a CloseReq, whereupon the client holds timewait
83	state. When this boolean socket option is on, the server sends a Close instead
84	and will enter TIMEWAIT. This option must be set after accept() returns.
85	
86	DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
87	partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
88	always cover the entire packet and that only fully covered application data is
89	accepted by the receiver. Hence, when using this feature on the sender, it must
90	be enabled at the receiver, too with suitable choice of CsCov.
91	
92	DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
93		range 0..15 are acceptable. The default setting is 0 (full coverage),
94		values between 1..15 indicate partial coverage.
95	DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
96		sets a threshold, where again values 0..15 are acceptable. The default
97		of 0 means that all packets with a partial coverage will be discarded.
98		Values in the range 1..15 indicate that packets with minimally such a
99		coverage value are also acceptable. The higher the number, the more
100		restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
101		settings are inherited to the child socket after accept().
102	
103	The following two options apply to CCID 3 exclusively and are getsockopt()-only.
104	In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
105	DCCP_SOCKOPT_CCID_RX_INFO
106		Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
107		optlen must be set to at least sizeof(struct tfrc_rx_info).
108	DCCP_SOCKOPT_CCID_TX_INFO
109		Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
110		optlen must be set to at least sizeof(struct tfrc_tx_info).
111	
112	On unidirectional connections it is useful to close the unused half-connection
113	via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
114	
115	Sysctl variables
116	================
117	Several DCCP default parameters can be managed by the following sysctls
118	(sysctl net.dccp.default or /proc/sys/net/dccp/default):
119	
120	request_retries
121		The number of active connection initiation retries (the number of
122		Requests minus one) before timing out. In addition, it also governs
123		the behaviour of the other, passive side: this variable also sets
124		the number of times DCCP repeats sending a Response when the initial
125		handshake does not progress from RESPOND to OPEN (i.e. when no Ack
126		is received after the initial Request).  This value should be greater
127		than 0, suggested is less than 10. Analogue of tcp_syn_retries.
128	
129	retries1
130		How often a DCCP Response is retransmitted until the listening DCCP
131		side considers its connecting peer dead. Analogue of tcp_retries1.
132	
133	retries2
134		The number of times a general DCCP packet is retransmitted. This has
135		importance for retransmitted acknowledgments and feature negotiation,
136		data packets are never retransmitted. Analogue of tcp_retries2.
137	
138	tx_ccid = 2
139		Default CCID for the sender-receiver half-connection. Depending on the
140		choice of CCID, the Send Ack Vector feature is enabled automatically.
141	
142	rx_ccid = 2
143		Default CCID for the receiver-sender half-connection; see tx_ccid.
144	
145	seq_window = 100
146		The initial sequence window (sec. 7.5.2) of the sender. This influences
147		the local ackno validity and the remote seqno validity windows (7.5.1).
148	
149	tx_qlen = 5
150		The size of the transmit buffer in packets. A value of 0 corresponds
151		to an unbounded transmit buffer.
152	
153	sync_ratelimit = 125 ms
154		The timeout between subsequent DCCP-Sync packets sent in response to
155		sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
156		of this parameter is milliseconds; a value of 0 disables rate-limiting.
157	
158	IOCTLS
159	======
160	FIONREAD
161		Works as in udp(7): returns in the `int' argument pointer the size of
162		the next pending datagram in bytes, or 0 when no datagram is pending.
163	
164	Notes
165	=====
166	
167	DCCP does not travel through NAT successfully at present on many boxes. This is
168	because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
169	support for DCCP has been added.
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