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Based on kernel version 3.9. Page generated on 2013-05-02 23:11 EST.

1	DCCP protocol
2	=============
3	
4	
5	Contents
6	========
7	- Introduction
8	- Missing features
9	- Socket options
10	- Sysctl variables
11	- IOCTLs
12	- Other tunables
13	- Notes
14	
15	
16	Introduction
17	============
18	Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
19	oriented protocol designed to solve issues present in UDP and TCP, particularly
20	for real-time and multimedia (streaming) traffic.
21	It divides into a base protocol (RFC 4340) and plugable congestion control
22	modules called CCIDs. Like plugable TCP congestion control, at least one CCID
23	needs to be enabled in order for the protocol to function properly. In the Linux
24	implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
25	the TCP-friendly CCID3 (RFC 4342), are optional.
26	For a brief introduction to CCIDs and suggestions for choosing a CCID to match
27	given applications, see section 10 of RFC 4340.
28	
29	It has a base protocol and pluggable congestion control IDs (CCIDs).
30	
31	DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
32	is at http://www.ietf.org/html.charters/dccp-charter.html
33	
34	
35	Missing features
36	================
37	The Linux DCCP implementation does not currently support all the features that are
38	specified in RFCs 4340...42.
39	
40	The known bugs are at:
41		http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP
42	
43	For more up-to-date versions of the DCCP implementation, please consider using
44	the experimental DCCP test tree; instructions for checking this out are on:
45	http://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree
46	
47	
48	Socket options
49	==============
50	DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
51	a policy ID as argument and can only be set before the connection (i.e. changes
52	during an established connection are not supported). Currently, two policies are
53	defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
54	and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
55	u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
56	a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
57	be formatted using a cmsg(3) message header filled in as follows:
58		cmsg->cmsg_level = SOL_DCCP;
59		cmsg->cmsg_type	 = DCCP_SCM_PRIORITY;
60		cmsg->cmsg_len	 = CMSG_LEN(sizeof(uint32_t));	/* or CMSG_LEN(4) */
61	
62	DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
63	value is always interpreted as unbounded queue length. If different from zero,
64	the interpretation of this parameter depends on the current dequeuing policy
65	(see above): the "simple" policy will enforce a fixed queue size by returning
66	EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
67	lowest-priority packet first. The default value for this parameter is
68	initialised from /proc/sys/net/dccp/default/tx_qlen.
69	
70	DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
71	service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
72	the socket will fall back to 0 (which means that no meaningful service code
73	is present). On active sockets this is set before connect(); specifying more
74	than one code has no effect (all subsequent service codes are ignored). The
75	case is different for passive sockets, where multiple service codes (up to 32)
76	can be set before calling bind().
77	
78	DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
79	size (application payload size) in bytes, see RFC 4340, section 14.
80	
81	DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
82	supported by the endpoint. The option value is an array of type uint8_t whose
83	size is passed as option length. The minimum array size is 4 elements, the
84	value returned in the optlen argument always reflects the true number of
85	built-in CCIDs.
86	
87	DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
88	time, combining the operation of the next two socket options. This option is
89	preferrable over the latter two, since often applications will use the same
90	type of CCID for both directions; and mixed use of CCIDs is not currently well
91	understood. This socket option takes as argument at least one uint8_t value, or
92	an array of uint8_t values, which must match available CCIDS (see above). CCIDs
93	must be registered on the socket before calling connect() or listen().
94	
95	DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
96	the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
97	Please note that the getsockopt argument type here is `int', not uint8_t.
98	
99	DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
100	
101	DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
102	timewait state when closing the connection (RFC 4340, 8.3). The usual case is
103	that the closing server sends a CloseReq, whereupon the client holds timewait
104	state. When this boolean socket option is on, the server sends a Close instead
105	and will enter TIMEWAIT. This option must be set after accept() returns.
106	
107	DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
108	partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
109	always cover the entire packet and that only fully covered application data is
110	accepted by the receiver. Hence, when using this feature on the sender, it must
111	be enabled at the receiver, too with suitable choice of CsCov.
112	
113	DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
114		range 0..15 are acceptable. The default setting is 0 (full coverage),
115		values between 1..15 indicate partial coverage.
116	DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
117		sets a threshold, where again values 0..15 are acceptable. The default
118		of 0 means that all packets with a partial coverage will be discarded.
119		Values in the range 1..15 indicate that packets with minimally such a
120		coverage value are also acceptable. The higher the number, the more
121		restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
122		settings are inherited to the child socket after accept().
123	
124	The following two options apply to CCID 3 exclusively and are getsockopt()-only.
125	In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
126	DCCP_SOCKOPT_CCID_RX_INFO
127		Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
128		optlen must be set to at least sizeof(struct tfrc_rx_info).
129	DCCP_SOCKOPT_CCID_TX_INFO
130		Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
131		optlen must be set to at least sizeof(struct tfrc_tx_info).
132	
133	On unidirectional connections it is useful to close the unused half-connection
134	via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
135	
136	
137	Sysctl variables
138	================
139	Several DCCP default parameters can be managed by the following sysctls
140	(sysctl net.dccp.default or /proc/sys/net/dccp/default):
141	
142	request_retries
143		The number of active connection initiation retries (the number of
144		Requests minus one) before timing out. In addition, it also governs
145		the behaviour of the other, passive side: this variable also sets
146		the number of times DCCP repeats sending a Response when the initial
147		handshake does not progress from RESPOND to OPEN (i.e. when no Ack
148		is received after the initial Request).  This value should be greater
149		than 0, suggested is less than 10. Analogue of tcp_syn_retries.
150	
151	retries1
152		How often a DCCP Response is retransmitted until the listening DCCP
153		side considers its connecting peer dead. Analogue of tcp_retries1.
154	
155	retries2
156		The number of times a general DCCP packet is retransmitted. This has
157		importance for retransmitted acknowledgments and feature negotiation,
158		data packets are never retransmitted. Analogue of tcp_retries2.
159	
160	tx_ccid = 2
161		Default CCID for the sender-receiver half-connection. Depending on the
162		choice of CCID, the Send Ack Vector feature is enabled automatically.
163	
164	rx_ccid = 2
165		Default CCID for the receiver-sender half-connection; see tx_ccid.
166	
167	seq_window = 100
168		The initial sequence window (sec. 7.5.2) of the sender. This influences
169		the local ackno validity and the remote seqno validity windows (7.5.1).
170		Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
171	
172	tx_qlen = 5
173		The size of the transmit buffer in packets. A value of 0 corresponds
174		to an unbounded transmit buffer.
175	
176	sync_ratelimit = 125 ms
177		The timeout between subsequent DCCP-Sync packets sent in response to
178		sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
179		of this parameter is milliseconds; a value of 0 disables rate-limiting.
180	
181	
182	IOCTLS
183	======
184	FIONREAD
185		Works as in udp(7): returns in the `int' argument pointer the size of
186		the next pending datagram in bytes, or 0 when no datagram is pending.
187	
188	
189	Other tunables
190	==============
191	Per-route rto_min support
192		CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
193		of the RTO timer. This setting can be modified via the 'rto_min' option
194		of iproute2; for example:
195			> ip route change 10.0.0.0/24   rto_min 250j dev wlan0
196			> ip route add    10.0.0.254/32 rto_min 800j dev wlan0
197			> ip route show dev wlan0
198		CCID-3 also supports the rto_min setting: it is used to define the lower
199		bound for the expiry of the nofeedback timer. This can be useful on LANs
200		with very low RTTs (e.g., loopback, Gbit ethernet).
201	
202	
203	Notes
204	=====
205	DCCP does not travel through NAT successfully at present on many boxes. This is
206	because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
207	support for DCCP has been added.
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