1.. SPDX-License-Identifier: GPL-2.0
2
3=============
4DCCP protocol
5=============
6
7
8.. Contents
9   - Introduction
10   - Missing features
11   - Socket options
12   - Sysctl variables
13   - IOCTLs
14   - Other tunables
15   - Notes
16
17
18Introduction
19============
20Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
21oriented protocol designed to solve issues present in UDP and TCP, particularly
22for real-time and multimedia (streaming) traffic.
23It divides into a base protocol (RFC 4340) and pluggable congestion control
24modules called CCIDs. Like pluggable TCP congestion control, at least one CCID
25needs to be enabled in order for the protocol to function properly. In the Linux
26implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
27the TCP-friendly CCID3 (RFC 4342), are optional.
28For a brief introduction to CCIDs and suggestions for choosing a CCID to match
29given applications, see section 10 of RFC 4340.
30
31It has a base protocol and pluggable congestion control IDs (CCIDs).
32
33DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
34is at http://www.ietf.org/html.charters/dccp-charter.html
35
36
37Missing features
38================
39The Linux DCCP implementation does not currently support all the features that are
40specified in RFCs 4340...42.
41
42The known bugs are at:
43
44	http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP
45
46For more up-to-date versions of the DCCP implementation, please consider using
47the experimental DCCP test tree; instructions for checking this out are on:
48http://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree
49
50
51Socket options
52==============
53DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
54a policy ID as argument and can only be set before the connection (i.e. changes
55during an established connection are not supported). Currently, two policies are
56defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
57and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
58u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
59a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
60be formatted using a cmsg(3) message header filled in as follows::
61
62	cmsg->cmsg_level = SOL_DCCP;
63	cmsg->cmsg_type	 = DCCP_SCM_PRIORITY;
64	cmsg->cmsg_len	 = CMSG_LEN(sizeof(uint32_t));	/* or CMSG_LEN(4) */
65
66DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
67value is always interpreted as unbounded queue length. If different from zero,
68the interpretation of this parameter depends on the current dequeuing policy
69(see above): the "simple" policy will enforce a fixed queue size by returning
70EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
71lowest-priority packet first. The default value for this parameter is
72initialised from /proc/sys/net/dccp/default/tx_qlen.
73
74DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
75service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
76the socket will fall back to 0 (which means that no meaningful service code
77is present). On active sockets this is set before connect(); specifying more
78than one code has no effect (all subsequent service codes are ignored). The
79case is different for passive sockets, where multiple service codes (up to 32)
80can be set before calling bind().
81
82DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
83size (application payload size) in bytes, see RFC 4340, section 14.
84
85DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
86supported by the endpoint. The option value is an array of type uint8_t whose
87size is passed as option length. The minimum array size is 4 elements, the
88value returned in the optlen argument always reflects the true number of
89built-in CCIDs.
90
91DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
92time, combining the operation of the next two socket options. This option is
93preferable over the latter two, since often applications will use the same
94type of CCID for both directions; and mixed use of CCIDs is not currently well
95understood. This socket option takes as argument at least one uint8_t value, or
96an array of uint8_t values, which must match available CCIDS (see above). CCIDs
97must be registered on the socket before calling connect() or listen().
98
99DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
100the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
101Please note that the getsockopt argument type here is ``int``, not uint8_t.
102
103DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
104
105DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
106timewait state when closing the connection (RFC 4340, 8.3). The usual case is
107that the closing server sends a CloseReq, whereupon the client holds timewait
108state. When this boolean socket option is on, the server sends a Close instead
109and will enter TIMEWAIT. This option must be set after accept() returns.
110
111DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
112partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
113always cover the entire packet and that only fully covered application data is
114accepted by the receiver. Hence, when using this feature on the sender, it must
115be enabled at the receiver, too with suitable choice of CsCov.
116
117DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
118	range 0..15 are acceptable. The default setting is 0 (full coverage),
119	values between 1..15 indicate partial coverage.
120
121DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
122	sets a threshold, where again values 0..15 are acceptable. The default
123	of 0 means that all packets with a partial coverage will be discarded.
124	Values in the range 1..15 indicate that packets with minimally such a
125	coverage value are also acceptable. The higher the number, the more
126	restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
127	settings are inherited to the child socket after accept().
128
129The following two options apply to CCID 3 exclusively and are getsockopt()-only.
130In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
131
132DCCP_SOCKOPT_CCID_RX_INFO
133	Returns a ``struct tfrc_rx_info`` in optval; the buffer for optval and
134	optlen must be set to at least sizeof(struct tfrc_rx_info).
135
136DCCP_SOCKOPT_CCID_TX_INFO
137	Returns a ``struct tfrc_tx_info`` in optval; the buffer for optval and
138	optlen must be set to at least sizeof(struct tfrc_tx_info).
139
140On unidirectional connections it is useful to close the unused half-connection
141via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
142
143
144Sysctl variables
145================
146Several DCCP default parameters can be managed by the following sysctls
147(sysctl net.dccp.default or /proc/sys/net/dccp/default):
148
149request_retries
150	The number of active connection initiation retries (the number of
151	Requests minus one) before timing out. In addition, it also governs
152	the behaviour of the other, passive side: this variable also sets
153	the number of times DCCP repeats sending a Response when the initial
154	handshake does not progress from RESPOND to OPEN (i.e. when no Ack
155	is received after the initial Request).  This value should be greater
156	than 0, suggested is less than 10. Analogue of tcp_syn_retries.
157
158retries1
159	How often a DCCP Response is retransmitted until the listening DCCP
160	side considers its connecting peer dead. Analogue of tcp_retries1.
161
162retries2
163	The number of times a general DCCP packet is retransmitted. This has
164	importance for retransmitted acknowledgments and feature negotiation,
165	data packets are never retransmitted. Analogue of tcp_retries2.
166
167tx_ccid = 2
168	Default CCID for the sender-receiver half-connection. Depending on the
169	choice of CCID, the Send Ack Vector feature is enabled automatically.
170
171rx_ccid = 2
172	Default CCID for the receiver-sender half-connection; see tx_ccid.
173
174seq_window = 100
175	The initial sequence window (sec. 7.5.2) of the sender. This influences
176	the local ackno validity and the remote seqno validity windows (7.5.1).
177	Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
178
179tx_qlen = 5
180	The size of the transmit buffer in packets. A value of 0 corresponds
181	to an unbounded transmit buffer.
182
183sync_ratelimit = 125 ms
184	The timeout between subsequent DCCP-Sync packets sent in response to
185	sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
186	of this parameter is milliseconds; a value of 0 disables rate-limiting.
187
188
189IOCTLS
190======
191FIONREAD
192	Works as in udp(7): returns in the ``int`` argument pointer the size of
193	the next pending datagram in bytes, or 0 when no datagram is pending.
194
195
196Other tunables
197==============
198Per-route rto_min support
199	CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
200	of the RTO timer. This setting can be modified via the 'rto_min' option
201	of iproute2; for example::
202
203		> ip route change 10.0.0.0/24   rto_min 250j dev wlan0
204		> ip route add    10.0.0.254/32 rto_min 800j dev wlan0
205		> ip route show dev wlan0
206
207	CCID-3 also supports the rto_min setting: it is used to define the lower
208	bound for the expiry of the nofeedback timer. This can be useful on LANs
209	with very low RTTs (e.g., loopback, Gbit ethernet).
210
211
212Notes
213=====
214DCCP does not travel through NAT successfully at present on many boxes. This is
215because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
216support for DCCP has been added.
217