Root/Documentation/networking/dccp.txt

1DCCP protocol
2============
3
4
5Contents
6========
7
8- Introduction
9- Missing features
10- Socket options
11- Notes
12
13Introduction
14============
15
16Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
17oriented protocol designed to solve issues present in UDP and TCP, particularly
18for real-time and multimedia (streaming) traffic.
19It divides into a base protocol (RFC 4340) and plugable congestion control
20modules called CCIDs. Like plugable TCP congestion control, at least one CCID
21needs to be enabled in order for the protocol to function properly. In the Linux
22implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
23the TCP-friendly CCID3 (RFC 4342), are optional.
24For a brief introduction to CCIDs and suggestions for choosing a CCID to match
25given applications, see section 10 of RFC 4340.
26
27It has a base protocol and pluggable congestion control IDs (CCIDs).
28
29DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
30is at http://www.ietf.org/html.charters/dccp-charter.html
31
32Missing features
33================
34
35The Linux DCCP implementation does not currently support all the features that are
36specified in RFCs 4340...42.
37
38The known bugs are at:
39    http://linux-net.osdl.org/index.php/TODO#DCCP
40
41For more up-to-date versions of the DCCP implementation, please consider using
42the experimental DCCP test tree; instructions for checking this out are on:
43http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
44
45
46Socket options
47==============
48
49DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
50service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
51the socket will fall back to 0 (which means that no meaningful service code
52is present). On active sockets this is set before connect(); specifying more
53than one code has no effect (all subsequent service codes are ignored). The
54case is different for passive sockets, where multiple service codes (up to 32)
55can be set before calling bind().
56
57DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
58size (application payload size) in bytes, see RFC 4340, section 14.
59
60DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
61supported by the endpoint. The option value is an array of type uint8_t whose
62size is passed as option length. The minimum array size is 4 elements, the
63value returned in the optlen argument always reflects the true number of
64built-in CCIDs.
65
66DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
67time, combining the operation of the next two socket options. This option is
68preferrable over the latter two, since often applications will use the same
69type of CCID for both directions; and mixed use of CCIDs is not currently well
70understood. This socket option takes as argument at least one uint8_t value, or
71an array of uint8_t values, which must match available CCIDS (see above). CCIDs
72must be registered on the socket before calling connect() or listen().
73
74DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
75the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
76Please note that the getsockopt argument type here is `int', not uint8_t.
77
78DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
79
80DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
81timewait state when closing the connection (RFC 4340, 8.3). The usual case is
82that the closing server sends a CloseReq, whereupon the client holds timewait
83state. When this boolean socket option is on, the server sends a Close instead
84and will enter TIMEWAIT. This option must be set after accept() returns.
85
86DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
87partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
88always cover the entire packet and that only fully covered application data is
89accepted by the receiver. Hence, when using this feature on the sender, it must
90be enabled at the receiver, too with suitable choice of CsCov.
91
92DCCP_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.
95DCCP_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
103The following two options apply to CCID 3 exclusively and are getsockopt()-only.
104In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
105DCCP_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).
108DCCP_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
112On unidirectional connections it is useful to close the unused half-connection
113via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
114
115Sysctl variables
116================
117Several DCCP default parameters can be managed by the following sysctls
118(sysctl net.dccp.default or /proc/sys/net/dccp/default):
119
120request_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
129retries1
130    How often a DCCP Response is retransmitted until the listening DCCP
131    side considers its connecting peer dead. Analogue of tcp_retries1.
132
133retries2
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
138tx_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
142rx_ccid = 2
143    Default CCID for the receiver-sender half-connection; see tx_ccid.
144
145seq_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
149tx_qlen = 5
150    The size of the transmit buffer in packets. A value of 0 corresponds
151    to an unbounded transmit buffer.
152
153sync_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
158IOCTLS
159======
160FIONREAD
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
164Notes
165=====
166
167DCCP does not travel through NAT successfully at present on many boxes. This is
168because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
169support for DCCP has been added.
170

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