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1 | DCCP protocol |
2 | ============ | |
3 | ||
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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 | |
e333b3ed GR |
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. | |
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26 | |
27 | It has a base protocol and pluggable congestion control IDs (CCIDs). | |
28 | ||
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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 | |
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31 | |
32 | Missing features | |
33 | ================ | |
34 | ||
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35 | The Linux DCCP implementation does not currently support all the features that are |
36 | specified in RFCs 4340...42. | |
98069ff4 | 37 | |
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38 | The known bugs are at: |
39 | http://linux-net.osdl.org/index.php/TODO#DCCP | |
98069ff4 | 40 | |
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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 | ||
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46 | Socket options |
47 | ============== | |
48 | ||
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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 | |
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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(). | |
98069ff4 | 56 | |
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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 | ||
d90ebcbf | 60 | DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs |
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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. | |
d90ebcbf | 65 | |
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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 | ||
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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 | ||
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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. | |
2bfd754d | 95 | DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it |
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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 | |
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100 | restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage |
101 | settings are inherited to the child socket after accept(). | |
6f4e5fff | 102 | |
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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 | ||
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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. | |
f2645101 | 114 | |
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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 | ||
2e2e9e92 | 138 | tx_ccid = 2 |
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139 | Default CCID for the sender-receiver half-connection. Depending on the |
140 | choice of CCID, the Send Ack Vector feature is enabled automatically. | |
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141 | |
142 | rx_ccid = 2 | |
0049bab5 | 143 | Default CCID for the receiver-sender half-connection; see tx_ccid. |
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144 | |
145 | seq_window = 100 | |
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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). | |
2e2e9e92 | 148 | |
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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 | ||
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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 | ||
c2814901 GR |
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 | ||
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164 | Notes |
165 | ===== | |
166 | ||
ddfe10b8 | 167 | DCCP does not travel through NAT successfully at present on many boxes. This is |
126acd5b | 168 | because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT |
ddfe10b8 | 169 | support for DCCP has been added. |