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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 | |
17 | based protocol designed to solve issues present in UDP and TCP particularly | |
18 | for real time and multimedia traffic. | |
19 | ||
20 | It has a base protocol and pluggable congestion control IDs (CCIDs). | |
21 | ||
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22 | It is at proposed standard RFC status and the homepage for DCCP as a protocol |
23 | is at: | |
ddfe10b8 | 24 | http://www.read.cs.ucla.edu/dccp/ |
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25 | |
26 | Missing features | |
27 | ================ | |
28 | ||
29 | The DCCP implementation does not currently have all the features that are in | |
ddfe10b8 | 30 | the RFC. |
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32 | The known bugs are at: |
33 | http://linux-net.osdl.org/index.php/TODO#DCCP | |
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34 | |
35 | Socket options | |
36 | ============== | |
37 | ||
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38 | DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of |
39 | service codes (RFC 4340, sec. 8.1.2); if this socket option is not set, | |
40 | the socket will fall back to 0 (which means that no meaningful service code | |
41 | is present). Connecting sockets set at most one service option; for | |
42 | listening sockets, multiple service codes can be specified. | |
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44 | DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the |
45 | partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums | |
46 | always cover the entire packet and that only fully covered application data is | |
47 | accepted by the receiver. Hence, when using this feature on the sender, it must | |
48 | be enabled at the receiver, too with suitable choice of CsCov. | |
49 | ||
50 | DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the | |
51 | range 0..15 are acceptable. The default setting is 0 (full coverage), | |
52 | values between 1..15 indicate partial coverage. | |
53 | DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it | |
54 | sets a threshold, where again values 0..15 are acceptable. The default | |
55 | of 0 means that all packets with a partial coverage will be discarded. | |
56 | Values in the range 1..15 indicate that packets with minimally such a | |
57 | coverage value are also acceptable. The higher the number, the more | |
58 | restrictive this setting (see [RFC 4340, sec. 9.2.1]). | |
59 | ||
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60 | The following two options apply to CCID 3 exclusively and are getsockopt()-only. |
61 | In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned. | |
62 | DCCP_SOCKOPT_CCID_RX_INFO | |
63 | Returns a `struct tfrc_rx_info' in optval; the buffer for optval and | |
64 | optlen must be set to at least sizeof(struct tfrc_rx_info). | |
65 | DCCP_SOCKOPT_CCID_TX_INFO | |
66 | Returns a `struct tfrc_tx_info' in optval; the buffer for optval and | |
67 | optlen must be set to at least sizeof(struct tfrc_tx_info). | |
68 | ||
69 | ||
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70 | Sysctl variables |
71 | ================ | |
72 | Several DCCP default parameters can be managed by the following sysctls | |
73 | (sysctl net.dccp.default or /proc/sys/net/dccp/default): | |
74 | ||
75 | request_retries | |
76 | The number of active connection initiation retries (the number of | |
77 | Requests minus one) before timing out. In addition, it also governs | |
78 | the behaviour of the other, passive side: this variable also sets | |
79 | the number of times DCCP repeats sending a Response when the initial | |
80 | handshake does not progress from RESPOND to OPEN (i.e. when no Ack | |
81 | is received after the initial Request). This value should be greater | |
82 | than 0, suggested is less than 10. Analogue of tcp_syn_retries. | |
83 | ||
84 | retries1 | |
85 | How often a DCCP Response is retransmitted until the listening DCCP | |
86 | side considers its connecting peer dead. Analogue of tcp_retries1. | |
87 | ||
88 | retries2 | |
89 | The number of times a general DCCP packet is retransmitted. This has | |
90 | importance for retransmitted acknowledgments and feature negotiation, | |
91 | data packets are never retransmitted. Analogue of tcp_retries2. | |
92 | ||
93 | send_ndp = 1 | |
94 | Whether or not to send NDP count options (sec. 7.7.2). | |
95 | ||
96 | send_ackvec = 1 | |
97 | Whether or not to send Ack Vector options (sec. 11.5). | |
98 | ||
99 | ack_ratio = 2 | |
100 | The default Ack Ratio (sec. 11.3) to use. | |
101 | ||
102 | tx_ccid = 2 | |
103 | Default CCID for the sender-receiver half-connection. | |
104 | ||
105 | rx_ccid = 2 | |
106 | Default CCID for the receiver-sender half-connection. | |
107 | ||
108 | seq_window = 100 | |
109 | The initial sequence window (sec. 7.5.2). | |
110 | ||
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111 | tx_qlen = 5 |
112 | The size of the transmit buffer in packets. A value of 0 corresponds | |
113 | to an unbounded transmit buffer. | |
114 | ||
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115 | Notes |
116 | ===== | |
117 | ||
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118 | DCCP does not travel through NAT successfully at present on many boxes. This is |
119 | because the checksum covers the psuedo-header as per TCP and UDP. Linux NAT | |
120 | support for DCCP has been added. |