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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 | ||
22 | It is at draft RFC status and the homepage for DCCP as a protocol is at: | |
23 | http://www.icir.org/kohler/dcp/ | |
24 | ||
25 | Missing features | |
26 | ================ | |
27 | ||
28 | The DCCP implementation does not currently have all the features that are in | |
29 | the draft RFC. | |
30 | ||
31 | In particular the following are missing: | |
32 | - CCID2 support | |
33 | - feature negotiation | |
34 | ||
35 | When testing against other implementations it appears that elapsed time | |
36 | options are not coded compliant to the specification. | |
37 | ||
38 | Socket options | |
39 | ============== | |
40 | ||
41 | DCCP_SOCKOPT_PACKET_SIZE is used for CCID3 to set default packet size for | |
42 | calculations. | |
43 | ||
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44 | DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of |
45 | service codes (RFC 4340, sec. 8.1.2); if this socket option is not set, | |
46 | the socket will fall back to 0 (which means that no meaningful service code | |
47 | is present). Connecting sockets set at most one service option; for | |
48 | listening sockets, multiple service codes can be specified. | |
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50 | DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the |
51 | partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums | |
52 | always cover the entire packet and that only fully covered application data is | |
53 | accepted by the receiver. Hence, when using this feature on the sender, it must | |
54 | be enabled at the receiver, too with suitable choice of CsCov. | |
55 | ||
56 | DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the | |
57 | range 0..15 are acceptable. The default setting is 0 (full coverage), | |
58 | values between 1..15 indicate partial coverage. | |
59 | DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it | |
60 | sets a threshold, where again values 0..15 are acceptable. The default | |
61 | of 0 means that all packets with a partial coverage will be discarded. | |
62 | Values in the range 1..15 indicate that packets with minimally such a | |
63 | coverage value are also acceptable. The higher the number, the more | |
64 | restrictive this setting (see [RFC 4340, sec. 9.2.1]). | |
65 | ||
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66 | Notes |
67 | ===== | |
68 | ||
69 | SELinux does not yet have support for DCCP. You will need to turn it off or | |
70 | else you will get EACCES. | |
71 | ||
72 | DCCP does not travel through NAT successfully at present. This is because | |
73 | the checksum covers the psuedo-header as per TCP and UDP. It should be | |
74 | relatively trivial to add Linux NAT support for DCCP. |