Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 1 | DCCP protocol |
| 2 | ============ |
| 3 | |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 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 |
Gerrit Renker | e333b3e | 2007-11-21 10:09:56 -0200 | [diff] [blame] | 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. |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 26 | |
| 27 | It has a base protocol and pluggable congestion control IDs (CCIDs). |
| 28 | |
Gerrit Renker | ebe6f7e | 2007-11-21 10:00:17 -0200 | [diff] [blame] | 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 |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 31 | |
| 32 | Missing features |
| 33 | ================ |
| 34 | |
Gerrit Renker | ebe6f7e | 2007-11-21 10:00:17 -0200 | [diff] [blame] | 35 | The Linux DCCP implementation does not currently support all the features that are |
| 36 | specified in RFCs 4340...42. |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 37 | |
Ian McDonald | ddfe10b | 2006-11-20 18:42:45 -0200 | [diff] [blame] | 38 | The known bugs are at: |
| 39 | http://linux-net.osdl.org/index.php/TODO#DCCP |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 40 | |
Gerrit Renker | ebe6f7e | 2007-11-21 10:00:17 -0200 | [diff] [blame] | 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 | |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 46 | Socket options |
| 47 | ============== |
| 48 | |
Gerrit Renker | 00e4d11 | 2006-09-22 09:33:58 +0100 | [diff] [blame] | 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 |
Gerrit Renker | 126acd5 | 2007-10-04 14:40:22 -0700 | [diff] [blame] | 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(). |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 56 | |
Gerrit Renker | 7c559a9 | 2007-10-04 14:39:22 -0700 | [diff] [blame] | 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 | |
Gerrit Renker | b8599d2 | 2007-12-13 12:25:01 -0200 | [diff] [blame] | 60 | DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold |
| 61 | timewait state when closing the connection (RFC 4340, 8.3). The usual case is |
| 62 | that the closing server sends a CloseReq, whereupon the client holds timewait |
| 63 | state. When this boolean socket option is on, the server sends a Close instead |
| 64 | and will enter TIMEWAIT. This option must be set after accept() returns. |
| 65 | |
Gerrit Renker | 6f4e5ff | 2006-11-10 17:43:06 -0200 | [diff] [blame] | 66 | DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the |
| 67 | partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums |
| 68 | always cover the entire packet and that only fully covered application data is |
| 69 | accepted by the receiver. Hence, when using this feature on the sender, it must |
| 70 | be enabled at the receiver, too with suitable choice of CsCov. |
| 71 | |
| 72 | DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the |
| 73 | range 0..15 are acceptable. The default setting is 0 (full coverage), |
| 74 | values between 1..15 indicate partial coverage. |
Gerrit Renker | 2bfd754 | 2007-10-04 14:50:57 -0700 | [diff] [blame] | 75 | DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it |
Gerrit Renker | 6f4e5ff | 2006-11-10 17:43:06 -0200 | [diff] [blame] | 76 | sets a threshold, where again values 0..15 are acceptable. The default |
| 77 | of 0 means that all packets with a partial coverage will be discarded. |
| 78 | Values in the range 1..15 indicate that packets with minimally such a |
| 79 | coverage value are also acceptable. The higher the number, the more |
Gerrit Renker | 2bfd754 | 2007-10-04 14:50:57 -0700 | [diff] [blame] | 80 | restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage |
| 81 | settings are inherited to the child socket after accept(). |
Gerrit Renker | 6f4e5ff | 2006-11-10 17:43:06 -0200 | [diff] [blame] | 82 | |
Gerrit Renker | f264510 | 2007-03-20 15:01:14 -0300 | [diff] [blame] | 83 | The following two options apply to CCID 3 exclusively and are getsockopt()-only. |
| 84 | In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned. |
| 85 | DCCP_SOCKOPT_CCID_RX_INFO |
| 86 | Returns a `struct tfrc_rx_info' in optval; the buffer for optval and |
| 87 | optlen must be set to at least sizeof(struct tfrc_rx_info). |
| 88 | DCCP_SOCKOPT_CCID_TX_INFO |
| 89 | Returns a `struct tfrc_tx_info' in optval; the buffer for optval and |
| 90 | optlen must be set to at least sizeof(struct tfrc_tx_info). |
| 91 | |
Gerrit Renker | 8e8c71f | 2007-11-21 09:56:48 -0200 | [diff] [blame] | 92 | On unidirectional connections it is useful to close the unused half-connection |
| 93 | via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs. |
Gerrit Renker | f264510 | 2007-03-20 15:01:14 -0300 | [diff] [blame] | 94 | |
Gerrit Renker | 2e2e9e9 | 2006-11-13 13:23:52 -0200 | [diff] [blame] | 95 | Sysctl variables |
| 96 | ================ |
| 97 | Several DCCP default parameters can be managed by the following sysctls |
| 98 | (sysctl net.dccp.default or /proc/sys/net/dccp/default): |
| 99 | |
| 100 | request_retries |
| 101 | The number of active connection initiation retries (the number of |
| 102 | Requests minus one) before timing out. In addition, it also governs |
| 103 | the behaviour of the other, passive side: this variable also sets |
| 104 | the number of times DCCP repeats sending a Response when the initial |
| 105 | handshake does not progress from RESPOND to OPEN (i.e. when no Ack |
| 106 | is received after the initial Request). This value should be greater |
| 107 | than 0, suggested is less than 10. Analogue of tcp_syn_retries. |
| 108 | |
| 109 | retries1 |
| 110 | How often a DCCP Response is retransmitted until the listening DCCP |
| 111 | side considers its connecting peer dead. Analogue of tcp_retries1. |
| 112 | |
| 113 | retries2 |
| 114 | The number of times a general DCCP packet is retransmitted. This has |
| 115 | importance for retransmitted acknowledgments and feature negotiation, |
| 116 | data packets are never retransmitted. Analogue of tcp_retries2. |
| 117 | |
Gerrit Renker | 410e27a | 2008-09-09 13:27:22 +0200 | [diff] [blame] | 118 | send_ndp = 1 |
| 119 | Whether or not to send NDP count options (sec. 7.7.2). |
| 120 | |
| 121 | send_ackvec = 1 |
| 122 | Whether or not to send Ack Vector options (sec. 11.5). |
| 123 | |
| 124 | ack_ratio = 2 |
| 125 | The default Ack Ratio (sec. 11.3) to use. |
| 126 | |
Gerrit Renker | 2e2e9e9 | 2006-11-13 13:23:52 -0200 | [diff] [blame] | 127 | tx_ccid = 2 |
Gerrit Renker | 410e27a | 2008-09-09 13:27:22 +0200 | [diff] [blame] | 128 | Default CCID for the sender-receiver half-connection. |
Gerrit Renker | 2e2e9e9 | 2006-11-13 13:23:52 -0200 | [diff] [blame] | 129 | |
| 130 | rx_ccid = 2 |
Gerrit Renker | 410e27a | 2008-09-09 13:27:22 +0200 | [diff] [blame] | 131 | Default CCID for the receiver-sender half-connection. |
Gerrit Renker | 2e2e9e9 | 2006-11-13 13:23:52 -0200 | [diff] [blame] | 132 | |
| 133 | seq_window = 100 |
Gerrit Renker | 410e27a | 2008-09-09 13:27:22 +0200 | [diff] [blame] | 134 | The initial sequence window (sec. 7.5.2). |
Gerrit Renker | 2e2e9e9 | 2006-11-13 13:23:52 -0200 | [diff] [blame] | 135 | |
Ian McDonald | 82e3ab9 | 2006-11-20 19:19:32 -0200 | [diff] [blame] | 136 | tx_qlen = 5 |
| 137 | The size of the transmit buffer in packets. A value of 0 corresponds |
| 138 | to an unbounded transmit buffer. |
| 139 | |
Gerrit Renker | a94f0f9 | 2007-09-26 11:31:49 -0300 | [diff] [blame] | 140 | sync_ratelimit = 125 ms |
| 141 | The timeout between subsequent DCCP-Sync packets sent in response to |
| 142 | sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit |
| 143 | of this parameter is milliseconds; a value of 0 disables rate-limiting. |
| 144 | |
Gerrit Renker | c281490 | 2007-11-21 10:14:31 -0200 | [diff] [blame] | 145 | IOCTLS |
| 146 | ====== |
| 147 | FIONREAD |
| 148 | Works as in udp(7): returns in the `int' argument pointer the size of |
| 149 | the next pending datagram in bytes, or 0 when no datagram is pending. |
| 150 | |
Ian McDonald | 98069ff | 2005-11-10 13:04:33 -0800 | [diff] [blame] | 151 | Notes |
| 152 | ===== |
| 153 | |
Ian McDonald | ddfe10b | 2006-11-20 18:42:45 -0200 | [diff] [blame] | 154 | DCCP does not travel through NAT successfully at present on many boxes. This is |
Gerrit Renker | 126acd5 | 2007-10-04 14:40:22 -0700 | [diff] [blame] | 155 | because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT |
Ian McDonald | ddfe10b | 2006-11-20 18:42:45 -0200 | [diff] [blame] | 156 | support for DCCP has been added. |