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-DCCP protocol
-=============
-
-
-Contents
-========
-- Introduction
-- Missing features
-- Socket options
-- Sysctl variables
-- IOCTLs
-- Other tunables
-- Notes
-
-
-Introduction
-============
-Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
-oriented protocol designed to solve issues present in UDP and TCP, particularly
-for real-time and multimedia (streaming) traffic.
-It divides into a base protocol (RFC 4340) and plugable congestion control
-modules called CCIDs. Like plugable TCP congestion control, at least one CCID
-needs to be enabled in order for the protocol to function properly. In the Linux
-implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
-the TCP-friendly CCID3 (RFC 4342), are optional.
-For a brief introduction to CCIDs and suggestions for choosing a CCID to match
-given applications, see section 10 of RFC 4340.
-
-It has a base protocol and pluggable congestion control IDs (CCIDs).
-
-DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
-is at http://www.ietf.org/html.charters/dccp-charter.html
-
-
-Missing features
-================
-The Linux DCCP implementation does not currently support all the features that are
-specified in RFCs 4340...42.
-
-The known bugs are at:
- http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP
-
-For more up-to-date versions of the DCCP implementation, please consider using
-the experimental DCCP test tree; instructions for checking this out are on:
-http://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree
-
-
-Socket options
-==============
-DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
-a policy ID as argument and can only be set before the connection (i.e. changes
-during an established connection are not supported). Currently, two policies are
-defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
-and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
-u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
-a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
-be formatted using a cmsg(3) message header filled in as follows:
- cmsg->cmsg_level = SOL_DCCP;
- cmsg->cmsg_type = DCCP_SCM_PRIORITY;
- cmsg->cmsg_len = CMSG_LEN(sizeof(uint32_t)); /* or CMSG_LEN(4) */
-
-DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
-value is always interpreted as unbounded queue length. If different from zero,
-the interpretation of this parameter depends on the current dequeuing policy
-(see above): the "simple" policy will enforce a fixed queue size by returning
-EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
-lowest-priority packet first. The default value for this parameter is
-initialised from /proc/sys/net/dccp/default/tx_qlen.
-
-DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
-service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
-the socket will fall back to 0 (which means that no meaningful service code
-is present). On active sockets this is set before connect(); specifying more
-than one code has no effect (all subsequent service codes are ignored). The
-case is different for passive sockets, where multiple service codes (up to 32)
-can be set before calling bind().
-
-DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
-size (application payload size) in bytes, see RFC 4340, section 14.
-
-DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
-supported by the endpoint. The option value is an array of type uint8_t whose
-size is passed as option length. The minimum array size is 4 elements, the
-value returned in the optlen argument always reflects the true number of
-built-in CCIDs.
-
-DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
-time, combining the operation of the next two socket options. This option is
-preferrable over the latter two, since often applications will use the same
-type of CCID for both directions; and mixed use of CCIDs is not currently well
-understood. This socket option takes as argument at least one uint8_t value, or
-an array of uint8_t values, which must match available CCIDS (see above). CCIDs
-must be registered on the socket before calling connect() or listen().
-
-DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
-the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
-Please note that the getsockopt argument type here is `int', not uint8_t.
-
-DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
-
-DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
-timewait state when closing the connection (RFC 4340, 8.3). The usual case is
-that the closing server sends a CloseReq, whereupon the client holds timewait
-state. When this boolean socket option is on, the server sends a Close instead
-and will enter TIMEWAIT. This option must be set after accept() returns.
-
-DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
-partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
-always cover the entire packet and that only fully covered application data is
-accepted by the receiver. Hence, when using this feature on the sender, it must
-be enabled at the receiver, too with suitable choice of CsCov.
-
-DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
- range 0..15 are acceptable. The default setting is 0 (full coverage),
- values between 1..15 indicate partial coverage.
-DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
- sets a threshold, where again values 0..15 are acceptable. The default
- of 0 means that all packets with a partial coverage will be discarded.
- Values in the range 1..15 indicate that packets with minimally such a
- coverage value are also acceptable. The higher the number, the more
- restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
- settings are inherited to the child socket after accept().
-
-The following two options apply to CCID 3 exclusively and are getsockopt()-only.
-In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
-DCCP_SOCKOPT_CCID_RX_INFO
- Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
- optlen must be set to at least sizeof(struct tfrc_rx_info).
-DCCP_SOCKOPT_CCID_TX_INFO
- Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
- optlen must be set to at least sizeof(struct tfrc_tx_info).
-
-On unidirectional connections it is useful to close the unused half-connection
-via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
-
-
-Sysctl variables
-================
-Several DCCP default parameters can be managed by the following sysctls
-(sysctl net.dccp.default or /proc/sys/net/dccp/default):
-
-request_retries
- The number of active connection initiation retries (the number of
- Requests minus one) before timing out. In addition, it also governs
- the behaviour of the other, passive side: this variable also sets
- the number of times DCCP repeats sending a Response when the initial
- handshake does not progress from RESPOND to OPEN (i.e. when no Ack
- is received after the initial Request). This value should be greater
- than 0, suggested is less than 10. Analogue of tcp_syn_retries.
-
-retries1
- How often a DCCP Response is retransmitted until the listening DCCP
- side considers its connecting peer dead. Analogue of tcp_retries1.
-
-retries2
- The number of times a general DCCP packet is retransmitted. This has
- importance for retransmitted acknowledgments and feature negotiation,
- data packets are never retransmitted. Analogue of tcp_retries2.
-
-tx_ccid = 2
- Default CCID for the sender-receiver half-connection. Depending on the
- choice of CCID, the Send Ack Vector feature is enabled automatically.
-
-rx_ccid = 2
- Default CCID for the receiver-sender half-connection; see tx_ccid.
-
-seq_window = 100
- The initial sequence window (sec. 7.5.2) of the sender. This influences
- the local ackno validity and the remote seqno validity windows (7.5.1).
- Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
-
-tx_qlen = 5
- The size of the transmit buffer in packets. A value of 0 corresponds
- to an unbounded transmit buffer.
-
-sync_ratelimit = 125 ms
- The timeout between subsequent DCCP-Sync packets sent in response to
- sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
- of this parameter is milliseconds; a value of 0 disables rate-limiting.
-
-
-IOCTLS
-======
-FIONREAD
- Works as in udp(7): returns in the `int' argument pointer the size of
- the next pending datagram in bytes, or 0 when no datagram is pending.
-
-
-Other tunables
-==============
-Per-route rto_min support
- CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
- of the RTO timer. This setting can be modified via the 'rto_min' option
- of iproute2; for example:
- > ip route change 10.0.0.0/24 rto_min 250j dev wlan0
- > ip route add 10.0.0.254/32 rto_min 800j dev wlan0
- > ip route show dev wlan0
- CCID-3 also supports the rto_min setting: it is used to define the lower
- bound for the expiry of the nofeedback timer. This can be useful on LANs
- with very low RTTs (e.g., loopback, Gbit ethernet).
-
-
-Notes
-=====
-DCCP does not travel through NAT successfully at present on many boxes. This is
-because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
-support for DCCP has been added.