e333b3edc4
This patch addresses the following problems:
1. DCCP relies for its proper functioning on having at least one CCID module
enabled (as in TCP plugable congestion control). Currently it is possible to
disable both CCIDs and thus leave the DCCP module in a compiled, but entirely
non-functional state: no sockets can be created when no CCID is available.
Furthermore, the protocol is (again like TCP) not intended to be used without
CCIDs. Last, a non-empty CCID list is needed for doing CCID feature negotiation.
2. Internally the default CCID that is advertised by the Linux host is set to CCID2
(DCCPF_INITIAL_CCID in include/linux/dccp.h). Disabling CCID2 in the Kconfig
menu without changing the defaults leads to a failure `module not found' when
trying to load the dccp module (which internally tries to load the default CCID).
3. The specification (RFC 4340, sec. 10) treats CCID2 somewhat like a
`minimum common denominator'; the specification says that:
* "New connections start with CCID 2 for both endpoints"
* "A DCCP implementation intended for general use, such as an implementation in a
general-purpose operating system kernel, SHOULD implement at least CCID 2.
The intent is to make CCID 2 broadly available for interoperability [...]"
Providing CCID2 as minimum-required CCID (like Reno/Cubic in TCP) thus seems reasonable.
Hence this patch automatically selects CCID2 when DCCP is enabled. Documentation also added.
Discussions with Ian McDonald on this subject are gratefully acknowledged.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
144 lines
5.7 KiB
Text
144 lines
5.7 KiB
Text
DCCP protocol
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============
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Contents
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========
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- Introduction
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- Missing features
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- Socket options
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- Notes
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Introduction
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============
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Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
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oriented protocol designed to solve issues present in UDP and TCP, particularly
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for real-time and multimedia (streaming) traffic.
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It divides into a base protocol (RFC 4340) and plugable congestion control
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modules called CCIDs. Like plugable TCP congestion control, at least one CCID
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needs to be enabled in order for the protocol to function properly. In the Linux
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implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
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the TCP-friendly CCID3 (RFC 4342), are optional.
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For a brief introduction to CCIDs and suggestions for choosing a CCID to match
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given applications, see section 10 of RFC 4340.
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It has a base protocol and pluggable congestion control IDs (CCIDs).
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DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
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is at http://www.ietf.org/html.charters/dccp-charter.html
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Missing features
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================
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The Linux DCCP implementation does not currently support all the features that are
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specified in RFCs 4340...42.
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The known bugs are at:
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http://linux-net.osdl.org/index.php/TODO#DCCP
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For more up-to-date versions of the DCCP implementation, please consider using
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the experimental DCCP test tree; instructions for checking this out are on:
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http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
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Socket options
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==============
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DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
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service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
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the socket will fall back to 0 (which means that no meaningful service code
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is present). On active sockets this is set before connect(); specifying more
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than one code has no effect (all subsequent service codes are ignored). The
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case is different for passive sockets, where multiple service codes (up to 32)
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can be set before calling bind().
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DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
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size (application payload size) in bytes, see RFC 4340, section 14.
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DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
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partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
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always cover the entire packet and that only fully covered application data is
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accepted by the receiver. Hence, when using this feature on the sender, it must
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be enabled at the receiver, too with suitable choice of CsCov.
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DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
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range 0..15 are acceptable. The default setting is 0 (full coverage),
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values between 1..15 indicate partial coverage.
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DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
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sets a threshold, where again values 0..15 are acceptable. The default
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of 0 means that all packets with a partial coverage will be discarded.
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Values in the range 1..15 indicate that packets with minimally such a
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coverage value are also acceptable. The higher the number, the more
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restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
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settings are inherited to the child socket after accept().
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The following two options apply to CCID 3 exclusively and are getsockopt()-only.
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In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
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DCCP_SOCKOPT_CCID_RX_INFO
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Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
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optlen must be set to at least sizeof(struct tfrc_rx_info).
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DCCP_SOCKOPT_CCID_TX_INFO
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Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
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optlen must be set to at least sizeof(struct tfrc_tx_info).
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On unidirectional connections it is useful to close the unused half-connection
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via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
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Sysctl variables
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================
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Several DCCP default parameters can be managed by the following sysctls
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(sysctl net.dccp.default or /proc/sys/net/dccp/default):
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request_retries
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The number of active connection initiation retries (the number of
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Requests minus one) before timing out. In addition, it also governs
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the behaviour of the other, passive side: this variable also sets
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the number of times DCCP repeats sending a Response when the initial
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handshake does not progress from RESPOND to OPEN (i.e. when no Ack
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is received after the initial Request). This value should be greater
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than 0, suggested is less than 10. Analogue of tcp_syn_retries.
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retries1
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How often a DCCP Response is retransmitted until the listening DCCP
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side considers its connecting peer dead. Analogue of tcp_retries1.
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retries2
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The number of times a general DCCP packet is retransmitted. This has
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importance for retransmitted acknowledgments and feature negotiation,
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data packets are never retransmitted. Analogue of tcp_retries2.
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send_ndp = 1
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Whether or not to send NDP count options (sec. 7.7.2).
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send_ackvec = 1
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Whether or not to send Ack Vector options (sec. 11.5).
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ack_ratio = 2
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The default Ack Ratio (sec. 11.3) to use.
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tx_ccid = 2
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Default CCID for the sender-receiver half-connection.
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rx_ccid = 2
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Default CCID for the receiver-sender half-connection.
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seq_window = 100
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The initial sequence window (sec. 7.5.2).
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tx_qlen = 5
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The size of the transmit buffer in packets. A value of 0 corresponds
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to an unbounded transmit buffer.
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sync_ratelimit = 125 ms
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The timeout between subsequent DCCP-Sync packets sent in response to
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sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
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of this parameter is milliseconds; a value of 0 disables rate-limiting.
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Notes
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=====
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DCCP does not travel through NAT successfully at present on many boxes. This is
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because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
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support for DCCP has been added.
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