220 lines
9.6 KiB
ReStructuredText
220 lines
9.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=============
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DCCP protocol
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=============
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.. Contents
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- Introduction
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- Missing features
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- Socket options
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- Sysctl variables
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- IOCTLs
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- Other tunables
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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 pluggable congestion control
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modules called CCIDs. Like pluggable 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://www.linuxfoundation.org/collaborate/workgroups/networking/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://www.linuxfoundation.org/collaborate/workgroups/networking/dccp_testing#Experimental_DCCP_source_tree
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Socket options
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==============
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DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
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a policy ID as argument and can only be set before the connection (i.e. changes
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during an established connection are not supported). Currently, two policies are
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defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
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and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
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u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
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a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
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be formatted using a cmsg(3) message header filled in as follows::
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cmsg->cmsg_level = SOL_DCCP;
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cmsg->cmsg_type = DCCP_SCM_PRIORITY;
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cmsg->cmsg_len = CMSG_LEN(sizeof(uint32_t)); /* or CMSG_LEN(4) */
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DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
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value is always interpreted as unbounded queue length. If different from zero,
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the interpretation of this parameter depends on the current dequeuing policy
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(see above): the "simple" policy will enforce a fixed queue size by returning
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EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
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lowest-priority packet first. The default value for this parameter is
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initialised from /proc/sys/net/dccp/default/tx_qlen.
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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_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
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supported by the endpoint. The option value is an array of type uint8_t whose
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size is passed as option length. The minimum array size is 4 elements, the
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value returned in the optlen argument always reflects the true number of
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built-in CCIDs.
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DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
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time, combining the operation of the next two socket options. This option is
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preferable over the latter two, since often applications will use the same
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type of CCID for both directions; and mixed use of CCIDs is not currently well
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understood. This socket option takes as argument at least one uint8_t value, or
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an array of uint8_t values, which must match available CCIDS (see above). CCIDs
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must be registered on the socket before calling connect() or listen().
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DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
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the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
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Please note that the getsockopt argument type here is ``int``, not uint8_t.
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DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
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DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
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timewait state when closing the connection (RFC 4340, 8.3). The usual case is
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that the closing server sends a CloseReq, whereupon the client holds timewait
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state. When this boolean socket option is on, the server sends a Close instead
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and will enter TIMEWAIT. This option must be set after accept() returns.
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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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tx_ccid = 2
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Default CCID for the sender-receiver half-connection. Depending on the
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choice of CCID, the Send Ack Vector feature is enabled automatically.
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rx_ccid = 2
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Default CCID for the receiver-sender half-connection; see tx_ccid.
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seq_window = 100
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The initial sequence window (sec. 7.5.2) of the sender. This influences
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the local ackno validity and the remote seqno validity windows (7.5.1).
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Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
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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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IOCTLS
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======
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FIONREAD
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Works as in udp(7): returns in the ``int`` argument pointer the size of
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the next pending datagram in bytes, or 0 when no datagram is pending.
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SIOCOUTQ
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Returns the number of unsent data bytes in the socket send queue as ``int``
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into the buffer specified by the argument pointer.
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Other tunables
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==============
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Per-route rto_min support
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CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
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of the RTO timer. This setting can be modified via the 'rto_min' option
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of iproute2; for example::
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> ip route change 10.0.0.0/24 rto_min 250j dev wlan0
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> ip route add 10.0.0.254/32 rto_min 800j dev wlan0
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> ip route show dev wlan0
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CCID-3 also supports the rto_min setting: it is used to define the lower
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bound for the expiry of the nofeedback timer. This can be useful on LANs
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with very low RTTs (e.g., loopback, Gbit ethernet).
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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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