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/*
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* net/dccp/ccids/ccid3.h
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*
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* Copyright (c) 2005-7 The University of Waikato, Hamilton, New Zealand.
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* Copyright (c) 2007 The University of Aberdeen, Scotland, UK
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*
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* An implementation of the DCCP protocol
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*
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* This code has been developed by the University of Waikato WAND
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* research group. For further information please see http://www.wand.net.nz/
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* or e-mail Ian McDonald - ian.mcdonald@jandi.co.nz
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*
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* This code also uses code from Lulea University, rereleased as GPL by its
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* authors:
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* Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon
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*
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* Changes to meet Linux coding standards, to make it meet latest ccid3 draft
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* and to make it work as a loadable module in the DCCP stack written by
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* Arnaldo Carvalho de Melo <acme@conectiva.com.br>.
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*
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* Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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#ifndef _DCCP_CCID3_H_
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#define _DCCP_CCID3_H_
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#include <linux/ktime.h>
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#include <linux/list.h>
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#include <linux/types.h>
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#include <linux/tfrc.h>
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#include "lib/tfrc.h"
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#include "../ccid.h"
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/* Two seconds as per RFC 3448 4.2 */
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#define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC)
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/* In usecs - half the scheduling granularity as per RFC3448 4.6 */
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#define TFRC_OPSYS_HALF_TIME_GRAN (USEC_PER_SEC / (2 * HZ))
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/* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
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#define TFRC_T_MBI 64
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dccp ccid-3: Bug fix for the inter-packet scheduling algorithm
This fixes a subtle bug in the calculation of the inter-packet gap and shows
that t_delta, as it is currently used, is not needed. And hence replaced.
The algorithm from RFC 3448, 4.6 below continually computes a send time t_nom,
which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies
the scheduling granularity, s the packet size, and X the sending rate:
t_distance = t_nom - t_now; // in microseconds
t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds
if (t_distance >= t_delta) {
reschedule after (t_distance / 1000) milliseconds;
} else {
t_ipi = s / X; // inter-packet interval in usec
t_nom += t_ipi; // compute the next send time
send packet now;
}
1) Description of the bug
-------------------------
Rescheduling requires a conversion into milliseconds, due to this call chain:
* ccid3_hc_tx_send_packet() returns a timeout in milliseconds,
* this value is converted by msecs_to_jiffies() in dccp_write_xmit(),
* and finally used as jiffy-expires-value for sk_reset_timer().
The highest jiffy resolution with HZ=1000 is 1 millisecond, so using a higher
granularity does not make much sense here.
As a consequence, values of t_distance < 1000 are truncated to 0. This issue
has so far been resolved by using instead
if (t_distance >= t_delta + 1000)
reschedule after (t_distance / 1000) milliseconds;
The bug is in artificially inflating t_delta to t_delta' = t_delta + 1000. This
is unnecessarily large, a more adequate value is t_delta' = max(t_delta, 1000).
2) Consequences of using the corrected t_delta'
-----------------------------------------------
Since t_delta <= t_gran/2 = 10^6/(2*HZ), we have t_delta <= 1000 as long as
HZ >= 500. This means that t_delta' = max(1000, t_delta) is constant at 1000.
On the other hand, when using a coarse HZ value of HZ < 500, we have three
sub-cases that can all be reduced to using another constant of t_gran/2.
(a) The first case arises when t_ipi > t_gran. Here t_delta' is the constant
t_delta' = max(1000, t_gran/2) = t_gran/2.
(b) If t_ipi <= 2000 < t_gran = 10^6/HZ usec, then t_delta = t_ipi/2 <= 1000,
so that t_delta' = max(1000, t_delta) = 1000 < t_gran/2.
(c) If 2000 < t_ipi <= t_gran, we have t_delta' = max(t_delta, 1000) = t_ipi/2.
In the second and third cases we have delay values less than t_gran/2, which is
in the order of less than or equal to half a jiffy.
How these are treated depends on how fractions of a jiffy are handled: they
are either always rounded down to 0, or always rounded up to 1 jiffy (assuming
non-zero values). In both cases the error is on average in the order of 50%.
Thus we are not increasing the error when in the second/third case we replace
a value less than t_gran/2 with 0, by setting t_delta' to the constant t_gran/2.
3) Summary
----------
Fixing (1) and considering (2), the patch replaces t_delta with a constant,
whose value depends on CONFIG_HZ, changing the above algorithm to:
if (t_distance >= t_delta')
reschedule after (t_distance / 1000) milliseconds;
where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
17 years ago
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enum ccid3_options {
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TFRC_OPT_LOSS_EVENT_RATE = 192,
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TFRC_OPT_LOSS_INTERVALS = 193,
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TFRC_OPT_RECEIVE_RATE = 194,
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};
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struct ccid3_options_received {
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u64 ccid3or_seqno:48,
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ccid3or_loss_intervals_idx:16;
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u16 ccid3or_loss_intervals_len;
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u32 ccid3or_loss_event_rate;
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u32 ccid3or_receive_rate;
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};
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/* TFRC sender states */
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enum ccid3_hc_tx_states {
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TFRC_SSTATE_NO_SENT = 1,
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TFRC_SSTATE_NO_FBACK,
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TFRC_SSTATE_FBACK,
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TFRC_SSTATE_TERM,
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};
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/** struct ccid3_hc_tx_sock - CCID3 sender half-connection socket
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*
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* @ccid3hctx_x - Current sending rate in 64 * bytes per second
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* @ccid3hctx_x_recv - Receive rate in 64 * bytes per second
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* @ccid3hctx_x_calc - Calculated rate in bytes per second
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* @ccid3hctx_rtt - Estimate of current round trip time in usecs
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* @ccid3hctx_p - Current loss event rate (0-1) scaled by 1000000
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* @ccid3hctx_s - Packet size in bytes
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* @ccid3hctx_t_rto - Nofeedback Timer setting in usecs
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* @ccid3hctx_t_ipi - Interpacket (send) interval (RFC 3448, 4.6) in usecs
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* @ccid3hctx_state - Sender state, one of %ccid3_hc_tx_states
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* @ccid3hctx_last_win_count - Last window counter sent
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* @ccid3hctx_t_last_win_count - Timestamp of earliest packet
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* with last_win_count value sent
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* @ccid3hctx_no_feedback_timer - Handle to no feedback timer
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* @ccid3hctx_t_ld - Time last doubled during slow start
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* @ccid3hctx_t_nom - Nominal send time of next packet
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* @ccid3hctx_delta - Send timer delta (RFC 3448, 4.6) in usecs
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* @ccid3hctx_hist - Packet history
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* @ccid3hctx_options_received - Parsed set of retrieved options
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*/
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struct ccid3_hc_tx_sock {
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struct tfrc_tx_info ccid3hctx_tfrc;
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#define ccid3hctx_x ccid3hctx_tfrc.tfrctx_x
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#define ccid3hctx_x_recv ccid3hctx_tfrc.tfrctx_x_recv
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#define ccid3hctx_x_calc ccid3hctx_tfrc.tfrctx_x_calc
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#define ccid3hctx_rtt ccid3hctx_tfrc.tfrctx_rtt
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#define ccid3hctx_p ccid3hctx_tfrc.tfrctx_p
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#define ccid3hctx_t_rto ccid3hctx_tfrc.tfrctx_rto
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#define ccid3hctx_t_ipi ccid3hctx_tfrc.tfrctx_ipi
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u16 ccid3hctx_s;
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enum ccid3_hc_tx_states ccid3hctx_state:8;
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u8 ccid3hctx_last_win_count;
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ktime_t ccid3hctx_t_last_win_count;
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struct timer_list ccid3hctx_no_feedback_timer;
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ktime_t ccid3hctx_t_ld;
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ktime_t ccid3hctx_t_nom;
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u32 ccid3hctx_delta;
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struct tfrc_tx_hist_entry *ccid3hctx_hist;
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struct ccid3_options_received ccid3hctx_options_received;
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};
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static inline struct ccid3_hc_tx_sock *ccid3_hc_tx_sk(const struct sock *sk)
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{
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struct ccid3_hc_tx_sock *hctx = ccid_priv(dccp_sk(sk)->dccps_hc_tx_ccid);
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BUG_ON(hctx == NULL);
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return hctx;
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}
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/* TFRC receiver states */
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enum ccid3_hc_rx_states {
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TFRC_RSTATE_NO_DATA = 1,
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TFRC_RSTATE_DATA,
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TFRC_RSTATE_TERM = 127,
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};
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/** struct ccid3_hc_rx_sock - CCID3 receiver half-connection socket
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*
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* @ccid3hcrx_x_recv - Receiver estimate of send rate (RFC 3448 4.3)
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* @ccid3hcrx_rtt - Receiver estimate of rtt (non-standard)
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* @ccid3hcrx_p - Current loss event rate (RFC 3448 5.4)
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* @ccid3hcrx_last_counter - Tracks window counter (RFC 4342, 8.1)
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* @ccid3hcrx_state - Receiver state, one of %ccid3_hc_rx_states
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* @ccid3hcrx_bytes_recv - Total sum of DCCP payload bytes
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* @ccid3hcrx_x_recv - Receiver estimate of send rate (RFC 3448, sec. 4.3)
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* @ccid3hcrx_rtt - Receiver estimate of RTT
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* @ccid3hcrx_tstamp_last_feedback - Time at which last feedback was sent
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* @ccid3hcrx_tstamp_last_ack - Time at which last feedback was sent
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* @ccid3hcrx_hist - Packet history (loss detection + RTT sampling)
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* @ccid3hcrx_li_hist - Loss Interval database
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* @ccid3hcrx_s - Received packet size in bytes
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* @ccid3hcrx_pinv - Inverse of Loss Event Rate (RFC 4342, sec. 8.5)
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*/
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struct ccid3_hc_rx_sock {
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u8 ccid3hcrx_last_counter:4;
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enum ccid3_hc_rx_states ccid3hcrx_state:8;
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u32 ccid3hcrx_bytes_recv;
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u32 ccid3hcrx_x_recv;
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u32 ccid3hcrx_rtt;
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ktime_t ccid3hcrx_tstamp_last_feedback;
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struct tfrc_rx_hist ccid3hcrx_hist;
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struct tfrc_loss_hist ccid3hcrx_li_hist;
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u16 ccid3hcrx_s;
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#define ccid3hcrx_pinv ccid3hcrx_li_hist.i_mean
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};
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static inline struct ccid3_hc_rx_sock *ccid3_hc_rx_sk(const struct sock *sk)
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{
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struct ccid3_hc_rx_sock *hcrx = ccid_priv(dccp_sk(sk)->dccps_hc_rx_ccid);
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BUG_ON(hcrx == NULL);
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return hcrx;
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}
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#endif /* _DCCP_CCID3_H_ */
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