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![Eric Dumazet](/assets/img/avatar_default.png)
Upcoming congestion controls for TCP require usec resolution for RTT estimations. Millisecond resolution is simply not enough these days. FQ/pacing in DC environments also require this change for finer control and removal of bimodal behavior due to the current hack in tcp_update_pacing_rate() for 'small rtt' TCP_CONG_RTT_STAMP is no longer needed. As Julian Anastasov pointed out, we need to keep user compatibility : tcp_metrics used to export RTT and RTTVAR in msec resolution, so we added RTT_US and RTTVAR_US. An iproute2 patch is needed to use the new attributes if provided by the kernel. In this example ss command displays a srtt of 32 usecs (10Gbit link) lpk51:~# ./ss -i dst lpk52 Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port tcp ESTAB 0 1 10.246.11.51:42959 10.246.11.52:64614 cubic wscale:6,6 rto:201 rtt:0.032/0.001 ato:40 mss:1448 cwnd:10 send 3620.0Mbps pacing_rate 7240.0Mbps unacked:1 rcv_rtt:993 rcv_space:29559 Updated iproute2 ip command displays : lpk51:~# ./ip tcp_metrics | grep 10.246.11.52 10.246.11.52 age 561.914sec cwnd 10 rtt 274us rttvar 213us source 10.246.11.51 Old binary displays : lpk51:~# ip tcp_metrics | grep 10.246.11.52 10.246.11.52 age 561.914sec cwnd 10 rtt 250us rttvar 125us source 10.246.11.51 With help from Julian Anastasov, Stephen Hemminger and Yuchung Cheng Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Cc: Stephen Hemminger <stephen@networkplumber.org> Cc: Yuchung Cheng <ycheng@google.com> Cc: Larry Brakmo <brakmo@google.com> Cc: Julian Anastasov <ja@ssi.bg> Signed-off-by: David S. Miller <davem@davemloft.net>
234 lines
5.7 KiB
C
234 lines
5.7 KiB
C
/*
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* TCP Veno congestion control
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*
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* This is based on the congestion detection/avoidance scheme described in
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* C. P. Fu, S. C. Liew.
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* "TCP Veno: TCP Enhancement for Transmission over Wireless Access Networks."
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* IEEE Journal on Selected Areas in Communication,
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* Feb. 2003.
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* See http://www.ie.cuhk.edu.hk/fileadmin/staff_upload/soung/Journal/J3.pdf
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*/
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/skbuff.h>
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#include <linux/inet_diag.h>
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#include <net/tcp.h>
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/* Default values of the Veno variables, in fixed-point representation
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* with V_PARAM_SHIFT bits to the right of the binary point.
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*/
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#define V_PARAM_SHIFT 1
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static const int beta = 3 << V_PARAM_SHIFT;
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/* Veno variables */
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struct veno {
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u8 doing_veno_now; /* if true, do veno for this rtt */
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u16 cntrtt; /* # of rtts measured within last rtt */
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u32 minrtt; /* min of rtts measured within last rtt (in usec) */
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u32 basertt; /* the min of all Veno rtt measurements seen (in usec) */
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u32 inc; /* decide whether to increase cwnd */
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u32 diff; /* calculate the diff rate */
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};
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/* There are several situations when we must "re-start" Veno:
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*
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* o when a connection is established
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* o after an RTO
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* o after fast recovery
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* o when we send a packet and there is no outstanding
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* unacknowledged data (restarting an idle connection)
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*
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*/
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static inline void veno_enable(struct sock *sk)
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{
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struct veno *veno = inet_csk_ca(sk);
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/* turn on Veno */
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veno->doing_veno_now = 1;
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veno->minrtt = 0x7fffffff;
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}
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static inline void veno_disable(struct sock *sk)
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{
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struct veno *veno = inet_csk_ca(sk);
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/* turn off Veno */
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veno->doing_veno_now = 0;
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}
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static void tcp_veno_init(struct sock *sk)
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{
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struct veno *veno = inet_csk_ca(sk);
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veno->basertt = 0x7fffffff;
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veno->inc = 1;
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veno_enable(sk);
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}
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/* Do rtt sampling needed for Veno. */
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static void tcp_veno_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us)
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{
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struct veno *veno = inet_csk_ca(sk);
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u32 vrtt;
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if (rtt_us < 0)
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return;
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/* Never allow zero rtt or baseRTT */
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vrtt = rtt_us + 1;
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/* Filter to find propagation delay: */
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if (vrtt < veno->basertt)
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veno->basertt = vrtt;
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/* Find the min rtt during the last rtt to find
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* the current prop. delay + queuing delay:
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*/
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veno->minrtt = min(veno->minrtt, vrtt);
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veno->cntrtt++;
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}
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static void tcp_veno_state(struct sock *sk, u8 ca_state)
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{
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if (ca_state == TCP_CA_Open)
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veno_enable(sk);
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else
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veno_disable(sk);
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}
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/*
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* If the connection is idle and we are restarting,
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* then we don't want to do any Veno calculations
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* until we get fresh rtt samples. So when we
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* restart, we reset our Veno state to a clean
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* state. After we get acks for this flight of
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* packets, _then_ we can make Veno calculations
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* again.
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*/
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static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
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{
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if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
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tcp_veno_init(sk);
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}
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static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked,
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u32 in_flight)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct veno *veno = inet_csk_ca(sk);
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if (!veno->doing_veno_now) {
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tcp_reno_cong_avoid(sk, ack, acked, in_flight);
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return;
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}
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/* limited by applications */
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if (!tcp_is_cwnd_limited(sk, in_flight))
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return;
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/* We do the Veno calculations only if we got enough rtt samples */
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if (veno->cntrtt <= 2) {
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/* We don't have enough rtt samples to do the Veno
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* calculation, so we'll behave like Reno.
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*/
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tcp_reno_cong_avoid(sk, ack, acked, in_flight);
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} else {
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u64 target_cwnd;
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u32 rtt;
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/* We have enough rtt samples, so, using the Veno
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* algorithm, we determine the state of the network.
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*/
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rtt = veno->minrtt;
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target_cwnd = (tp->snd_cwnd * veno->basertt);
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target_cwnd <<= V_PARAM_SHIFT;
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do_div(target_cwnd, rtt);
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veno->diff = (tp->snd_cwnd << V_PARAM_SHIFT) - target_cwnd;
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if (tp->snd_cwnd <= tp->snd_ssthresh) {
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/* Slow start. */
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tcp_slow_start(tp, acked);
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} else {
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/* Congestion avoidance. */
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if (veno->diff < beta) {
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/* In the "non-congestive state", increase cwnd
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* every rtt.
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*/
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tcp_cong_avoid_ai(tp, tp->snd_cwnd);
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} else {
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/* In the "congestive state", increase cwnd
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* every other rtt.
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*/
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if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
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if (veno->inc &&
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tp->snd_cwnd < tp->snd_cwnd_clamp) {
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tp->snd_cwnd++;
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veno->inc = 0;
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} else
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veno->inc = 1;
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tp->snd_cwnd_cnt = 0;
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} else
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tp->snd_cwnd_cnt++;
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}
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}
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if (tp->snd_cwnd < 2)
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tp->snd_cwnd = 2;
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else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
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tp->snd_cwnd = tp->snd_cwnd_clamp;
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}
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/* Wipe the slate clean for the next rtt. */
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/* veno->cntrtt = 0; */
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veno->minrtt = 0x7fffffff;
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}
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/* Veno MD phase */
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static u32 tcp_veno_ssthresh(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct veno *veno = inet_csk_ca(sk);
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if (veno->diff < beta)
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/* in "non-congestive state", cut cwnd by 1/5 */
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return max(tp->snd_cwnd * 4 / 5, 2U);
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else
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/* in "congestive state", cut cwnd by 1/2 */
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return max(tp->snd_cwnd >> 1U, 2U);
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}
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static struct tcp_congestion_ops tcp_veno __read_mostly = {
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.init = tcp_veno_init,
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.ssthresh = tcp_veno_ssthresh,
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.cong_avoid = tcp_veno_cong_avoid,
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.pkts_acked = tcp_veno_pkts_acked,
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.set_state = tcp_veno_state,
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.cwnd_event = tcp_veno_cwnd_event,
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.owner = THIS_MODULE,
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.name = "veno",
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};
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static int __init tcp_veno_register(void)
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{
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BUILD_BUG_ON(sizeof(struct veno) > ICSK_CA_PRIV_SIZE);
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tcp_register_congestion_control(&tcp_veno);
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return 0;
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}
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static void __exit tcp_veno_unregister(void)
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{
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tcp_unregister_congestion_control(&tcp_veno);
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}
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module_init(tcp_veno_register);
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module_exit(tcp_veno_unregister);
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MODULE_AUTHOR("Bin Zhou, Cheng Peng Fu");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("TCP Veno");
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