#include <linux/tcp.h>
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#include <net/tcp.h>
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int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS;
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/* Marks a packet lost, if some packet sent later has been (s)acked.
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* The underlying idea is similar to the traditional dupthresh and FACK
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* but they look at different metrics:
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*
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* dupthresh: 3 OOO packets delivered (packet count)
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* FACK: sequence delta to highest sacked sequence (sequence space)
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* RACK: sent time delta to the latest delivered packet (time domain)
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*
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* The advantage of RACK is it applies to both original and retransmitted
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* packet and therefore is robust against tail losses. Another advantage
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* is being more resilient to reordering by simply allowing some
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* "settling delay", instead of tweaking the dupthresh.
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*
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* The current version is only used after recovery starts but can be
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* easily extended to detect the first loss.
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*/
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int tcp_rack_mark_lost(struct sock *sk)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct sk_buff *skb;
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u32 reo_wnd, prior_retrans = tp->retrans_out;
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if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced)
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return 0;
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/* Reset the advanced flag to avoid unnecessary queue scanning */
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tp->rack.advanced = 0;
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/* To be more reordering resilient, allow min_rtt/4 settling delay
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* (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
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* RTT because reordering is often a path property and less related
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* to queuing or delayed ACKs.
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*
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* TODO: measure and adapt to the observed reordering delay, and
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* use a timer to retransmit like the delayed early retransmit.
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*/
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reo_wnd = 1000;
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if (tp->rack.reord && tcp_min_rtt(tp) != ~0U)
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reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
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tcp_for_write_queue(skb, sk) {
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struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
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if (skb == tcp_send_head(sk))
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break;
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/* Skip ones already (s)acked */
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if (!after(scb->end_seq, tp->snd_una) ||
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scb->sacked & TCPCB_SACKED_ACKED)
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continue;
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if (skb_mstamp_after(&tp->rack.mstamp, &skb->skb_mstamp)) {
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if (skb_mstamp_us_delta(&tp->rack.mstamp,
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&skb->skb_mstamp) <= reo_wnd)
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continue;
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/* skb is lost if packet sent later is sacked */
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tcp_skb_mark_lost_uncond_verify(tp, skb);
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if (scb->sacked & TCPCB_SACKED_RETRANS) {
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scb->sacked &= ~TCPCB_SACKED_RETRANS;
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tp->retrans_out -= tcp_skb_pcount(skb);
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NET_INC_STATS(sock_net(sk),
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LINUX_MIB_TCPLOSTRETRANSMIT);
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}
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} else if (!(scb->sacked & TCPCB_RETRANS)) {
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/* Original data are sent sequentially so stop early
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* b/c the rest are all sent after rack_sent
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*/
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break;
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}
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}
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return prior_retrans - tp->retrans_out;
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}
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/* Record the most recently (re)sent time among the (s)acked packets */
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void tcp_rack_advance(struct tcp_sock *tp,
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const struct skb_mstamp *xmit_time, u8 sacked)
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{
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if (tp->rack.mstamp.v64 &&
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!skb_mstamp_after(xmit_time, &tp->rack.mstamp))
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return;
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if (sacked & TCPCB_RETRANS) {
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struct skb_mstamp now;
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/* If the sacked packet was retransmitted, it's ambiguous
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* whether the retransmission or the original (or the prior
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* retransmission) was sacked.
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*
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* If the original is lost, there is no ambiguity. Otherwise
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* we assume the original can be delayed up to aRTT + min_rtt.
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* the aRTT term is bounded by the fast recovery or timeout,
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* so it's at least one RTT (i.e., retransmission is at least
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* an RTT later).
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*/
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skb_mstamp_get(&now);
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if (skb_mstamp_us_delta(&now, xmit_time) < tcp_min_rtt(tp))
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return;
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}
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tp->rack.mstamp = *xmit_time;
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tp->rack.advanced = 1;
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}
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