1#include <linux/tcp.h>
  2#include <net/tcp.h>
  3
  4int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS;
  5
  6/* Marks a packet lost, if some packet sent later has been (s)acked.
  7 * The underlying idea is similar to the traditional dupthresh and FACK
  8 * but they look at different metrics:
  9 *
 10 * dupthresh: 3 OOO packets delivered (packet count)
 11 * FACK: sequence delta to highest sacked sequence (sequence space)
 12 * RACK: sent time delta to the latest delivered packet (time domain)
 13 *
 14 * The advantage of RACK is it applies to both original and retransmitted
 15 * packet and therefore is robust against tail losses. Another advantage
 16 * is being more resilient to reordering by simply allowing some
 17 * "settling delay", instead of tweaking the dupthresh.
 18 *
 19 * The current version is only used after recovery starts but can be
 20 * easily extended to detect the first loss.
 21 */
 22int tcp_rack_mark_lost(struct sock *sk)
 23{
 24	struct tcp_sock *tp = tcp_sk(sk);
 25	struct sk_buff *skb;
 26	u32 reo_wnd, prior_retrans = tp->retrans_out;
 27
 28	if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced)
 29		return 0;
 30
 31	/* Reset the advanced flag to avoid unnecessary queue scanning */
 32	tp->rack.advanced = 0;
 33
 34	/* To be more reordering resilient, allow min_rtt/4 settling delay
 35	 * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
 36	 * RTT because reordering is often a path property and less related
 37	 * to queuing or delayed ACKs.
 38	 *
 39	 * TODO: measure and adapt to the observed reordering delay, and
 40	 * use a timer to retransmit like the delayed early retransmit.
 41	 */
 42	reo_wnd = 1000;
 43	if (tp->rack.reord && tcp_min_rtt(tp) != ~0U)
 44		reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
 45
 46	tcp_for_write_queue(skb, sk) {
 47		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
 48
 49		if (skb == tcp_send_head(sk))
 50			break;
 51
 52		/* Skip ones already (s)acked */
 53		if (!after(scb->end_seq, tp->snd_una) ||
 54		    scb->sacked & TCPCB_SACKED_ACKED)
 55			continue;
 56
 57		if (skb_mstamp_after(&tp->rack.mstamp, &skb->skb_mstamp)) {
 58
 59			if (skb_mstamp_us_delta(&tp->rack.mstamp,
 60						&skb->skb_mstamp) <= reo_wnd)
 61				continue;
 62
 63			/* skb is lost if packet sent later is sacked */
 64			tcp_skb_mark_lost_uncond_verify(tp, skb);
 65			if (scb->sacked & TCPCB_SACKED_RETRANS) {
 66				scb->sacked &= ~TCPCB_SACKED_RETRANS;
 67				tp->retrans_out -= tcp_skb_pcount(skb);
 68				NET_INC_STATS(sock_net(sk),
 69					      LINUX_MIB_TCPLOSTRETRANSMIT);
 70			}
 71		} else if (!(scb->sacked & TCPCB_RETRANS)) {
 72			/* Original data are sent sequentially so stop early
 73			 * b/c the rest are all sent after rack_sent
 74			 */
 75			break;
 76		}
 77	}
 78	return prior_retrans - tp->retrans_out;
 79}
 80
 81/* Record the most recently (re)sent time among the (s)acked packets */
 82void tcp_rack_advance(struct tcp_sock *tp,
 83		      const struct skb_mstamp *xmit_time, u8 sacked)
 84{
 85	if (tp->rack.mstamp.v64 &&
 86	    !skb_mstamp_after(xmit_time, &tp->rack.mstamp))
 87		return;
 88
 89	if (sacked & TCPCB_RETRANS) {
 90		struct skb_mstamp now;
 91
 92		/* If the sacked packet was retransmitted, it's ambiguous
 93		 * whether the retransmission or the original (or the prior
 94		 * retransmission) was sacked.
 95		 *
 96		 * If the original is lost, there is no ambiguity. Otherwise
 97		 * we assume the original can be delayed up to aRTT + min_rtt.
 98		 * the aRTT term is bounded by the fast recovery or timeout,
 99		 * so it's at least one RTT (i.e., retransmission is at least
100		 * an RTT later).
101		 */
102		skb_mstamp_get(&now);
103		if (skb_mstamp_us_delta(&now, xmit_time) < tcp_min_rtt(tp))
104			return;
105	}
106
107	tp->rack.mstamp = *xmit_time;
108	tp->rack.advanced = 1;
109}