@@ -433,6 +433,15 @@ tcp_orphan_retries - INTEGER
you should think about lowering this value, such sockets
may consume significant resources. Cf. tcp_max_orphans.
+tcp_recovery - INTEGER
+ This value is a bitmap to enable various experimental loss recovery
+ features.
+
+ RACK: 0x1 enables the RACK loss detection for fast detection of lost
+ retransmissions and tail drops.
+
+ Default: 0x1
+
tcp_reordering - INTEGER
Initial reordering level of packets in a TCP stream.
TCP stack can then dynamically adjust flow reordering level
@@ -567,6 +567,7 @@ void tcp_resume_early_retransmit(struct sock *sk);
void tcp_rearm_rto(struct sock *sk);
void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req);
void tcp_reset(struct sock *sk);
+void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb);
/* tcp_timer.c */
void tcp_init_xmit_timers(struct sock *);
@@ -1752,6 +1753,14 @@ void tcp_init(void);
/* tcp_recovery.c */
+/* Flags to enable various loss recovery features. See below */
+extern int sysctl_tcp_recovery;
+
+/* Use TCP RACK to detect (some) tail and retransmit losses */
+#define TCP_RACK_LOST_RETRANS 0x1
+
+extern int tcp_rack_mark_lost(struct sock *sk);
+
extern void tcp_rack_advance(struct tcp_sock *tp,
const struct skb_mstamp *xmit_time, u8 sacked);
@@ -496,6 +496,13 @@ static struct ctl_table ipv4_table[] = {
.proc_handler = proc_dointvec
},
{
+ .procname = "tcp_recovery",
+ .data = &sysctl_tcp_recovery,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec,
+ },
+ {
.procname = "tcp_reordering",
.data = &sysctl_tcp_reordering,
.maxlen = sizeof(int),
@@ -881,6 +881,7 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
if (metric > 0)
tcp_disable_early_retrans(tp);
+ tp->rack.reord = 1;
}
/* This must be called before lost_out is incremented */
@@ -906,8 +907,7 @@ static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
}
}
-static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp,
- struct sk_buff *skb)
+void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb)
{
tcp_verify_retransmit_hint(tp, skb);
@@ -2806,6 +2806,11 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
}
}
+ /* Use RACK to detect loss */
+ if (sysctl_tcp_recovery & TCP_RACK_LOST_RETRANS &&
+ tcp_rack_mark_lost(sk))
+ flag |= FLAG_LOST_RETRANS;
+
/* E. Process state. */
switch (icsk->icsk_ca_state) {
case TCP_CA_Recovery:
@@ -1,6 +1,83 @@
#include <linux/tcp.h>
#include <net/tcp.h>
+int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS;
+
+/* Marks a packet lost, if some packet sent later has been (s)acked.
+ * The underlying idea is similar to the traditional dupthresh and FACK
+ * but they look at different metrics:
+ *
+ * dupthresh: 3 OOO packets delivered (packet count)
+ * FACK: sequence delta to highest sacked sequence (sequence space)
+ * RACK: sent time delta to the latest delivered packet (time domain)
+ *
+ * The advantage of RACK is it applies to both original and retransmitted
+ * packet and therefore is robust against tail losses. Another advantage
+ * is being more resilient to reordering by simply allowing some
+ * "settling delay", instead of tweaking the dupthresh.
+ *
+ * The current version is only used after recovery starts but can be
+ * easily extended to detect the first loss.
+ */
+int tcp_rack_mark_lost(struct sock *sk)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ struct sk_buff *skb;
+ u32 reo_wnd, prior_retrans = tp->retrans_out;
+
+ if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced)
+ return 0;
+
+ /* Reset the advanced flag to avoid unnecessary queue scanning */
+ tp->rack.advanced = 0;
+
+ /* To be more reordering resilient, allow min_rtt/4 settling delay
+ * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
+ * RTT because reordering is often a path property and less related
+ * to queuing or delayed ACKs.
+ *
+ * TODO: measure and adapt to the observed reordering delay, and
+ * use a timer to retransmit like the delayed early retransmit.
+ */
+ reo_wnd = 1000;
+ if (tp->rack.reord && tcp_min_rtt(tp) != ~0U)
+ reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
+
+ tcp_for_write_queue(skb, sk) {
+ struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
+
+ if (skb == tcp_send_head(sk))
+ break;
+
+ /* Skip ones already (s)acked */
+ if (!after(scb->end_seq, tp->snd_una) ||
+ scb->sacked & TCPCB_SACKED_ACKED)
+ continue;
+
+ if (skb_mstamp_after(&tp->rack.mstamp, &skb->skb_mstamp)) {
+
+ if (skb_mstamp_us_delta(&tp->rack.mstamp,
+ &skb->skb_mstamp) <= reo_wnd)
+ continue;
+
+ /* skb is lost if packet sent later is sacked */
+ tcp_skb_mark_lost_uncond_verify(tp, skb);
+ if (scb->sacked & TCPCB_SACKED_RETRANS) {
+ scb->sacked &= ~TCPCB_SACKED_RETRANS;
+ tp->retrans_out -= tcp_skb_pcount(skb);
+ NET_INC_STATS_BH(sock_net(sk),
+ LINUX_MIB_TCPLOSTRETRANSMIT);
+ }
+ } else if (!(scb->sacked & TCPCB_RETRANS)) {
+ /* Original data are sent sequentially so stop early
+ * b/c the rest are all sent after rack_sent
+ */
+ break;
+ }
+ }
+ return prior_retrans - tp->retrans_out;
+}
+
/* Record the most recently (re)sent time among the (s)acked packets */
void tcp_rack_advance(struct tcp_sock *tp,
const struct skb_mstamp *xmit_time, u8 sacked)