1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
|
#include <linux/tcp.h>
#include <net/tcp.h>
int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOSS_DETECTION;
static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
tcp_skb_mark_lost_uncond_verify(tp, skb);
if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
/* Account for retransmits that are lost again */
TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
tp->retrans_out -= tcp_skb_pcount(skb);
NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
tcp_skb_pcount(skb));
}
}
static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
{
return t1 > t2 || (t1 == t2 && after(seq1, seq2));
}
/* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
*
* 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.
*
* When tcp_rack_detect_loss() detects some packets are lost and we
* are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
* or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
* make us enter the CA_Recovery state.
*/
static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
u32 reo_wnd;
*reo_timeout = 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.
*/
reo_wnd = 1000;
if ((tp->rack.reord || !tp->lost_out) && 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 (tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
tp->rack.end_seq, scb->end_seq)) {
/* Step 3 in draft-cheng-tcpm-rack-00.txt:
* A packet is lost if its elapsed time is beyond
* the recent RTT plus the reordering window.
*/
u32 elapsed = tcp_stamp_us_delta(tp->tcp_mstamp,
skb->skb_mstamp);
s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed;
if (remaining < 0) {
tcp_rack_mark_skb_lost(sk, skb);
continue;
}
/* Skip ones marked lost but not yet retransmitted */
if ((scb->sacked & TCPCB_LOST) &&
!(scb->sacked & TCPCB_SACKED_RETRANS))
continue;
/* Record maximum wait time (+1 to avoid 0) */
*reo_timeout = max_t(u32, *reo_timeout, 1 + remaining);
} 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;
}
}
}
void tcp_rack_mark_lost(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout;
if (!tp->rack.advanced)
return;
/* Reset the advanced flag to avoid unnecessary queue scanning */
tp->rack.advanced = 0;
tcp_rack_detect_loss(sk, &timeout);
if (timeout) {
timeout = usecs_to_jiffies(timeout + TCP_REO_TIMEOUT_MIN);
inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
timeout, inet_csk(sk)->icsk_rto);
}
}
/* Record the most recently (re)sent time among the (s)acked packets
* This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
* draft-cheng-tcpm-rack-00.txt
*/
void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
u64 xmit_time)
{
u32 rtt_us;
if (tp->rack.mstamp &&
!tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
end_seq, tp->rack.end_seq))
return;
rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
if (sacked & TCPCB_RETRANS) {
/* If the sacked packet was retransmitted, it's ambiguous
* whether the retransmission or the original (or the prior
* retransmission) was sacked.
*
* If the original is lost, there is no ambiguity. Otherwise
* we assume the original can be delayed up to aRTT + min_rtt.
* the aRTT term is bounded by the fast recovery or timeout,
* so it's at least one RTT (i.e., retransmission is at least
* an RTT later).
*/
if (rtt_us < tcp_min_rtt(tp))
return;
}
tp->rack.rtt_us = rtt_us;
tp->rack.mstamp = xmit_time;
tp->rack.end_seq = end_seq;
tp->rack.advanced = 1;
}
/* We have waited long enough to accommodate reordering. Mark the expired
* packets lost and retransmit them.
*/
void tcp_rack_reo_timeout(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout, prior_inflight;
prior_inflight = tcp_packets_in_flight(tp);
tcp_rack_detect_loss(sk, &timeout);
if (prior_inflight != tcp_packets_in_flight(tp)) {
if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
tcp_enter_recovery(sk, false);
if (!inet_csk(sk)->icsk_ca_ops->cong_control)
tcp_cwnd_reduction(sk, 1, 0);
}
tcp_xmit_retransmit_queue(sk);
}
if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
tcp_rearm_rto(sk);
}
|
