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
|
#include <linux/tcp.h>
#include <net/tcp.h>
int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS;
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_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT);
}
}
/* 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.
*/
static void tcp_rack_detect_loss(struct sock *sk, const struct skb_mstamp *now)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
u32 reo_wnd;
/* 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)) {
/* 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.
*/
if (skb_mstamp_us_delta(now, &skb->skb_mstamp) >
tp->rack.rtt_us + reo_wnd) {
tcp_rack_mark_skb_lost(sk, skb);
}
} 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, const struct skb_mstamp *now)
{
struct tcp_sock *tp = tcp_sk(sk);
if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced)
return;
/* Reset the advanced flag to avoid unnecessary queue scanning */
tp->rack.advanced = 0;
tcp_rack_detect_loss(sk, now);
}
/* 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,
const struct skb_mstamp *xmit_time,
const struct skb_mstamp *ack_time)
{
u32 rtt_us;
if (tp->rack.mstamp.v64 &&
!skb_mstamp_after(xmit_time, &tp->rack.mstamp))
return;
rtt_us = skb_mstamp_us_delta(ack_time, 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.advanced = 1;
}
|
