tcp: detect loss above high_seq in recovery
Correctly implement a loss detection heuristic: New sequences (above
high_seq) sent during the fast recovery are deemed lost when higher
sequences are SACKed.
Current code does not catch these losses, because tcp_mark_head_lost()
does not check packets beyond high_seq. The fix is straight-forward by
checking packets until the highest sacked packet. In addition, all the
FLAG_DATA_LOST logic are in-effective and redundant and can be removed.
Update the loss heuristic comments. The algorithm above is documented
as heuristic B, but it is redundant too because heuristic A already
covers B.
Note that this change only marks some forward-retransmitted packets LOST.
It does NOT forbid TCP performing further CWR on new losses. A potential
follow-up patch under preparation is to perform another CWR on "new"
losses such as
1) sequence above high_seq is lost (by resetting high_seq to snd_nxt)
2) retransmission is lost.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/snmp.h b/include/linux/snmp.h
index e16557a..c1241c42 100644
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -192,7 +192,6 @@
LINUX_MIB_TCPPARTIALUNDO, /* TCPPartialUndo */
LINUX_MIB_TCPDSACKUNDO, /* TCPDSACKUndo */
LINUX_MIB_TCPLOSSUNDO, /* TCPLossUndo */
- LINUX_MIB_TCPLOSS, /* TCPLoss */
LINUX_MIB_TCPLOSTRETRANSMIT, /* TCPLostRetransmit */
LINUX_MIB_TCPRENOFAILURES, /* TCPRenoFailures */
LINUX_MIB_TCPSACKFAILURES, /* TCPSackFailures */
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 3569d8e..6afc807 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -216,7 +216,6 @@
SNMP_MIB_ITEM("TCPPartialUndo", LINUX_MIB_TCPPARTIALUNDO),
SNMP_MIB_ITEM("TCPDSACKUndo", LINUX_MIB_TCPDSACKUNDO),
SNMP_MIB_ITEM("TCPLossUndo", LINUX_MIB_TCPLOSSUNDO),
- SNMP_MIB_ITEM("TCPLoss", LINUX_MIB_TCPLOSS),
SNMP_MIB_ITEM("TCPLostRetransmit", LINUX_MIB_TCPLOSTRETRANSMIT),
SNMP_MIB_ITEM("TCPRenoFailures", LINUX_MIB_TCPRENOFAILURES),
SNMP_MIB_ITEM("TCPSackFailures", LINUX_MIB_TCPSACKFAILURES),
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2877c3e..976034f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -105,7 +105,6 @@
#define FLAG_SYN_ACKED 0x10 /* This ACK acknowledged SYN. */
#define FLAG_DATA_SACKED 0x20 /* New SACK. */
#define FLAG_ECE 0x40 /* ECE in this ACK */
-#define FLAG_DATA_LOST 0x80 /* SACK detected data lossage. */
#define FLAG_SLOWPATH 0x100 /* Do not skip RFC checks for window update.*/
#define FLAG_ONLY_ORIG_SACKED 0x200 /* SACKs only non-rexmit sent before RTO */
#define FLAG_SND_UNA_ADVANCED 0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
@@ -1040,13 +1039,11 @@
* These 6 states form finite state machine, controlled by the following events:
* 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
* 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
- * 3. Loss detection event of one of three flavors:
+ * 3. Loss detection event of two flavors:
* A. Scoreboard estimator decided the packet is lost.
* A'. Reno "three dupacks" marks head of queue lost.
- * A''. Its FACK modfication, head until snd.fack is lost.
- * B. SACK arrives sacking data transmitted after never retransmitted
- * hole was sent out.
- * C. SACK arrives sacking SND.NXT at the moment, when the
+ * A''. Its FACK modification, head until snd.fack is lost.
+ * B. SACK arrives sacking SND.NXT at the moment, when the
* segment was retransmitted.
* 4. D-SACK added new rule: D-SACK changes any tag to S.
*
@@ -1153,7 +1150,7 @@
}
/* Check for lost retransmit. This superb idea is borrowed from "ratehalving".
- * Event "C". Later note: FACK people cheated me again 8), we have to account
+ * Event "B". Later note: FACK people cheated me again 8), we have to account
* for reordering! Ugly, but should help.
*
* Search retransmitted skbs from write_queue that were sent when snd_nxt was
@@ -1844,10 +1841,6 @@
if (found_dup_sack && ((i + 1) == first_sack_index))
next_dup = &sp[i + 1];
- /* Event "B" in the comment above. */
- if (after(end_seq, tp->high_seq))
- state.flag |= FLAG_DATA_LOST;
-
/* Skip too early cached blocks */
while (tcp_sack_cache_ok(tp, cache) &&
!before(start_seq, cache->end_seq))
@@ -2515,8 +2508,11 @@
tcp_verify_left_out(tp);
}
-/* Mark head of queue up as lost. With RFC3517 SACK, the packets is
- * is against sacked "cnt", otherwise it's against facked "cnt"
+/* Detect loss in event "A" above by marking head of queue up as lost.
+ * For FACK or non-SACK(Reno) senders, the first "packets" number of segments
+ * are considered lost. For RFC3517 SACK, a segment is considered lost if it
+ * has at least tp->reordering SACKed seqments above it; "packets" refers to
+ * the maximum SACKed segments to pass before reaching this limit.
*/
static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
{
@@ -2525,6 +2521,8 @@
int cnt, oldcnt;
int err;
unsigned int mss;
+ /* Use SACK to deduce losses of new sequences sent during recovery */
+ const u32 loss_high = tcp_is_sack(tp) ? tp->snd_nxt : tp->high_seq;
WARN_ON(packets > tp->packets_out);
if (tp->lost_skb_hint) {
@@ -2546,7 +2544,7 @@
tp->lost_skb_hint = skb;
tp->lost_cnt_hint = cnt;
- if (after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
+ if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
break;
oldcnt = cnt;
@@ -3033,19 +3031,10 @@
if (tcp_check_sack_reneging(sk, flag))
return;
- /* C. Process data loss notification, provided it is valid. */
- if (tcp_is_fack(tp) && (flag & FLAG_DATA_LOST) &&
- before(tp->snd_una, tp->high_seq) &&
- icsk->icsk_ca_state != TCP_CA_Open &&
- tp->fackets_out > tp->reordering) {
- tcp_mark_head_lost(sk, tp->fackets_out - tp->reordering, 0);
- NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSS);
- }
-
- /* D. Check consistency of the current state. */
+ /* C. Check consistency of the current state. */
tcp_verify_left_out(tp);
- /* E. Check state exit conditions. State can be terminated
+ /* D. Check state exit conditions. State can be terminated
* when high_seq is ACKed. */
if (icsk->icsk_ca_state == TCP_CA_Open) {
WARN_ON(tp->retrans_out != 0);
@@ -3077,7 +3066,7 @@
}
}
- /* F. Process state. */
+ /* E. Process state. */
switch (icsk->icsk_ca_state) {
case TCP_CA_Recovery:
if (!(flag & FLAG_SND_UNA_ADVANCED)) {