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..c1241c4 100644 (file)
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -192,7 +192,6 @@ enum
        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 (file)
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -216,7 +216,6 @@ static const struct snmp_mib snmp4_net_list[] = {
        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 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -105,7 +105,6 @@ int sysctl_tcp_abc __read_mostly;
 #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 @@ static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp,
  * 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 @@ static int tcp_is_sackblock_valid(struct tcp_sock *tp, int is_dsack,
 }
 
 /* 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 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
                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 @@ static void tcp_timeout_skbs(struct sock *sk)
        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 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
        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 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
                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 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
        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 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
                }
        }
 
-       /* F. Process state. */
+       /* E. Process state. */
        switch (icsk->icsk_ca_state) {
        case TCP_CA_Recovery:
                if (!(flag & FLAG_SND_UNA_ADVANCED)) {