Implements the SACK-enhanced FRTO given in RFC4138 using the
variant given in Appendix B.
RFC4138, Appendix B:
"This means that in order to declare timeout spurious, the TCP
sender must receive an acknowledgment for non-retransmitted
segment between SND.UNA and RecoveryPoint in algorithm step 3.
RecoveryPoint is defined in conservative SACK-recovery
algorithm [RFC3517]"
The basic version of the FRTO algorithm can still be used also
when SACK is enabled. To enabled SACK-enhanced version, tcp_frto
sysctl is set to 2.
Signed-off-by: Ilpo Järvinen <[EMAIL PROTECTED]>
---
net/ipv4/tcp_input.c | 76 +++++++++++++++++++++++++++++++++++++++++++-------
1 files changed, 65 insertions(+), 11 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 356de02..3ce4019 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -100,6 +100,7 @@ #define FLAG_DATA_SACKED 0x20 /* New SAC
#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_ACKED (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
#define FLAG_NOT_DUP (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
@@ -110,6 +111,8 @@ #define IsReno(tp) ((tp)->rx_opt.sack_ok
#define IsFack(tp) ((tp)->rx_opt.sack_ok & 2)
#define IsDSack(tp) ((tp)->rx_opt.sack_ok & 4)
+#define IsSackFrto() (sysctl_tcp_frto == 0x2)
+
#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
/* Adapt the MSS value used to make delayed ack decision to the
@@ -1159,6 +1162,18 @@ tcp_sacktag_write_queue(struct sock *sk,
/* clear lost hint */
tp->retransmit_skb_hint = NULL;
}
+ /* SACK enhanced F-RTO detection.
+ * Set flag if and only if non-rexmitted
+ * segments below frto_highmark are
+ * SACKed (RFC4138; Appendix B).
+ * Clearing correct due to in-order walk
+ */
+ if (after(end_seq, tp->frto_highmark)) {
+ flag &= ~FLAG_ONLY_ORIG_SACKED;
+ } else {
+ if (!(sacked & TCPCB_RETRANS))
+ flag |=
FLAG_ONLY_ORIG_SACKED;
+ }
}
TCP_SKB_CB(skb)->sacked |= TCPCB_SACKED_ACKED;
@@ -1240,7 +1255,8 @@ #endif
/* F-RTO can only be used if these conditions are satisfied:
* - there must be some unsent new data
* - the advertised window should allow sending it
- * - TCP has never retransmitted anything other than head
+ * - TCP has never retransmitted anything other than head (SACK enhanced
+ * variant from Appendix B of RFC4138 is more robust here)
*/
int tcp_use_frto(struct sock *sk)
{
@@ -1252,6 +1268,9 @@ int tcp_use_frto(struct sock *sk)
tp->snd_una + tp->snd_wnd))
return 0;
+ if (IsSackFrto())
+ return 1;
+
/* Avoid expensive walking of rexmit queue if possible */
if (tp->retrans_out > 1)
return 0;
@@ -1328,9 +1347,18 @@ void tcp_enter_frto(struct sock *sk)
}
tcp_sync_left_out(tp);
+ /* Earlier loss recovery underway (see RFC4138; Appendix B).
+ * The last condition is necessary at least in tp->frto_counter case.
+ */
+ if (IsSackFrto() && (tp->frto_counter ||
+ ((1 << icsk->icsk_ca_state) & (TCPF_CA_Recovery|TCPF_CA_Loss))) &&
+ after(tp->high_seq, tp->snd_una)) {
+ tp->frto_highmark = tp->high_seq;
+ } else {
+ tp->frto_highmark = tp->snd_nxt;
+ }
tcp_set_ca_state(sk, TCP_CA_Disorder);
tp->high_seq = tp->snd_nxt;
- tp->frto_highmark = tp->snd_nxt;
tp->frto_counter = 1;
}
@@ -2566,6 +2594,10 @@ static void tcp_conservative_spur_to_res
* Rationale: if the RTO was spurious, new ACKs should arrive from the
* original window even after we transmit two new data segments.
*
+ * SACK version:
+ * on first step, wait until first cumulative ACK arrives, then move to
+ * the second step. In second step, the next ACK decides.
+ *
* F-RTO is implemented (mainly) in four functions:
* - tcp_use_frto() is used to determine if TCP is can use F-RTO
* - tcp_enter_frto() prepares TCP state on RTO if F-RTO is used, it is
@@ -2590,16 +2622,38 @@ static int tcp_process_frto(struct sock
return 1;
}
- /* RFC4138 shortcoming in step 2; should also have case c): ACK isn't
- * duplicate nor advances window, e.g., opposite dir data, winupdate
- */
- if ((tp->snd_una == prior_snd_una) && (flag&FLAG_NOT_DUP) &&
- !(flag&FLAG_FORWARD_PROGRESS))
- return 1;
+ if (!IsSackFrto() || IsReno(tp)) {
+ /* RFC4138 shortcoming in step 2; should also have case c):
+ * ACK isn't duplicate nor advances window, e.g., opposite dir
+ * data, winupdate
+ */
+ if ((tp->snd_una == prior_snd_una) && (flag&FLAG_NOT_DUP) &&
+ !(flag&FLAG_FORWARD_PROGRESS))
+ return 1;
- if (!(flag&FLAG_DATA_ACKED)) {
- tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3), flag);
- return 1;
+ if (!(flag&FLAG_DATA_ACKED)) {
+ tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3),
+ flag);
+ return 1;
+ }
+ } else {
+ if (!(flag&FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {
+ /* Prevent sending of new data. */
+ tp->snd_cwnd = min(tp->snd_cwnd,
+ tcp_packets_in_flight(tp));
+ return 1;
+ }
+
+ if ((tp->frto_counter == 2) &&
+ (!(flag&FLAG_FORWARD_PROGRESS) ||
+ ((flag&FLAG_DATA_SACKED) &&
!(flag&FLAG_ONLY_ORIG_SACKED)))) {
+ /* RFC4138 shortcoming (see comment above) */
+ if (!(flag&FLAG_FORWARD_PROGRESS) &&
(flag&FLAG_NOT_DUP))
+ return 1;
+
+ tcp_enter_frto_loss(sk, 3, flag);
+ return 1;
+ }
}
if (tp->frto_counter == 1) {
--
1.4.2
-
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