rx: More improvments to RTT calculation

Move the decision about whether a packet contributes to the peer's
rount trip time into the CalculateRoundTripTime function, and improve
the criteria used.

Previously, we only computed the RTT if we had not retransmitted. This
is bad, because it means that places where we have backed off in order
to retransmit never actually lengthen the RTT, and so the RTT is kept
artificially low, and we see a large number of retransmits. Instead,
use the serial of the ACK packet to determine which transmission is
being acknowledged, and if it is the first, or the last, transmission
use the appropriate sent time to calculate the RTT.

If we have no serial in the ACK (for a delayed ack, for example), or
if the serial doesn't match (where a single acknowledgement is soft
acking a number of packets), fall back to only using the ack if the
packet has not be retransmitted.

Also, avoid multiple counting of packets which have arrived as part
of a jumbogram by only permitting the last packet in a jumbogram to
contribute to the RTT. This avoids giving the RTT of jumbograms more
weight than those of normal packets - doing so would pull down the
RTT, as it in effect favours packets which have not be retransmitted.

Change-Id: I6d978a118e2bd0694484376b586dd4eaa64cdd8c
Reviewed-on: http://gerrit.openafs.org/3029
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>

diff --git a/src/rx/rx.c b/src/rx/rx.c
index d545e7c..df01950 100644 (file)
--- a/src/rx/rx.c
+++ b/src/rx/rx.c
@@ -129,7 +129,7 @@ int (*swapNameProgram) (PROCESS, const char *, char *) = 0;
 
 /* Local static routines */
 static void rxi_DestroyConnectionNoLock(struct rx_connection *conn);
-static void rxi_ComputeRoundTripTime(struct rx_packet *, struct clock *,
+static void rxi_ComputeRoundTripTime(struct rx_packet *, struct rx_ackPacket *,
                                     struct rx_peer *, struct clock *);
 
 #ifdef RX_ENABLE_LOCKS
@@ -4061,11 +4061,8 @@ rxi_ReceiveAckPacket(struct rx_call *call, struct rx_packet *np,
 
        if (!(tp->flags & RX_PKTFLAG_ACKED)) {
            newAckCount++;
-           if (ap->reason != RX_ACK_DELAY &&
-               clock_Eq(&tp->timeSent, &tp->firstSent)) {
-               rxi_ComputeRoundTripTime(tp, &tp->timeSent, call->conn->peer,
-                                        &now);
-           }
+
+           rxi_ComputeRoundTripTime(tp, ap, call->conn->peer, &now);
        }
 
 #ifdef ADAPT_WINDOW
@@ -4142,11 +4139,7 @@ rxi_ReceiveAckPacket(struct rx_call *call, struct rx_packet *np,
                    newAckCount++;
                    tp->flags |= RX_PKTFLAG_ACKED;
 
-                   if (ap->reason != RX_ACK_DELAY &&
-                       clock_Eq(&tp->timeSent, &tp->firstSent)) {
-                       rxi_ComputeRoundTripTime(tp, &tp->timeSent,
-                                                call->conn->peer, &now);
-                   }
+                   rxi_ComputeRoundTripTime(tp, ap, call->conn->peer, &now);
 #ifdef ADAPT_WINDOW
                    rxi_ComputeRate(call->conn->peer, call, tp, np,
                                    ap->reason);
@@ -6538,12 +6531,44 @@ rxi_ChallengeOn(struct rx_connection *conn)
 /* sentp and/or peer may be null */
 static void
 rxi_ComputeRoundTripTime(struct rx_packet *p,
-                        struct clock *sentp,
+                        struct rx_ackPacket *ack,
                         struct rx_peer *peer,
                         struct clock *now)
 {
-    struct clock thisRtt, *rttp = &thisRtt;
+    struct clock thisRtt, *sentp, *rttp = &thisRtt;
     int rtt_timeout;
+    int serial;
+
+    /* If the ACK is delayed, then do nothing */
+    if (ack->reason == RX_ACK_DELAY)
+       return;
+
+    /* On the wire, jumbograms are a single UDP packet. We shouldn't count
+     * their RTT multiple times, so only include the RTT of the last packet
+     * in a jumbogram */
+    if (p->flags & RX_JUMBO_PACKET)
+       return;
+
+    /* Use the serial number to determine which transmission the ACK is for,
+     * and set the sent time to match this. If we have no serial number, then
+     * only use the ACK for RTT calculations if the packet has not been
+     * retransmitted
+     */
+
+    serial = ntohl(ack->serial);
+    if (serial) {
+       if (serial == p->header.serial) {
+           sentp = &p->timeSent;
+       } else if (serial == p->firstSerial) {
+           sentp = &p->firstSent;
+       } else
+           return;
+    } else {
+       if (clock_Eq(&p->timeSent, &p->firstSent)) {
+           sentp = &p->firstSent;
+       } else
+           return;
+    }
 
     thisRtt = *now;