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Quoting Alexander:

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Formulas described in RFC require high precision of floating point.
  Formulas of integer math implemented in ng_pptpgre give mistake in range
  of +0-7ms on RTT and +0-3ms on deviation. This leads to significant
  underestimation of real packet RTT.

  I have made a very simple patch to reduce mistake to +4-3ms on RTT and
  +2-1ms on deviation. Mistake in RTT is not good, but gets covered by
  deviation. To cover worst possible negative mistake in deviation I have
  added 2ms to it. Also this 2 ms cover the case when measured deviation
  is so small (about zero) that it can interfere with process scheduling
  delays or weather on Mars.

  My tests show decreasing of packet losses on 20ms RTT link from 2.5% to
  0.3% while speed increased un 1/3.

Reviewed by:	archie
This commit is contained in:
glebius 2007-02-02 09:45:23 +00:00
parent 325d4d7fda
commit fd07387e04
1 changed files with 5 additions and 4 deletions
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9
sys/netgraph/ng_pptpgre.c
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@ -132,8 +132,8 @@ typedef u_int64_t pptptime_t;
#define PPTP_MAX_ACK_DELAY (PPTP_TIME_SCALE / 2) /* 500 milliseconds */
/* See RFC 2637 section 4.4 */
#define PPTP_ACK_ALPHA(x) ((x) >> 3) /* alpha = 0.125 */
#define PPTP_ACK_BETA(x) ((x) >> 2) /* beta = 0.25 */
#define PPTP_ACK_ALPHA(x) (((x) + 4) >> 3) /* alpha = 0.125 */
#define PPTP_ACK_BETA(x) (((x) + 2) >> 2) /* beta = 0.25 */
#define PPTP_ACK_CHI(x) ((x) << 2) /* chi = 4 */
#define PPTP_ACK_DELTA(x) ((x) << 1) /* delta = 2 */
@ -676,7 +676,8 @@ ng_pptpgre_recv(node_p node, item_p item)
if (diff < 0)
diff = -diff;
a->dev += PPTP_ACK_BETA(diff - a->dev);
a->ato = a->rtt + PPTP_ACK_CHI(a->dev);
/* +2 to compensate low precision of int math */
a->ato = a->rtt + PPTP_ACK_CHI(a->dev + 2);
if (a->ato > PPTP_MAX_TIMEOUT)
a->ato = PPTP_MAX_TIMEOUT;
if (a->ato < PPTP_MIN_TIMEOUT)
@ -827,7 +828,7 @@ ng_pptpgre_recv_ack_timeout(node_p node, hook_p hook, void *arg1, int arg2)
/* Update adaptive timeout stuff */
priv->stats.recvAckTimeouts++;
a->rtt = PPTP_ACK_DELTA(a->rtt);
a->rtt = PPTP_ACK_DELTA(a->rtt) + 1; /* +1 to avoid delta*0 case */
a->ato = a->rtt + PPTP_ACK_CHI(a->dev);
if (a->ato > PPTP_MAX_TIMEOUT)
a->ato = PPTP_MAX_TIMEOUT;