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![]() This fixes a subtle bug in the calculation of the inter-packet gap and shows that t_delta, as it is currently used, is not needed. And hence replaced. The algorithm from RFC 3448, 4.6 below continually computes a send time t_nom, which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies the scheduling granularity, s the packet size, and X the sending rate: t_distance = t_nom - t_now; // in microseconds t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds if (t_distance >= t_delta) { reschedule after (t_distance / 1000) milliseconds; } else { t_ipi = s / X; // inter-packet interval in usec t_nom += t_ipi; // compute the next send time send packet now; } 1) Description of the bug ------------------------- Rescheduling requires a conversion into milliseconds, due to this call chain: * ccid3_hc_tx_send_packet() returns a timeout in milliseconds, * this value is converted by msecs_to_jiffies() in dccp_write_xmit(), * and finally used as jiffy-expires-value for sk_reset_timer(). The highest jiffy resolution with HZ=1000 is 1 millisecond, so using a higher granularity does not make much sense here. As a consequence, values of t_distance < 1000 are truncated to 0. This issue has so far been resolved by using instead if (t_distance >= t_delta + 1000) reschedule after (t_distance / 1000) milliseconds; The bug is in artificially inflating t_delta to t_delta' = t_delta + 1000. This is unnecessarily large, a more adequate value is t_delta' = max(t_delta, 1000). 2) Consequences of using the corrected t_delta' ----------------------------------------------- Since t_delta <= t_gran/2 = 10^6/(2*HZ), we have t_delta <= 1000 as long as HZ >= 500. This means that t_delta' = max(1000, t_delta) is constant at 1000. On the other hand, when using a coarse HZ value of HZ < 500, we have three sub-cases that can all be reduced to using another constant of t_gran/2. (a) The first case arises when t_ipi > t_gran. Here t_delta' is the constant t_delta' = max(1000, t_gran/2) = t_gran/2. (b) If t_ipi <= 2000 < t_gran = 10^6/HZ usec, then t_delta = t_ipi/2 <= 1000, so that t_delta' = max(1000, t_delta) = 1000 < t_gran/2. (c) If 2000 < t_ipi <= t_gran, we have t_delta' = max(t_delta, 1000) = t_ipi/2. In the second and third cases we have delay values less than t_gran/2, which is in the order of less than or equal to half a jiffy. How these are treated depends on how fractions of a jiffy are handled: they are either always rounded down to 0, or always rounded up to 1 jiffy (assuming non-zero values). In both cases the error is on average in the order of 50%. Thus we are not increasing the error when in the second/third case we replace a value less than t_gran/2 with 0, by setting t_delta' to the constant t_gran/2. 3) Summary ---------- Fixing (1) and considering (2), the patch replaces t_delta with a constant, whose value depends on CONFIG_HZ, changing the above algorithm to: if (t_distance >= t_delta') reschedule after (t_distance / 1000) milliseconds; where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise. Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk> |
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.. | ||
ccids | ||
ackvec.c | ||
ackvec.h | ||
ccid.c | ||
ccid.h | ||
dccp.h | ||
diag.c | ||
feat.c | ||
feat.h | ||
input.c | ||
ipv4.c | ||
ipv6.c | ||
ipv6.h | ||
Kconfig | ||
Makefile | ||
minisocks.c | ||
options.c | ||
output.c | ||
probe.c | ||
proto.c | ||
sysctl.c | ||
timer.c |