/* * See RFC2988 for a discussion of RTO calculation; comments assume * familiarity with that document. * * The smoothed round-trip time and estimated variance are stored as * fixed point numbers. Historically, srtt was scaled by * TCP_RTT_SHIFT bits, and rttvar by TCP_RTTVAR_SHIFT bits. Because * the values coincide with the alpha and beta parameters suggested * for RTO calculation (1/8 for srtt, 1/4 for rttvar), the combination * of computing 1/8 of the new value and transforming it to the * fixed-point representation required zero instructions. However, * the storage representations no longer coincide with the alpha/beta * shifts; instead, more fractional bits are present. * * The storage representation of srtt is 1/32 slow ticks, or 1/64 s. * (The assumption that a slow tick is 500 ms should not be present in * the code.) * * The storage representation of rttvar is 1/16 slow ticks, or 1/32 s. * There may be some confusion about this in the code. * * For historical reasons, these scales are also used in smoothing the * average (smoothed = (1/scale)sample + ((scale-1)/scale)smoothed). * This results in alpha of 0.125 and beta of 0.25, following RFC2988 * section 2.3 * * XXX Change SHIFT values to LGWEIGHT and REP_SHIFT, and adjust * the code to use the correct ones. */