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author | Gerrit Renker <gerrit@erg.abdn.ac.uk> | 2008-09-04 07:30:19 +0200 |
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committer | Gerrit Renker <gerrit@erg.abdn.ac.uk> | 2008-09-04 07:45:43 +0200 |
commit | a3cbdde8e9c38b66b4f13ac5d6ff1939ded0ff20 (patch) | |
tree | 8e66f40579776dbc07fdacb206c4d56e1b351e86 /net/dccp/ccids/lib | |
parent | 53ac9570c8145710aaed9e1eb850c2e991a4ebc1 (diff) | |
download | op-kernel-dev-a3cbdde8e9c38b66b4f13ac5d6ff1939ded0ff20.zip op-kernel-dev-a3cbdde8e9c38b66b4f13ac5d6ff1939ded0ff20.tar.gz |
dccp ccid-3: Preventing Oscillations
This implements [RFC 3448, 4.5], which performs congestion avoidance behaviour
by reducing the transmit rate as the queueing delay (measured in terms of
long-term RTT) increases.
Oscillation can be turned on/off via a module option (do_osc_prev) and via sysfs
(using mode 0644), the default is off.
Overflow analysis:
------------------
* oscillation prevention is done after update_x(), so that t_ipi <= 64000;
* hence the multiplication "t_ipi * sqrt(R_sample)" needs 64 bits;
* done using u64 for sqrt_sample and explicit typecast of t_ipi;
* the divisor, R_sqmean, is non-zero because oscillation prevention is first
called when receiving the second feedback packet, and tfrc_scaled_rtt() > 0.
A detailed discussion of the algorithm (with plots) is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid3/sender_notes/oscillation_prevention/
The algorithm has negative side effects:
* when allowing to decrease t_ipi (leads to a large RTT) and
* when using it during slow-start;
both uses are therefore disabled.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Diffstat (limited to 'net/dccp/ccids/lib')
-rw-r--r-- | net/dccp/ccids/lib/tfrc.h | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/net/dccp/ccids/lib/tfrc.h b/net/dccp/ccids/lib/tfrc.h index bb47146..ede12f5 100644 --- a/net/dccp/ccids/lib/tfrc.h +++ b/net/dccp/ccids/lib/tfrc.h @@ -48,6 +48,21 @@ static inline u32 scaled_div32(u64 a, u64 b) } /** + * tfrc_scaled_sqrt - Compute scaled integer sqrt(x) for 0 < x < 2^22-1 + * Uses scaling to improve accuracy of the integer approximation of sqrt(). The + * scaling factor of 2^10 limits the maximum @sample to 4e6; this is okay for + * clamped RTT samples (dccp_sample_rtt). + * Should best be used for expressions of type sqrt(x)/sqrt(y), since then the + * scaling factor is neutralised. For this purpose, it avoids returning zero. + */ +static inline u16 tfrc_scaled_sqrt(const u32 sample) +{ + const unsigned long non_zero_sample = sample ? : 1; + + return int_sqrt(non_zero_sample << 10); +} + +/** * tfrc_ewma - Exponentially weighted moving average * @weight: Weight to be used as damping factor, in units of 1/10 */ |