Commit 825de27d authored by Gerrit Renker's avatar Gerrit Renker Committed by David S. Miller

dccp ccid-3: Fix "t_ipi explosion" bug

The identification of this bug is thanks to Cheng Wei and Tomasz
Grobelny.

To avoid divide-by-zero, the implementation previously ignored RTTs
smaller than 4 microseconds when performing integer division RTT/4.

When the RTT reached a value less than 4 microseconds (as observed on
loopback), this prevented the Window Counter CCVal value from
advancing. As a result, the receiver stopped sending feedback. This in
turn caused non-ending expiries of the nofeedback timer at the sender,
so that the sending rate was progressively reduced until reaching the
minimum of one packet per 64 seconds.

The patch fixes this bug by handling integer division more
intelligently. Due to consistent use of dccp_sample_rtt(),
divide-by-zero-RTT is avoided.
Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 6079a463
...@@ -193,22 +193,17 @@ static inline void ccid3_hc_tx_update_s(struct ccid3_hc_tx_sock *hctx, int len) ...@@ -193,22 +193,17 @@ static inline void ccid3_hc_tx_update_s(struct ccid3_hc_tx_sock *hctx, int len)
/* /*
* Update Window Counter using the algorithm from [RFC 4342, 8.1]. * Update Window Counter using the algorithm from [RFC 4342, 8.1].
* The algorithm is not applicable if RTT < 4 microseconds. * As elsewhere, RTT > 0 is assumed by using dccp_sample_rtt().
*/ */
static inline void ccid3_hc_tx_update_win_count(struct ccid3_hc_tx_sock *hctx, static inline void ccid3_hc_tx_update_win_count(struct ccid3_hc_tx_sock *hctx,
ktime_t now) ktime_t now)
{ {
u32 quarter_rtts; u32 delta = ktime_us_delta(now, hctx->ccid3hctx_t_last_win_count),
quarter_rtts = (4 * delta) / hctx->ccid3hctx_rtt;
if (unlikely(hctx->ccid3hctx_rtt < 4)) /* avoid divide-by-zero */
return;
quarter_rtts = ktime_us_delta(now, hctx->ccid3hctx_t_last_win_count);
quarter_rtts /= hctx->ccid3hctx_rtt / 4;
if (quarter_rtts > 0) { if (quarter_rtts > 0) {
hctx->ccid3hctx_t_last_win_count = now; hctx->ccid3hctx_t_last_win_count = now;
hctx->ccid3hctx_last_win_count += min_t(u32, quarter_rtts, 5); hctx->ccid3hctx_last_win_count += min(quarter_rtts, 5U);
hctx->ccid3hctx_last_win_count &= 0xF; /* mod 16 */ hctx->ccid3hctx_last_win_count &= 0xF; /* mod 16 */
} }
} }
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment