Quoting Eddie Kohler:
| Same comment (it's probably safe to use the Req-Response exchange for an
| initial RTT estimate).
Same answer: this is part of draft-ietf-dccp-rfc3448bis-00.txt, but not part of
RFC 3448; therefore I am against it.
|
| E
|
|
| Gerrit Renker wrote:
| > [CCID 3]: Avoid `division by zero' errors
| >
| > Several places of the code divide by the current RTT value. A division-by-zero
| > error results if this value is 0.
| > To protect against this,
| > * DCCP_BUG_ON conditions are added throughout the tx code;
| > * in ccid3_hc_tx_packet_recv, a code block is shifted, to avoid not updating
| > p and x_recv in case the RTT estimation results in an error;
| >
| > Since this could probably be done in a smarter way, a FIXME is added. Lastly,
| > some minor code comment updates.
| >
| > Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
| > ---
| > net/dccp/ccids/ccid3.c | 53 ++++++++++++++++++++-----------------
| > net/dccp/ccids/ccid3.h | 2 -
| > net/dccp/ccids/lib/tfrc_equation.c | 7 +---
| > 3 files changed, 33 insertions(+), 29 deletions(-)
| >
| > --- a/net/dccp/ccids/ccid3.c
| > +++ b/net/dccp/ccids/ccid3.c
| > @@ -117,7 +117,7 @@ static inline void ccid3_update_send_tim
| > TFRC_OPSYS_HALF_TIME_GRAN);
| > }
| > /*
| > - * Update X by
| > + * Update X by (this implements step (4) of [RFC 3448, 4.3]):
| > * If (p > 0)
| > * x_calc = calcX(s, R, p);
| > * X = max(min(X_calc, 2 * X_recv), s / t_mbi);
| > @@ -125,12 +125,17 @@ static inline void ccid3_update_send_tim
| > * If (now - tld >= R)
| > * X = max(min(2 * X, 2 * X_recv), s / R);
| > * tld = now;
| > + *
| > + * If X has changed, we also update the scheduled send time t_now,
| > + * the inter-packet interval t_ipi, and the delta value.
| > */
| > static void ccid3_hc_tx_update_x(struct sock *sk)
| > {
| > struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
| > const __u32 old_x = hctx->ccid3hctx_x;
| >
| > + DCCP_BUG_ON(hctx->ccid3hctx_rtt == 0);
| > +
| > /* To avoid large error in calcX */
| > if (hctx->ccid3hctx_p >= TFRC_SMALLEST_P) {
| > hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
| > @@ -421,7 +426,6 @@ static void ccid3_hc_tx_packet_recv(stru
| > unsigned long next_tmout;
| > u32 t_elapsed;
| > u32 pinv;
| > - u32 x_recv;
| > u32 r_sample;
| >
| > BUG_ON(hctx == NULL);
| > @@ -434,15 +438,11 @@ static void ccid3_hc_tx_packet_recv(stru
| > opt_recv = &hctx->ccid3hctx_options_received;
| >
| > t_elapsed = dp->dccps_options_received.dccpor_elapsed_time * 10;
| > - x_recv = opt_recv->ccid3or_receive_rate;
| > pinv = opt_recv->ccid3or_loss_event_rate;
| >
| > switch (hctx->ccid3hctx_state) {
| > case TFRC_SSTATE_NO_FBACK:
| > case TFRC_SSTATE_FBACK:
| > - /* Calculate new round trip sample by
| > - * R_sample = (now - t_recvdata) - t_delay */
| > - /* get t_recvdata from history */
| > packet = dccp_tx_hist_find_entry(&hctx->ccid3hctx_hist,
| > DCCP_SKB_CB(skb)->dccpd_ack_seq);
| > if (unlikely(packet == NULL)) {
| > @@ -453,7 +453,25 @@ static void ccid3_hc_tx_packet_recv(stru
| > return;
| > }
| >
| > - /* Update RTT */
| > + /* Update receive rate */
| > + hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate;
| > +
| > + /* Update loss event rate */
| > + if (pinv == ~0 || pinv == 0)
| > + hctx->ccid3hctx_p = 0;
| > + else {
| > + hctx->ccid3hctx_p = 1000000 / pinv;
| > +
| > + if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
| > + hctx->ccid3hctx_p = TFRC_SMALLEST_P;
| > + ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
| > + dccp_role(sk), sk);
| > + }
| > + }
| > +
| > + /* Calculate new round trip sample by
| > + * R_sample = (t_now - t_recvdata) - t_delay
| > + * t_recvdata is taken from packet history */
| > dccp_timestamp(sk, &now);
| > r_sample = timeval_delta(&now, &packet->dccphtx_tstamp);
| > if (unlikely(r_sample <= t_elapsed))
| > @@ -462,7 +480,7 @@ static void ccid3_hc_tx_packet_recv(stru
| > else
| > r_sample -= t_elapsed;
| >
| > - /* Update RTT estimate by
| > + /* Update RTT estimate by (step (2) of [RFC 3448, 4.3]):
| > * If (No feedback recv)
| > * R = R_sample;
| > * Else
| > @@ -481,21 +499,10 @@ static void ccid3_hc_tx_packet_recv(stru
| > "r_sample=%us\n", dccp_role(sk), sk,
| > hctx->ccid3hctx_rtt, r_sample);
| >
| > - /* Update receive rate */
| > - hctx->ccid3hctx_x_recv = x_recv;/* X_recv in bytes per sec */
| > -
| > - /* Update loss event rate */
| > - if (pinv == ~0 || pinv == 0)
| > - hctx->ccid3hctx_p = 0;
| > - else {
| > - hctx->ccid3hctx_p = 1000000 / pinv;
| > -
| > - if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
| > - hctx->ccid3hctx_p = TFRC_SMALLEST_P;
| > - ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
| > - dccp_role(sk), sk);
| > - }
| > - }
| > + /* Avoid `division-by zero' errors.
| > + * FIXME: We need a smarter and more robust way to
| > + * protect against this possibility */
| > + DCCP_BUG_ON(hctx->ccid3hctx_rtt == 0);
| >
| > /* unschedule no feedback timer */
| > sk_stop_timer(sk, &hctx->ccid3hctx_no_feedback_timer);
| > --- a/net/dccp/ccids/lib/tfrc_equation.c
| > +++ b/net/dccp/ccids/lib/tfrc_equation.c
| > @@ -578,6 +578,8 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
| > u32 f;
| > u64 tmp1, tmp2;
| >
| > + DCCP_BUG_ON(R == 0); /* RTT can't be zero or else divide by zero */
| > +
| > if (p < TFRC_CALC_X_SPLIT)
| > index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;
| > else
| > @@ -587,11 +589,6 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
| > /* p should be 0 unless there is a bug in my code */
| > index = 0;
| >
| > - if (R == 0) {
| > - DCCP_WARN("RTT==0, setting to 1\n");
| > - R = 1; /* RTT can't be zero or else divide by zero */
| > - }
| > -
| > BUG_ON(index >= TFRC_CALC_X_ARRSIZE);
| >
| > if (p >= TFRC_CALC_X_SPLIT)
| > --- a/net/dccp/ccids/ccid3.h
| > +++ b/net/dccp/ccids/ccid3.h
| > @@ -78,7 +78,7 @@ enum ccid3_hc_tx_states {
| > /** struct ccid3_hc_tx_sock - CCID3 sender half-connection socket
| > *
| > * @ccid3hctx_x - Current sending rate
| > - * @ccid3hctx_x_recv - Receive rate
| > + * @ccid3hctx_x_recv - Receive rate in bytes per second
| > * @ccid3hctx_x_calc - Calculated send rate (RFC 3448, 3.1)
| > * @ccid3hctx_rtt - Estimate of current round trip time in usecs
| > * @ccid3hctx_p - Current loss event rate (0-1) scaled by 1000000
| > -
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|
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