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[DCCP] ccid3: Finer-grained resolution of sending rates
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This patch
 * resolves a bug where packets smaller than 32/64 bytes resulted in sending rates of 0
 * supports all sending rates from 1/64 bytes/second up to 4Gbyte/second
 * simplifies the present overflow problems in calculations

Current sending rate X and the cached value X_recv of the receiver-estimated
sending rate are both scaled by 64 (2^6) in order to
 * cope with low sending rates (minimally 1 byte/second)
 * allow upgrading to use a packets-per-second implementation of CCID 3
 * avoid calculation errors due to integer arithmetic cut-off

The patch implements a revised strategy from
http://www.mail-archive.com/dccp@vger.kernel.org/msg01040.html

The only difference with regard to that strategy is that t_ipi is already
used in the calculation of the nofeedback timeout, which saves one division.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
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Gerrit Renker authored and David S. Miller committed Dec 11, 2006
1 parent 179ebc9 commit 1a21e49
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Showing 3 changed files with 78 additions and 39 deletions.
8 changes: 6 additions & 2 deletions include/linux/tfrc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -37,10 +37,14 @@ struct tfrc_rx_info {
* @tfrctx_p: current loss event rate (5.4)
* @tfrctx_rto: estimate of RTO, equals 4*RTT (4.3)
* @tfrctx_ipi: inter-packet interval (4.6)
*
* Note: X and X_recv are both maintained in units of 64 * bytes/second. This
* enables a finer resolution of sending rates and avoids problems with
* integer arithmetic; u32 is not sufficient as scaling consumes 6 bits.
*/
struct tfrc_tx_info {
__u32 tfrctx_x;
__u32 tfrctx_x_recv;
__u64 tfrctx_x;
__u64 tfrctx_x_recv;
__u32 tfrctx_x_calc;
__u32 tfrctx_rtt;
__u32 tfrctx_p;
Expand Down
76 changes: 46 additions & 30 deletions net/dccp/ccids/ccid3.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -108,8 +108,9 @@ static inline void ccid3_update_send_time(struct ccid3_hc_tx_sock *hctx)
{
timeval_sub_usecs(&hctx->ccid3hctx_t_nom, hctx->ccid3hctx_t_ipi);

/* Calculate new t_ipi (inter packet interval) by t_ipi = s / X_inst */
hctx->ccid3hctx_t_ipi = usecs_div(hctx->ccid3hctx_s, hctx->ccid3hctx_x);
/* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */
hctx->ccid3hctx_t_ipi = scaled_div(hctx->ccid3hctx_s,
hctx->ccid3hctx_x >> 6);

/* Update nominal send time with regard to the new t_ipi */
timeval_add_usecs(&hctx->ccid3hctx_t_nom, hctx->ccid3hctx_t_ipi);
Expand All @@ -128,26 +129,33 @@ static inline void ccid3_update_send_time(struct ccid3_hc_tx_sock *hctx)
* X = max(min(2 * X, 2 * X_recv), s / R);
* tld = now;
*
* Note: X and X_recv are both stored in units of 64 * bytes/second, to support
* fine-grained resolution of sending rates. This requires scaling by 2^6
* throughout the code. Only X_calc is unscaled (in bytes/second).
*
* 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 timeval *now)

{
struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
const __u32 old_x = hctx->ccid3hctx_x;
const __u64 old_x = hctx->ccid3hctx_x;

if (hctx->ccid3hctx_p > 0) {
hctx->ccid3hctx_x = max_t(u32, min(hctx->ccid3hctx_x_calc,
hctx->ccid3hctx_x_recv * 2),
hctx->ccid3hctx_s / TFRC_T_MBI);

hctx->ccid3hctx_x = min_t(u64, hctx->ccid3hctx_x_calc << 6,
hctx->ccid3hctx_x_recv * 2 );
hctx->ccid3hctx_x = max_t(u64, hctx->ccid3hctx_x,
(hctx->ccid3hctx_s << 6)/TFRC_T_MBI);

} else if (timeval_delta(now, &hctx->ccid3hctx_t_ld) >=
hctx->ccid3hctx_rtt) {
hctx->ccid3hctx_x = max(min(hctx->ccid3hctx_x_recv,
hctx->ccid3hctx_x ) * 2,
usecs_div(hctx->ccid3hctx_s,
hctx->ccid3hctx_rtt) );

hctx->ccid3hctx_x = max(2 * min(hctx->ccid3hctx_x,
hctx->ccid3hctx_x_recv),
scaled_div(hctx->ccid3hctx_s << 6,
hctx->ccid3hctx_rtt ));
hctx->ccid3hctx_t_ld = *now;
}

Expand Down Expand Up @@ -194,13 +202,13 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
case TFRC_SSTATE_NO_FBACK:
/* RFC 3448, 4.4: Halve send rate directly */
hctx->ccid3hctx_x = max_t(u32, hctx->ccid3hctx_x / 2,
hctx->ccid3hctx_s / TFRC_T_MBI);
(hctx->ccid3hctx_s << 6)/TFRC_T_MBI);

ccid3_pr_debug("%s, sk=%p, state=%s, updated tx rate to %d "
ccid3_pr_debug("%s, sk=%p, state=%s, updated tx rate to %u "
"bytes/s\n",
dccp_role(sk), sk,
ccid3_tx_state_name(hctx->ccid3hctx_state),
hctx->ccid3hctx_x);
(unsigned)(hctx->ccid3hctx_x >> 6));
/* The value of R is still undefined and so we can not recompute
* the timout value. Keep initial value as per [RFC 4342, 5]. */
t_nfb = TFRC_INITIAL_TIMEOUT;
Expand All @@ -209,11 +217,11 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
case TFRC_SSTATE_FBACK:
/*
* Check if IDLE since last timeout and recv rate is less than
* 4 packets per RTT
* 4 packets (in units of 64*bytes/sec) per RTT
*/
if (!hctx->ccid3hctx_idle ||
(hctx->ccid3hctx_x_recv >=
4 * usecs_div(hctx->ccid3hctx_s, hctx->ccid3hctx_rtt))) {
(hctx->ccid3hctx_x_recv >= 4 *
scaled_div(hctx->ccid3hctx_s << 6, hctx->ccid3hctx_rtt))) {
struct timeval now;

ccid3_pr_debug("%s, sk=%p, state=%s, not idle\n",
Expand All @@ -227,17 +235,23 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
* X_recv = max(X_recv / 2, s / (2 * t_mbi));
* Else
* X_recv = X_calc / 4;
*
* Note that X_recv is scaled by 2^6 while X_calc is not
*/
BUG_ON(hctx->ccid3hctx_p && !hctx->ccid3hctx_x_calc);

if (hctx->ccid3hctx_p == 0 ||
hctx->ccid3hctx_x_calc > 2 * hctx->ccid3hctx_x_recv) {
hctx->ccid3hctx_x_recv = max_t(u32, hctx->ccid3hctx_x_recv / 2,
hctx->ccid3hctx_s / (2 * TFRC_T_MBI));
hctx->ccid3hctx_x_calc > (hctx->ccid3hctx_x_recv >> 5)) {

hctx->ccid3hctx_x_recv =
max_t(u64, hctx->ccid3hctx_x_recv / 2,
(hctx->ccid3hctx_s << 6) /
(2*TFRC_T_MBI));

if (hctx->ccid3hctx_p == 0)
dccp_timestamp(sk, &now);
} else
hctx->ccid3hctx_x_recv = hctx->ccid3hctx_x_calc / 4;
hctx->ccid3hctx_x_recv = hctx->ccid3hctx_x_calc << 4;

/* Now recalculate X [RFC 3448, 4.3, step (4)] */
ccid3_hc_tx_update_x(sk, &now);
Expand Down Expand Up @@ -315,9 +329,9 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
hctx->ccid3hctx_t_last_win_count = now;
ccid3_hc_tx_set_state(sk, TFRC_SSTATE_NO_FBACK);

/* Set initial sending rate to 1 packet per second */
/* Set initial sending rate X/s to 1pps (X is scaled by 2^6) */
ccid3_hc_tx_update_s(hctx, skb->len);
hctx->ccid3hctx_x = hctx->ccid3hctx_s;
hctx->ccid3hctx_x = hctx->ccid3hctx_s << 6;

/* First timeout, according to [RFC 3448, 4.2], is 1 second */
hctx->ccid3hctx_t_ipi = USEC_PER_SEC;
Expand Down Expand Up @@ -438,8 +452,8 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
return;
}

/* Update receive rate */
hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate;
/* Update receive rate in units of 64 * bytes/second */
hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate << 6;

/* Update loss event rate */
pinv = opt_recv->ccid3or_loss_event_rate;
Expand Down Expand Up @@ -475,20 +489,22 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
* q is a constant, RFC 3448 recomments 0.9
*/
if (hctx->ccid3hctx_state == TFRC_SSTATE_NO_FBACK) {
/* Use Larger Initial Windows [RFC 4342, sec. 5]
* We deviate in that we use `s' instead of `MSS'. */
/*
* Larger Initial Windows [RFC 4342, sec. 5]
* We deviate in that we use `s' instead of `MSS'.
*/
u16 w_init = min( 4 * hctx->ccid3hctx_s,
max(2 * hctx->ccid3hctx_s, 4380));
hctx->ccid3hctx_rtt = r_sample;
hctx->ccid3hctx_x = usecs_div(w_init, r_sample);
hctx->ccid3hctx_x = scaled_div(w_init<< 6, r_sample);
hctx->ccid3hctx_t_ld = now;

ccid3_update_send_time(hctx);

ccid3_pr_debug("%s(%p), s=%u, w_init=%u, "
"R_sample=%ldus, X=%u\n", dccp_role(sk),
sk, hctx->ccid3hctx_s, w_init, r_sample,
hctx->ccid3hctx_x);
(unsigned)(hctx->ccid3hctx_x >> 6));

ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
} else {
Expand All @@ -508,7 +524,7 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
sk, hctx->ccid3hctx_rtt, r_sample,
hctx->ccid3hctx_s, hctx->ccid3hctx_p,
hctx->ccid3hctx_x_calc,
hctx->ccid3hctx_x);
(unsigned)(hctx->ccid3hctx_x >> 6));
}

/* unschedule no feedback timer */
Expand Down
33 changes: 26 additions & 7 deletions net/dccp/ccids/ccid3.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -75,14 +75,14 @@ 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_calc - Calculated send rate (RFC 3448, 3.1)
* @ccid3hctx_x - Current sending rate in 64 * bytes per second
* @ccid3hctx_x_recv - Receive rate in 64 * bytes per second
* @ccid3hctx_x_calc - Calculated rate in bytes per second
* @ccid3hctx_rtt - Estimate of current round trip time in usecs
* @ccid3hctx_p - Current loss event rate (0-1) scaled by 1000000
* @ccid3hctx_s - Packet size
* @ccid3hctx_t_rto - Retransmission Timeout (RFC 3448, 3.1)
* @ccid3hctx_t_ipi - Interpacket (send) interval (RFC 3448, 4.6)
* @ccid3hctx_s - Packet size in bytes
* @ccid3hctx_t_rto - Nofeedback Timer setting in usecs
* @ccid3hctx_t_ipi - Interpacket (send) interval (RFC 3448, 4.6) in usecs
* @ccid3hctx_state - Sender state, one of %ccid3_hc_tx_states
* @ccid3hctx_last_win_count - Last window counter sent
* @ccid3hctx_t_last_win_count - Timestamp of earliest packet
Expand All @@ -91,7 +91,7 @@ enum ccid3_hc_tx_states {
* @ccid3hctx_idle - Flag indicating that sender is idling
* @ccid3hctx_t_ld - Time last doubled during slow start
* @ccid3hctx_t_nom - Nominal send time of next packet
* @ccid3hctx_delta - Send timer delta
* @ccid3hctx_delta - Send timer delta (RFC 3448, 4.6) in usecs
* @ccid3hctx_hist - Packet history
* @ccid3hctx_options_received - Parsed set of retrieved options
*/
Expand Down Expand Up @@ -171,4 +171,23 @@ static inline struct ccid3_hc_rx_sock *ccid3_hc_rx_sk(const struct sock *sk)
return ccid_priv(dccp_sk(sk)->dccps_hc_rx_ccid);
}

static inline u64 scaled_div(u64 a, u32 b)
{
BUG_ON(b==0);
a *= 1000000;
do_div(a, b);
return a;
}

static inline u32 scaled_div32(u64 a, u32 b)
{
u64 result = scaled_div(a, b);

if (result > UINT_MAX) {
DCCP_CRIT("Overflow: a(%llu)/b(%u) > ~0U",
(unsigned long long)a, b);
return UINT_MAX;
}
return result;
}
#endif /* _DCCP_CCID3_H_ */

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