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dccp ccid-3: Update the computation of X_recv

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This updates the computation of X_recv with regard to Errata 610/611 for
RFC 4342 and draft rfc3448bis-06, ensuring that at least an interval of 1
RTT is used to compute X_recv.  The change is wrapped into a new function
ccid3_hc_rx_x_recv().

Further changes:
----------------
 * feedback is not sent when no data packets arrived (bytes_recv == 0), as per
   rfc3448bis-06, 6.2;
 * take the timestamp for the feedback /after/ dccp_send_ack() returns, to avoid
   taking the transmission time into account (in case layer-2 is busy);
 * clearer handling of failure in ccid3_first_li().

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This commit is contained in:
Gerrit Renker 2008-09-04 07:30:19 +02:00
parent 22338f09bd
commit 68c89ee535
3 changed files with 66 additions and 41 deletions
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64
net/dccp/ccids/ccid3.c
View File

@ -533,9 +533,6 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
enum ccid3_fback_type fbtype)
{
struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
struct dccp_sock *dp = dccp_sk(sk);
ktime_t now = ktime_get_real();
s64 delta = 0;
switch (fbtype) {
case CCID3_FBACK_INITIAL:
@ -565,42 +562,33 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
/*
* When parameters change (new loss or p > p_prev), we do not
* have a reliable estimate for R_m of [RFC 3448, 6.2] and so
* need to reuse the previous value of X_recv. However, when
* X_recv was 0 (due to early loss), this would kill X down to
* s/t_mbi (i.e. one packet in 64 seconds).
* To avoid such drastic reduction, we approximate X_recv as
* the number of bytes since last feedback.
* This is a safe fallback, since X is bounded above by X_calc.
* always check whether at least RTT time units were covered.
*/
if (hcrx->x_recv > 0)
break;
/* fall through */
hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
break;
case CCID3_FBACK_PERIODIC:
/*
* FIXME: check if delta is less than or equal to 1 RTT using
* the receiver RTT sample. This is described in Errata 610/611
* of RFC 4342 which reference section 6.2 of RFC 3448.
* Step (2) of rfc3448bis-06, 6.2:
* - if no data packets have been received, just restart timer
* - if data packets have been received, re-compute X_recv
*/
delta = ktime_us_delta(now, hcrx->tstamp_last_feedback);
if (delta <= 0)
DCCP_BUG("delta (%ld) <= 0", (long)delta);
else
hcrx->x_recv = scaled_div32(hcrx->hist.bytes_recvd, delta);
if (hcrx->hist.bytes_recvd == 0)
goto prepare_for_next_time;
hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
break;
default:
return;
}
ccid3_pr_debug("Interval %ldusec, X_recv=%u, 1/p=%u\n",
(long)delta, hcrx->x_recv, hcrx->p_inverse);
ccid3_pr_debug("X_recv=%u, 1/p=%u\n", hcrx->x_recv, hcrx->p_inverse);
hcrx->tstamp_last_feedback = now;
hcrx->last_counter = dccp_hdr(skb)->dccph_ccval;
hcrx->hist.bytes_recvd = 0;
hcrx->feedback = fbtype;
dp->dccps_hc_rx_insert_options = 1;
dccp_sk(sk)->dccps_hc_rx_insert_options = 1;
dccp_send_ack(sk);
prepare_for_next_time:
tfrc_rx_hist_restart_byte_counter(&hcrx->hist);
hcrx->last_counter = dccp_hdr(skb)->dccph_ccval;
hcrx->feedback = fbtype;
}
static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
@ -639,7 +627,7 @@ static u32 ccid3_first_li(struct sock *sk)
{
struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p, delta;
rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p;
u64 fval;
/*
@ -650,16 +638,9 @@ static u32 ccid3_first_li(struct sock *sk)
if (unlikely(hcrx->feedback == CCID3_FBACK_NONE))
return 5;
delta = ktime_to_us(net_timedelta(hcrx->tstamp_last_feedback));
x_recv = scaled_div32(hcrx->hist.bytes_recvd, delta);
if (x_recv == 0) { /* would also trigger divide-by-zero */
DCCP_WARN("X_recv==0\n");
if (hcrx->x_recv == 0) {
DCCP_BUG("stored value of X_recv is zero");
return ~0U;
}
x_recv = hcrx->x_recv;
}
x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
if (x_recv == 0)
goto failed;
fval = scaled_div32(scaled_div(s, rtt), x_recv);
p = tfrc_calc_x_reverse_lookup(fval);
@ -667,7 +648,10 @@ static u32 ccid3_first_li(struct sock *sk)
ccid3_pr_debug("%s(%p), receive rate=%u bytes/s, implied "
"loss rate=%u\n", dccp_role(sk), sk, x_recv, p);
return p == 0 ? ~0U : scaled_div(1, p);
if (p > 0)
return scaled_div(1, p);
failed:
return UINT_MAX;
}
static void ccid3_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)

30
net/dccp/ccids/lib/packet_history.c
View File

@ -385,6 +385,36 @@ int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
}
EXPORT_SYMBOL_GPL(tfrc_rx_handle_loss);
/* Compute the sending rate X_recv measured between feedback intervals */
u32 tfrc_rx_hist_x_recv(struct tfrc_rx_hist *h, const u32 last_x_recv)
{
u64 bytes = h->bytes_recvd, last_rtt = h->rtt_estimate;
s64 delta = ktime_to_us(net_timedelta(h->bytes_start));
WARN_ON(delta <= 0);
/*
* Ensure that the sampling interval for X_recv is at least one RTT,
* by extending the sampling interval backwards in time, over the last
* R_(m-1) seconds, as per rfc3448bis-06, 6.2.
* To reduce noise (e.g. when the RTT changes often), this is only
* done when delta is smaller than RTT/2.
*/
if (last_x_recv > 0 && delta < last_rtt/2) {
tfrc_pr_debug("delta < RTT ==> %ld us < %u us\n",
(long)delta, (unsigned)last_rtt);
delta = (bytes ? delta : 0) + last_rtt;
bytes += div_u64((u64)last_x_recv * last_rtt, USEC_PER_SEC);
}
if (unlikely(bytes == 0)) {
DCCP_WARN("X_recv == 0, using old value of %u\n", last_x_recv);
return last_x_recv;
}
return scaled_div32(bytes, delta);
}
EXPORT_SYMBOL_GPL(tfrc_rx_hist_x_recv);
void tfrc_rx_hist_purge(struct tfrc_rx_hist *h)
{
int i;

13
net/dccp/ccids/lib/packet_history.h
View File

@ -93,7 +93,8 @@ struct tfrc_rx_hist_entry {
* @rtt_sample_prev: Used during RTT sampling, points to candidate entry
* @rtt_estimate: Receiver RTT estimate
* @packet_size: Packet size in bytes (as per RFC 3448, 3.1)
* @bytes_recvd: Number of bytes received since last sending feedback
* @bytes_recvd: Number of bytes received since @bytes_start
* @bytes_start: Start time for counting @bytes_recvd
*/
struct tfrc_rx_hist {
struct tfrc_rx_hist_entry *ring[TFRC_NDUPACK + 1];
@ -105,6 +106,7 @@ struct tfrc_rx_hist {
/* Receiver sampling of application payload lengths */
u32 packet_size,
bytes_recvd;
ktime_t bytes_start;
};
/**
@ -169,6 +171,15 @@ static inline u32 tfrc_rx_hist_rtt(const struct tfrc_rx_hist *h)
return h->rtt_estimate;
}
static inline void tfrc_rx_hist_restart_byte_counter(struct tfrc_rx_hist *h)
{
h->bytes_recvd = 0;
h->bytes_start = ktime_get_real();
}
extern u32 tfrc_rx_hist_x_recv(struct tfrc_rx_hist *h, const u32 last_x_recv);
extern void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
const struct sk_buff *skb, const u64 ndp);