Commit 68c89ee5 authored by Gerrit Renker's avatar Gerrit Renker
Browse files

dccp ccid-3: Update the computation of X_recv 



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: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 22338f09

net/dccp/ccids/ccid3.c

+24 −40
Original line number Diff line number Diff line
@@ -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.

		 */

		if (hcrx->x_recv > 0)

		 * always check whether at least RTT time units were covered.

		 */

		hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);

		break;

		/* fall through */

	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);



	dccp_sk(sk)->dccps_hc_rx_insert_options = 1;

	dccp_send_ack(sk);



	hcrx->tstamp_last_feedback = now;

prepare_for_next_time:

	tfrc_rx_hist_restart_byte_counter(&hcrx->hist);

	hcrx->last_counter = dccp_hdr(skb)->dccph_ccval;

	hcrx->hist.bytes_recvd	   = 0;

	hcrx->feedback	   = fbtype;



	dp->dccps_hc_rx_insert_options = 1;

	dccp_send_ack(sk);

}



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)

net/dccp/ccids/lib/packet_history.c

+30 −0
Original line number Diff line number Diff line
@@ -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;

net/dccp/ccids/lib/packet_history.h

+12 −1
Original line number Diff line number Diff line
@@ -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);

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