Commit bfe24a6c authored by Gerrit Renker's avatar Gerrit Renker Committed by David S. Miller
Browse files

[DCCP] ccid3: Simplify calculation for reverse lookup of p 



 This simplifies the calculation of a value p for a given fval when the
 first loss interval is computed (RFC 3448, 6.3.1). It makes use of the
 two new functions scaled_div/scaled_div32 to provide overflow protection.

 Additionally, protection against divide-by-zero is extended - in this
 case the function will return the maximally possible value of p=100%.

Background:

 The maximum fval, f(100%), is approximately 244, i.e. the scaled value of fval
 should never exceed 244E6, which fits easily into u32. The problem is the scaling
 by 10^6, since additionally R(TT) is in microseconds.
 This is resolved by breaking the division into two stages: the first stage
 computes fval=(s*10^6)/R, stores that into u64; the second stage computes
 fval = (fval*10^6)/X_recv and complains if overflow is reached for u32.
 This case is safe since the TFRC reverse-lookup routine then returns p=100%.

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: default avatarIan McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@mandriva.com>
parent b9039a2a

net/dccp/ccids/ccid3.c

+22 −17
Original line number Diff line number Diff line
@@ -785,12 +785,12 @@ static u32 ccid3_hc_rx_calc_first_li(struct sock *sk) 
{

	struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);

	struct dccp_rx_hist_entry *entry, *next, *tail = NULL;

	u32 rtt, delta, x_recv, fval, p, tmp2;

	u32 rtt, delta, x_recv, p;

	struct timeval tstamp = { 0, };

	int interval = 0;

	int win_count = 0;

	int step = 0;

	u64 tmp1;

	u64 fval;



	list_for_each_entry_safe(entry, next, &hcrx->ccid3hcrx_hist,

				 dccphrx_node) {
@@ -834,30 +834,35 @@ found: 
	ccid3_pr_debug("%s, sk=%p, approximated RTT to %uus\n",

		       dccp_role(sk), sk, rtt);



	if (rtt == 0) {

		DCCP_WARN("RTT==0, setting to 1\n");

 		rtt = 1;

	/*

	 * Determine the length of the first loss interval via inverse lookup.

	 * Assume that X_recv can be computed by the throughput equation

	 *      	    s

	 * 	X_recv = --------

	 * 		 R * fval

	 * Find some p such that f(p) = fval; return 1/p [RFC 3448, 6.3.1].

	 */

	if (rtt == 0) {			/* would result in divide-by-zero */

		DCCP_WARN("RTT==0, returning 1/p = 1\n");

		return 1000000;

	}



	dccp_timestamp(sk, &tstamp);

	delta = timeval_delta(&tstamp, &hcrx->ccid3hcrx_tstamp_last_feedback);

	x_recv = scaled_div32(hcrx->ccid3hcrx_bytes_recv, delta);



	if (x_recv == 0)

		x_recv = hcrx->ccid3hcrx_x_recv;



	tmp1 = (u64)x_recv * (u64)rtt;

	do_div(tmp1,10000000);

	tmp2 = (u32)tmp1;



	if (!tmp2) {

		DCCP_CRIT("tmp2 = 0, x_recv = %u, rtt =%u\n", x_recv, rtt);

		return ~0;

	if (x_recv == 0) {		/* would also trigger divide-by-zero */

		DCCP_WARN("X_recv==0\n");

		if ((x_recv = hcrx->ccid3hcrx_x_recv) == 0) {

			DCCP_BUG("stored value of X_recv is zero");

			return 1000000;

		}

	}



	fval = (hcrx->ccid3hcrx_s * 100000) / tmp2;

	/* do not alter order above or you will get overflow on 32 bit */

	fval = scaled_div(hcrx->ccid3hcrx_s, rtt);

	fval = scaled_div32(fval, x_recv);

	p = tfrc_calc_x_reverse_lookup(fval);



	ccid3_pr_debug("%s, sk=%p, receive rate=%u bytes/s, implied "

		       "loss rate=%u\n", dccp_role(sk), sk, x_recv, p);

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