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Commit 113ced1f authored by Gerrit Renker's avatar Gerrit Renker
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dccp ccid-2: Perform congestion-window validation 



CCID-2's cwnd increases like TCP during slow-start, which has implications for
 * the local Sequence Window value (should be > cwnd),
 * the Ack Ratio value.
Hence an exponential growth, if it does not reflect the actual network
conditions, can quickly lead to instability.

This patch adds congestion-window validation (RFC2861) to CCID-2:
 * cwnd is constrained if the sender is application limited;
 * cwnd is reduced after a long idle period, as suggested in the '90 paper
   by Van Jacobson, in RFC 2581 (sec. 4.1);
 * cwnd is never reduced below the RFC 3390 initial window.

As marked in the comments, the code is actually almost a direct copy of the
TCP congestion-window-validation algorithms. By continuing this work, it may
in future be possible to use the TCP code (not possible at the moment).

The mechanism can be turned off using a module parameter. Sampling of the
currently-used window (moving-maximum) is however done constantly; this is
used to determine the expected window, which can be exploited to regulate
DCCP's Sequence Window value.

This patch also sets slow-start-after-idle (RFC 4341, 5.1), i.e. it behaves like
TCP when net.ipv4.tcp_slow_start_after_idle = 1.

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 58fdea0f

net/dccp/ccids/ccid2.c

+81 −3
Original line number Diff line number Diff line
@@ -153,17 +153,93 @@ static void ccid2_hc_tx_rto_expire(unsigned long data) 
	sock_put(sk);

}



/*

 *	Congestion window validation (RFC 2861).

 */

static int ccid2_do_cwv = 1;

module_param(ccid2_do_cwv, bool, 0644);

MODULE_PARM_DESC(ccid2_do_cwv, "Perform RFC2861 Congestion Window Validation");



/**

 * ccid2_update_used_window  -  Track how much of cwnd is actually used

 * This is done in addition to CWV. The sender needs to have an idea of how many

 * packets may be in flight, to set the local Sequence Window value accordingly

 * (RFC 4340, 7.5.2). The CWV mechanism is exploited to keep track of the

 * maximum-used window. We use an EWMA low-pass filter to filter out noise.

 */

static void ccid2_update_used_window(struct ccid2_hc_tx_sock *hc, u32 new_wnd)

{

	hc->tx_expected_wnd = (3 * hc->tx_expected_wnd + new_wnd) / 4;

}



/* This borrows the code of tcp_cwnd_application_limited() */

static void ccid2_cwnd_application_limited(struct sock *sk, const u32 now)

{

	struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

	/* don't reduce cwnd below the initial window (IW) */

	u32 init_win = rfc3390_bytes_to_packets(dccp_sk(sk)->dccps_mss_cache),

	    win_used = max(hc->tx_cwnd_used, init_win);



	if (win_used < hc->tx_cwnd) {

		hc->tx_ssthresh = max(hc->tx_ssthresh,

				     (hc->tx_cwnd >> 1) + (hc->tx_cwnd >> 2));

		hc->tx_cwnd = (hc->tx_cwnd + win_used) >> 1;

	}

	hc->tx_cwnd_used  = 0;

	hc->tx_cwnd_stamp = now;

}



/* This borrows the code of tcp_cwnd_restart() */

static void ccid2_cwnd_restart(struct sock *sk, const u32 now)

{

	struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

	u32 cwnd = hc->tx_cwnd, restart_cwnd,

	    iwnd = rfc3390_bytes_to_packets(dccp_sk(sk)->dccps_mss_cache);



	hc->tx_ssthresh = max(hc->tx_ssthresh, (cwnd >> 1) + (cwnd >> 2));



	/* don't reduce cwnd below the initial window (IW) */

	restart_cwnd = min(cwnd, iwnd);

	cwnd >>= (now - hc->tx_lsndtime) / hc->tx_rto;

	hc->tx_cwnd = max(cwnd, restart_cwnd);



	hc->tx_cwnd_stamp = now;

	hc->tx_cwnd_used  = 0;

}



static void ccid2_hc_tx_packet_sent(struct sock *sk, unsigned int len)

{

	struct dccp_sock *dp = dccp_sk(sk);

	struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

	const u32 now = ccid2_time_stamp;

	struct ccid2_seq *next;



	hc->tx_pipe++;

	/* slow-start after idle periods (RFC 2581, RFC 2861) */

	if (ccid2_do_cwv && !hc->tx_pipe &&

	    (s32)(now - hc->tx_lsndtime) >= hc->tx_rto)

		ccid2_cwnd_restart(sk, now);



	hc->tx_lsndtime = now;

	hc->tx_pipe    += 1;



	/* see whether cwnd was fully used (RFC 2861), update expected window */

	if (ccid2_cwnd_network_limited(hc)) {

		ccid2_update_used_window(hc, hc->tx_cwnd);

		hc->tx_cwnd_used  = 0;

		hc->tx_cwnd_stamp = now;

	} else {

		if (hc->tx_pipe > hc->tx_cwnd_used)

			hc->tx_cwnd_used = hc->tx_pipe;



		ccid2_update_used_window(hc, hc->tx_cwnd_used);



		if (ccid2_do_cwv && (s32)(now - hc->tx_cwnd_stamp) >= hc->tx_rto)

			ccid2_cwnd_application_limited(sk, now);

	}



	hc->tx_seqh->ccid2s_seq   = dp->dccps_gss;

	hc->tx_seqh->ccid2s_acked = 0;

	hc->tx_seqh->ccid2s_sent  = ccid2_time_stamp;

	hc->tx_seqh->ccid2s_sent  = now;



	next = hc->tx_seqh->ccid2s_next;

	/* check if we need to alloc more space */
@@ -594,6 +670,7 @@ static int ccid2_hc_tx_init(struct ccid *ccid, struct sock *sk) 


	/* Use larger initial windows (RFC 4341, section 5). */

	hc->tx_cwnd = rfc3390_bytes_to_packets(dp->dccps_mss_cache);

	hc->tx_expected_wnd = hc->tx_cwnd;



	/* Make sure that Ack Ratio is enabled and within bounds. */

	max_ratio = DIV_ROUND_UP(hc->tx_cwnd, 2);
@@ -606,7 +683,8 @@ static int ccid2_hc_tx_init(struct ccid *ccid, struct sock *sk) 


	hc->tx_rto	 = DCCP_TIMEOUT_INIT;

	hc->tx_rpdupack  = -1;

	hc->tx_last_cong = ccid2_time_stamp;

	hc->tx_last_cong = hc->tx_lsndtime = hc->tx_cwnd_stamp = ccid2_time_stamp;

	hc->tx_cwnd_used = 0;

	setup_timer(&hc->tx_rtotimer, ccid2_hc_tx_rto_expire,

			(unsigned long)sk);

	INIT_LIST_HEAD(&hc->tx_av_chunks);

net/dccp/ccids/ccid2.h

+10 −0
Original line number Diff line number Diff line
@@ -53,6 +53,10 @@ struct ccid2_seq { 
 * @tx_rttvar:		     moving average/maximum of @mdev_max

 * @tx_rto:		     RTO value deriving from SRTT and RTTVAR (RFC 2988)

 * @tx_rtt_seq:		     to decay RTTVAR at most once per flight

 * @tx_cwnd_used:	     actually used cwnd, W_used of RFC 2861

 * @tx_expected_wnd:	     moving average of @tx_cwnd_used

 * @tx_cwnd_stamp:	     to track idle periods in CWV

 * @tx_lsndtime:	     last time (in jiffies) a data packet was sent

 * @tx_rpseq:		     last consecutive seqno

 * @tx_rpdupack:	     dupacks since rpseq

 * @tx_av_chunks:	     list of Ack Vectors received on current skb
@@ -76,6 +80,12 @@ struct ccid2_hc_tx_sock { 
	u64			tx_rtt_seq:48;

	struct timer_list	tx_rtotimer;



	/* Congestion Window validation (optional, RFC 2861) */

	u32			tx_cwnd_used,

				tx_expected_wnd,

				tx_cwnd_stamp,

				tx_lsndtime;



	u64			tx_rpseq;

	int			tx_rpdupack;

	u32			tx_last_cong;