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Commit d346d886 authored by Samuel Jero's avatar Samuel Jero Committed by Gerrit Renker
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dccp ccid-2: prevent cwnd > Sequence Window 



Add a check to prevent CCID-2 from increasing the cwnd greater than the
Sequence Window.

When the congestion window becomes bigger than the Sequence Window, CCID-2
will attempt to keep more data in the network than the DCCP Sequence Window
code considers possible. This results in the Sequence Window code issuing
a Sync, thereby inducing needless overhead. Further, if this occurs at the
sender, CCID-2 will never detect the problem because the Acks it receives
will indicate no losses. I have seen this cause a drop of 1/3rd in throughput
for a connection.

Also add code to adjust the Sequence Window to be about 5 times the number of
packets in the network (RFC 4340, 7.5.2) and to adjust the Ack Ratio so that
the remote Sequence Window will hold about 5 times the number of packets in
the network. This allows the congestion window to increase correctly without
being limited by the Sequence Window.

Signed-off-by: default avatarSamuel Jero <sj323707@ohio.edu>
Acked-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 31daf039

net/dccp/ccids/ccid2.c

+35 −15
Original line number Diff line number Diff line
@@ -85,7 +85,6 @@ static int ccid2_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb) 


static void ccid2_change_l_ack_ratio(struct sock *sk, u32 val)

{

	struct dccp_sock *dp = dccp_sk(sk);

	u32 max_ratio = DIV_ROUND_UP(ccid2_hc_tx_sk(sk)->tx_cwnd, 2);



	/*
@@ -98,14 +97,15 @@ static void ccid2_change_l_ack_ratio(struct sock *sk, u32 val) 
		DCCP_WARN("Limiting Ack Ratio (%u) to %u\n", val, max_ratio);

		val = max_ratio;

	}

	if (val > DCCPF_ACK_RATIO_MAX)

		val = DCCPF_ACK_RATIO_MAX;



	if (val == dp->dccps_l_ack_ratio)

		return;

	dccp_feat_signal_nn_change(sk, DCCPF_ACK_RATIO,

				   min_t(u32, val, DCCPF_ACK_RATIO_MAX));

}



	ccid2_pr_debug("changing local ack ratio to %u\n", val);

	dp->dccps_l_ack_ratio = val;

static void ccid2_change_l_seq_window(struct sock *sk, u64 val)

{

	dccp_feat_signal_nn_change(sk, DCCPF_SEQUENCE_WINDOW,

				   clamp_val(val, DCCPF_SEQ_WMIN,

						  DCCPF_SEQ_WMAX));

}



static void ccid2_hc_tx_rto_expire(unsigned long data)
@@ -405,7 +405,11 @@ static void ccid2_new_ack(struct sock *sk, struct ccid2_seq *seqp, 
			  unsigned int *maxincr)

{

	struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

	struct dccp_sock *dp = dccp_sk(sk);

	int r_seq_used = hc->tx_cwnd / dp->dccps_l_ack_ratio;



	if (hc->tx_cwnd < dp->dccps_l_seq_win &&

	    r_seq_used < dp->dccps_r_seq_win) {

		if (hc->tx_cwnd < hc->tx_ssthresh) {

			if (*maxincr > 0 && ++hc->tx_packets_acked == 2) {

				hc->tx_cwnd += 1;
@@ -416,6 +420,22 @@ static void ccid2_new_ack(struct sock *sk, struct ccid2_seq *seqp, 
			hc->tx_cwnd += 1;

			hc->tx_packets_acked = 0;

		}

	}



	/*

	 * Adjust the local sequence window and the ack ratio to allow about

	 * 5 times the number of packets in the network (RFC 4340 7.5.2)

	 */

	if (r_seq_used * CCID2_WIN_CHANGE_FACTOR >= dp->dccps_r_seq_win)

		ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio * 2);

	else if (r_seq_used * CCID2_WIN_CHANGE_FACTOR < dp->dccps_r_seq_win/2)

		ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio / 2 ? : 1U);



	if (hc->tx_cwnd * CCID2_WIN_CHANGE_FACTOR >= dp->dccps_l_seq_win)

		ccid2_change_l_seq_window(sk, dp->dccps_l_seq_win * 2);

	else if (hc->tx_cwnd * CCID2_WIN_CHANGE_FACTOR < dp->dccps_l_seq_win/2)

		ccid2_change_l_seq_window(sk, dp->dccps_l_seq_win / 2);



	/*

	 * FIXME: RTT is sampled several times per acknowledgment (for each

	 * entry in the Ack Vector), instead of once per Ack (as in TCP SACK).

net/dccp/ccids/ccid2.h

+6 −0
Original line number Diff line number Diff line
@@ -43,6 +43,12 @@ struct ccid2_seq { 
#define CCID2_SEQBUF_LEN 1024

#define CCID2_SEQBUF_MAX 128



/*

 * Multiple of congestion window to keep the sequence window at

 * (RFC 4340 7.5.2)

 */

#define CCID2_WIN_CHANGE_FACTOR 5



/**

 * struct ccid2_hc_tx_sock - CCID2 TX half connection

 * @tx_{cwnd,ssthresh,pipe}: as per RFC 4341, section 5