[TCP] FRTO: fixes fallback to conventional recovery (95c4922b) · Commits · 戴 / test

The FRTO detection did not care how ACK pattern affects to cwnd calculation of the conventional recovery. This caused incorrect setting of cwnd when the fallback becames necessary. The knowledge tcp_process_frto() has about the incoming ACK is now passed on to tcp_enter_frto_loss() in allowed_segments parameter that gives the number of segments that must be added to packets-in-flight while calculating the new cwnd. Instead of snd_una we use FLAG_DATA_ACKED in duplicate ACK detection because RFC4138 states (in Section 2.2): If the first acknowledgment after the RTO retransmission does not acknowledge all of the data that was retransmitted in step 1, the TCP sender reverts to the conventional RTO recovery. Otherwise, a malicious receiver acknowledging partial segments could cause the sender to declare the timeout spurious in a case where data was lost. If the next ACK after RTO is duplicate, we do not retransmit anything, which is equal to what conservative conventional recovery does in such case. Signed-off-by:

Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

+9 −5

Original line number	Diff line number	Diff line
		@@ -1296,7 +1296,7 @@ void tcp_enter_frto(struct sock *sk)
		* which indicates that we should follow the traditional RTO recovery,
		* i.e. mark everything lost and do go-back-N retransmission.
		*/
		static void tcp_enter_frto_loss(struct sock *sk)
		static void tcp_enter_frto_loss(struct sock *sk, int allowed_segments)
		{
		struct tcp_sock *tp = tcp_sk(sk);
		struct sk_buff *skb;
		@@ -1326,7 +1326,7 @@ static void tcp_enter_frto_loss(struct sock *sk)
		}
		tcp_sync_left_out(tp);

		tp->snd_cwnd = tp->frto_counter + tcp_packets_in_flight(tp)+1;
		tp->snd_cwnd = tcp_packets_in_flight(tp) + allowed_segments;
		tp->snd_cwnd_cnt = 0;
		tp->snd_cwnd_stamp = tcp_time_stamp;
		tp->undo_marker = 0;
		@@ -2527,6 +2527,11 @@ static void tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
		if (flag&FLAG_DATA_ACKED)
		inet_csk(sk)->icsk_retransmits = 0;

		if (!before(tp->snd_una, tp->frto_highmark)) {
		tcp_enter_frto_loss(sk, tp->frto_counter + 1);
		return;
		}

		/* RFC4138 shortcoming in step 2; should also have case c): ACK isn't
		* duplicate nor advances window, e.g., opposite dir data, winupdate
		*/
		@@ -2534,9 +2539,8 @@ static void tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
		!(flag&FLAG_FORWARD_PROGRESS))
		return;

		if (tp->snd_una == prior_snd_una \|\|
		!before(tp->snd_una, tp->frto_highmark)) {
		tcp_enter_frto_loss(sk);
		if (!(flag&FLAG_DATA_ACKED)) {
		tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3));
		return;
		}

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