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Commit c84a5711 authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

tcp: fix bogus RTT on special retransmission 



RTT may be bogus with tall loss probe (TLP) when a packet
is retransmitted and latter (s)acked without TCPCB_SACKED_RETRANS flag.

For example, TLP calls __tcp_retransmit_skb() instead of
tcp_retransmit_skb(). The skb timestamps are updated but the sacked
flag is not marked with TCPCB_SACKED_RETRANS. As a result we'll
get bogus RTT in tcp_clean_rtx_queue() or in tcp_sacktag_one() on
spurious retransmission.

The fix is to apply the sticky flag TCP_EVER_RETRANS to enforce Karn's
check on RTT sampling. However this will disable F-RTO if timeout occurs
after TLP, by resetting undo_marker in tcp_enter_loss(). We relax this
check to only if any pending retransmists are still in-flight.

Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Acked-by: default avatarEric Dumazet <edumazet@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Acked-by: default avatarNandita Dukkipati <nanditad@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent de39d7a4

net/ipv4/tcp_input.c

+2 −1
Original line number Diff line number Diff line
@@ -1945,8 +1945,9 @@ void tcp_enter_loss(struct sock *sk, int how) 
		if (skb == tcp_send_head(sk))

			break;



		if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)

		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)

			tp->undo_marker = 0;



		TCP_SKB_CB(skb)->sacked &= (~TCPCB_TAGBITS)|TCPCB_SACKED_ACKED;

		if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED) || how) {

			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;

net/ipv4/tcp_output.c

+8 −3
Original line number Diff line number Diff line
@@ -2337,6 +2337,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb) 
	struct tcp_sock *tp = tcp_sk(sk);

	struct inet_connection_sock *icsk = inet_csk(sk);

	unsigned int cur_mss;

	int err;



	/* Inconslusive MTU probe */

	if (icsk->icsk_mtup.probe_size) {
@@ -2400,11 +2401,15 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb) 
		     skb_headroom(skb) >= 0xFFFF)) {

		struct sk_buff *nskb = __pskb_copy(skb, MAX_TCP_HEADER,

						   GFP_ATOMIC);

		return nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :

		err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :

			     -ENOBUFS;

	} else {

		return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);

		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);

	}



	if (likely(!err))

		TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;

	return err;

}



int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)