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tcp: tcp_sack_new_ofo_skb() should be more conservative

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Currently, tcp_sack_new_ofo_skb() sends an ack if prior
acks were 'compressed', if room has to be made in tp->selective_acks[]

But there is no guarantee all four sack ranges can be included
in SACK option. As a matter of fact, when TCP timestamps option
is used, only three SACK ranges can be included.

Lets assume only two ranges can be included, and force the ack:

- When we touch more than 2 ranges in the reordering
  done if tcp_sack_extend() could be done.

- If we have at least 2 ranges when adding a new one.

This enforces that before a range is in third or fourth
position, at least one ACK packet included it in first/second
position.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2020-04-30 10:35:42 -07:00 committed by David S. Miller
parent 2b19585012
commit ccd0628fca
1 changed files with 11 additions and 1 deletions
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12
net/ipv4/tcp_input.c
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@ -4348,6 +4348,12 @@ static void tcp_sack_compress_send_ack(struct sock *sk)
tcp_send_ack(sk);
}
/* Reasonable amount of sack blocks included in TCP SACK option
* The max is 4, but this becomes 3 if TCP timestamps are there.
* Given that SACK packets might be lost, be conservative and use 2.
*/
#define TCP_SACK_BLOCKS_EXPECTED 2
static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
{
struct tcp_sock *tp = tcp_sk(sk);
@ -4360,6 +4366,8 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
if (tcp_sack_extend(sp, seq, end_seq)) {
if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
tcp_sack_compress_send_ack(sk);
/* Rotate this_sack to the first one. */
for (; this_sack > 0; this_sack--, sp--)
swap(*sp, *(sp - 1));
@ -4369,6 +4377,9 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
}
}
if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
tcp_sack_compress_send_ack(sk);
/* Could not find an adjacent existing SACK, build a new one,
* put it at the front, and shift everyone else down. We
* always know there is at least one SACK present already here.
@ -4376,7 +4387,6 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
* If the sack array is full, forget about the last one.
*/
if (this_sack >= TCP_NUM_SACKS) {
tcp_sack_compress_send_ack(sk);
this_sack--;
tp->rx_opt.num_sacks--;
sp--;