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Subject[PATCH v2] net:ipv4: send an ack when seg.ack > snd.nxt
Date
From: Zhouyi Zhou <zhouzhouyi@gmail.com>

In RFC 793, page 72: "If the ACK acks something not yet sent
(SEG.ACK > SND.NXT) then send an ACK, drop the segment,
and return."

Fix Linux's behavior according to RFC 793.

Reported-by: Wei Xu <xuweihf@ustc.edu.cn>
Signed-off-by: Wei Xu <xuweihf@ustc.edu.cn>
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
---
Thank Florian Westphal for pointing out
the potential duplicated ack bug in patch version 1.
--
net/ipv4/tcp_input.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index bfe4112e000c..4bbf85d7ea8c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3771,11 +3771,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
goto old_ack;
}

- /* If the ack includes data we haven't sent yet, discard
- * this segment (RFC793 Section 3.9).
+ /* If the ack includes data we haven't sent yet, then send
+ * an ack, drop this segment, and return (RFC793 Section 3.9 page 72).
*/
- if (after(ack, tp->snd_nxt))
- return -1;
+ if (after(ack, tp->snd_nxt)) {
+ tcp_send_ack(sk);
+ return -2;
+ }

if (after(ack, prior_snd_una)) {
flag |= FLAG_SND_UNA_ADVANCED;
@@ -6385,6 +6387,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
struct request_sock *req;
int queued = 0;
bool acceptable;
+ int ret;

switch (sk->sk_state) {
case TCP_CLOSE:
@@ -6451,14 +6454,16 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
return 0;

/* step 5: check the ACK field */
- acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
- FLAG_UPDATE_TS_RECENT |
- FLAG_NO_CHALLENGE_ACK) > 0;
+ ret = tcp_ack(sk, skb, FLAG_SLOWPATH |
+ FLAG_UPDATE_TS_RECENT |
+ FLAG_NO_CHALLENGE_ACK);
+ acceptable = ret > 0;

if (!acceptable) {
if (sk->sk_state == TCP_SYN_RECV)
return 1; /* send one RST */
- tcp_send_challenge_ack(sk);
+ if (ret > -2)
+ tcp_send_challenge_ack(sk);
goto discard;
}
switch (sk->sk_state) {
--
2.25.1
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