Commit a9b2c06d authored by Neal Cardwell's avatar Neal Cardwell Committed by David S. Miller

tcp: mitigate ACK loops for connections as tcp_request_sock

In the SYN_RECV state, where the TCP connection is represented by
tcp_request_sock, we now rate-limit SYNACKs in response to a client's
retransmitted SYNs: we do not send a SYNACK in response to client SYN
if it has been less than sysctl_tcp_invalid_ratelimit (default 500ms)
since we last sent a SYNACK in response to a client's retransmitted
SYN.

This allows the vast majority of legitimate client connections to
proceed unimpeded, even for the most aggressive platforms, iOS and
MacOS, which actually retransmit SYNs 1-second intervals for several
times in a row. They use SYN RTO timeouts following the progression:
1,1,1,1,1,2,4,8,16,32.
Reported-by: default avatarAvery Fay <avery@mixpanel.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 032ee423
......@@ -115,6 +115,7 @@ struct tcp_request_sock {
u32 rcv_isn;
u32 snt_isn;
u32 snt_synack; /* synack sent time */
u32 last_oow_ack_time; /* last SYNACK */
u32 rcv_nxt; /* the ack # by SYNACK. For
* FastOpen it's the seq#
* after data-in-SYN.
......
......@@ -1145,6 +1145,7 @@ static inline void tcp_openreq_init(struct request_sock *req,
tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
tcp_rsk(req)->snt_synack = tcp_time_stamp;
tcp_rsk(req)->last_oow_ack_time = 0;
req->mss = rx_opt->mss_clamp;
req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
ireq->tstamp_ok = rx_opt->tstamp_ok;
......
......@@ -605,7 +605,11 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
* Reset timer after retransmitting SYNACK, similar to
* the idea of fast retransmit in recovery.
*/
if (!inet_rtx_syn_ack(sk, req))
if (!tcp_oow_rate_limited(sock_net(sk), skb,
LINUX_MIB_TCPACKSKIPPEDSYNRECV,
&tcp_rsk(req)->last_oow_ack_time) &&
!inet_rtx_syn_ack(sk, req))
req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout,
TCP_RTO_MAX) + jiffies;
return NULL;
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment