On Fri, Dec 11, 2020 at 2:55 AM Alexander Duyck
<alexander.du...@gmail.com> wrote:
>
> From: Alexander Duyck <alexanderdu...@fb.com>
>
> In the case of a fastopen SYN there are cases where it may trigger either a
> ICMP_TOOBIG message in the case of IPv6 or a fragmentation request in the
> case of IPv4. This results in the socket stalling for a second or more as
> it does not respond to the message by retransmitting the SYN frame.
>
> Normally a SYN frame should not be able to trigger a ICMP_TOOBIG or
> ICMP_FRAG_NEEDED however in the case of fastopen we can have a frame that
> makes use of the entire MTU. In the case of fastopen it does, and an
> additional complication is that the retransmit queue doesn't contain the
> original frames. As a result when tcp_simple_retransmit is called and
> walks the list of frames in the queue it may not mark the frames as lost
> because both the SYN and the data packet each individually are smaller than
> the MSS size after the adjustment. This results in the socket being stalled
> until the retransmit timer kicks in and forces the SYN frame out again
> without the data attached.
>
> In order to resolve this we need to mark the SYN frame as lost if it is the
> first packet in the queue. Doing this allows the socket to recover much
> more quickly without the retransmit timeout stall.
>
> Signed-off-by: Alexander Duyck <alexanderdu...@fb.com>
I do not think it is net candidate, but net-next
Yuchung might correct me, but I think TCP Fastopen standard was very
conservative about payload len in the SYN packet
So receiving an ICMP was never considered.
> ---
> include/net/tcp.h | 1 +
> net/ipv4/tcp_input.c | 8 ++++++++
> net/ipv4/tcp_ipv4.c | 6 ++++++
> net/ipv6/tcp_ipv6.c | 4 ++++
> 4 files changed, 19 insertions(+)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index d4ef5bf94168..6181ad98727a 100644
> --- a/include/net/tcp.h
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -546,6 +546,12 @@ int tcp_v4_err(struct sk_buff *skb, u32 info)
> if (sk->sk_state == TCP_LISTEN)
> goto out;
>
> + /* fastopen SYN may have triggered the fragmentation
> + * request. Mark the SYN or SYN/ACK as lost.
> + */
> + if (sk->sk_state == TCP_SYN_SENT)
> + tcp_mark_syn_lost(sk);
This is going to crash in some cases, you do not know if you own the socket.
(Look a few lines below)
> +
> tp->mtu_info = info;
> if (!sock_owned_by_user(sk)) {
> tcp_v4_mtu_reduced(sk);
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 992cbf3eb9e3..d7b1346863e3 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -443,6 +443,10 @@ static int tcp_v6_err(struct sk_buff *skb, struct
> inet6_skb_parm *opt,
> if (!ip6_sk_accept_pmtu(sk))
> goto out;
>
> + /* fastopen SYN may have triggered TOOBIG, mark it lost. */
> + if (sk->sk_state == TCP_SYN_SENT)
> + tcp_mark_syn_lost(sk);
Same issue here.
> +
> tp->mtu_info = ntohl(info);
> if (!sock_owned_by_user(sk))
> tcp_v6_mtu_reduced(sk);
>
>
