Re: [net PATCH] tcp: Mark fastopen SYN packet as lost when receiving ICMP_TOOBIG/ICMP_FRAG_NEEDED

[Eric Dumazet]

On Fri, Dec 11, 2020 at 2:55 AM Alexander Duyck
<alexander.duyck@xxxxxxxxx> wrote:
>
> From: Alexander Duyck <alexanderduyck@xxxxxx>
>
> 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 <alexanderduyck@xxxxxx>


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);
>
>