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

[Alexander Duyck]

On Fri, Dec 11, 2020 at 8:22 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Fri, Dec 11, 2020 at 5:03 PM Alexander Duyck
> <alexander.duyck@xxxxxxxxx> wrote:
>
> > That's fine. I can target this for net-next. I had just selected net
> > since I had considered it a fix, but I suppose it could be considered
> > a behavioral change.
>
> We are very late in the 5.10 cycle, and we never handled ICMP in this
> state, so net-next is definitely better.
>
> Note that RFC 7413 states in 4.1.3 :
>
>  The client MUST cache cookies from servers for later Fast Open
>    connections.  For a multihomed client, the cookies are dependent on
>    the client and server IP addresses.  Hence, the client should cache
>    at most one (most recently received) cookie per client and server IP
>    address pair.
>
>    When caching cookies, we recommend that the client also cache the
>    Maximum Segment Size (MSS) advertised by the server.  The client can
>    cache the MSS advertised by the server in order to determine the
>    maximum amount of data that the client can fit in the SYN packet in
>    subsequent TFO connections.  Caching the server MSS is useful
>    because, with Fast Open, a client sends data in the SYN packet before
>    the server announces its MSS in the SYN-ACK packet.  If the client
>    sends more data in the SYN packet than the server will accept, this
>    will likely require the client to retransmit some or all of the data.
>    Hence, caching the server MSS can enhance performance.
>
>    Without a cached server MSS, the amount of data in the SYN packet is
>    limited to the default MSS of 536 bytes for IPv4 [RFC1122] and 1220
>    bytes for IPv6 [RFC2460].  Even if the client complies with this
>    limit when sending the SYN, it is known that an IPv4 receiver
>    advertising an MSS less than 536 bytes can receive a segment larger
>    than it is expecting.
>
>    If the cached MSS is larger than the typical size (1460 bytes for
>    IPv4 or 1440 bytes for IPv6), then the excess data in the SYN packet
>    may cause problems that offset the performance benefit of Fast Open.
>    For example, the unusually large SYN may trigger IP fragmentation and
>    may confuse firewalls or middleboxes, causing SYN retransmission and
>    other side effects.  Therefore, the client MAY limit the cached MSS
>    to 1460 bytes for IPv4 or 1440 for IPv6.
>
>
> Relying on ICMP is fragile, since they can be filtered in some way.

In this case I am not relying on the ICMP, but thought that since I
have it I should make use of it. WIthout the ICMP we would still just
be waiting on the retransmit timer.

The problem case has a v6-in-v6 tunnel between the client and the
endpoint so both ends assume an MTU 1500 and advertise a 1440 MSS
which works fine until they actually go to send a large packet between
the two. At that point the tunnel is triggering an ICMP_TOOBIG and the
endpoint is stalling since the MSS is dropped to 1400, but the SYN and
data payload were already smaller than that so no retransmits are
being triggered. This results in TFO being 1s slower than non-TFO
because of the failure to trigger the retransmit for the frame that
violated the PMTU. The patch is meant to get the two back into
comparable times.