Re: [PATCH net-next v2 0/4] tcp: fix tcp_base_mss on passive connections and test MTU probing

[Esko Mobius]

> How the issue was discovered
Hi,

This came up while adding packetdrill coverage for tcp_mtu_probing=2.
The test work involved comparing the initial send MSS and what happened
after the first probe was acknowledged on active and passive IPv4
connections.

With tcp_base_mss=1024, the active connection started at 1024 as
expected. The passive connection started at 1460, and its send MSS
dropped to 1242 after the first probe was acknowledged. That led to
checking the passive setup path, where tcp_v4_syn_recv_sock() called
tcp_sync_mss() before tcp_init_transfer() enabled MTU probing.

Thanks,
Yongjian