The server Dell PowerEdge R640 (baseboard 0PHYDR) with BIOS Version: 1.6.13 02/13/2019 [Hetzner 1.0.2] has Intel 1GB i350 network card:
igb0@pci0:1:0:0: class=0x020000 card=0x1f9a1028 chip=0x15218086 rev=0x01 hdr=0x00
vendor = 'Intel Corporation'
device = 'I350 Gigabit Network Connection'
class = network
subclass = ethernet
It runs FreeBSD 11.2-STABLE/amd64 r344922 with GENERIC kernel config plus following options only:
It also runs ipfw nat that is not compatible with TSO4 so this option is disabled for igb0. Still, running with tso4 disabled and txcsum enabled, it suffers from stalled TCP connections due to some TCP segments being sent with broken checksum occasionally.
ifconfig igb0 -txcsum solves the problem.