OS: FreeBSD 12.2-STABLE #56 r367863: Fri Nov 20 08:12:33 CET 2020 amd64
The box in question is a Fujitsu Celsius M750 system, dated 2015. Since the first occurence of iflib() in FreeBSD and the related new design, this specific chipset seems to have massive problems keeping up a connection open via ssh. Since then, the problem is present up to this day. We swapped all NICs of that type, if possible, with dedicated NIC boards carrying two i350 ports (igb) and the problem has been mitigated so far (still sporadic connection losses, but very rare).
For a reference, we used also some Xubunto hosts with 20.04 LTS on the same box connectiong to the very same hosts and thoese connections were rock stable and lasted days. Even connections with the igb/i350 chipsets lasted sometimes for days, but not i217-LM.
Below some hardware specific informations for the i217-LM:
em0@pci0:0:25:0: class=0x020000 card=0x11ed1734 chip=0x153a8086 rev=0x05 hdr=0x00
vendor = 'Intel Corporation'
device = 'Ethernet Connection I217-LM'
class = network
subclass = ethernet
bar  = type Memory, range 32, base rxfb400000, size 131072, enabled
bar  = type Memory, range 32, base rxfb439000, size 4096, enabled
bar  = type I/O Port, range 32, base rxf020, size 32, enabled
cap 01[c8] = powerspec 2 supports D0 D3 current D0
cap 05[d0] = MSI supports 1 message, 64 bit enabled with 1 message
cap 13[e0] = PCI Advanced Features: FLR TP
pci1: <simple comms> at device 22.0 (no driver attached)
em0: <Intel(R) PRO/1000 Network Connection> port 0xf020-0xf03f mem 0xfb400000-0xfb41ffff,0xfb439000-0xfb439fff at device 25.0 numa-domain 0 on pci1
em0: Using 1024 TX descriptors and 1024 RX descriptors
em0: Using an MSI interrupt
em0: Ethernet address: XXXX
em0: netmap queues/slots: TX 1/1024, RX 1/1024
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
inet 192.168.0.2 netmask 0xffffff00 broadcast 192.168.0.255
inet6 fe80::xxxx:eff:xxxx:xxxx%em00 prefixlen 64 scopeid 0x1
inet6 fd60:b403:101::233 prefixlen 64
media: Ethernet autoselect (1000baseT <full-duplex>)