The ss utility can be used to gather information for open connections such as TCP MSS/MTU and current rtt.
Example:
ss -tin
Output looks like:
State Recv-Q Send-Q Local Address:Port Peer Address:Port
Process
ESTAB 0 0 192.168.139.0:37896 52.74.12.98:443
cubic wscale:12,7 rto:304 rtt:103.604/14.523 ato:40 mss:1288 pmtu:1500 rcvmss:1288 advmss:1448 cwnd:10 bytes_sent:19351 bytes_retrans:164 bytes_acked:19188 bytes_received:35302 segs_out:542 segs_in:472 data_segs_out:308 data_segs_in:233 send 994556bps lastsnd:7868
lastrcv:7772 lastack:7772 pacing_rate 1989096bps delivery_rate 332920bps delivered:309 app_limited busy:30228ms retrans:0/3 dsack_dups:3 rcv_space:14480 rcv_ssthresh:64088 minrtt:87.029