Latency Without Loss

A home router with a 200 ms deep buffer sits on a 20 Mbit/s uplink. While a single large upload runs, an idle SSH session on the same link shows round trip times climbing from 18 ms to 780 ms. Packet loss is zero the entire time, and the upload does achieve close to 20 Mbit/s.

Which change addresses the cause rather than a symptom?

Enable ECN on the uploading host.

Put an active queue discipline such as fq_codel on the uplink so the queue drops or marks before it grows to 780 ms of data.

Increase the router's buffer so packets stop being delayed by a full queue.

Raise the uploader's SO_SNDBUF so it can keep more data in flight and finish sooner.