Current behavior to set DSCP EF code point by default is harmful

[Job Snijders]

Hi,

On Fri, Apr 03, 2026 at 12:24:51PM -0400, matt at theaddisons.us wrote:
> The current default behavior (changed in 10.1) to mark traffic EF by
> default is harmful

It is important to note that _only_ interactive traffic (i.e., real-time
traffic resulting from people typing into their terminal) is marked EF.
Any other SSH traffic (e.g., sftp/scp traffic, X11 forwarding, etc) is
marked with the operating system default, usually CS0.

And daintily, in order to prevent overuse of the network's priority
queues, OpenSSH automatically and continuously selects the appropriate
marker even in complicated scenarios such as when multiple types of
sessions share a single network connection through ControlMaster.

The joy and quality of interactive work is latency sensitive, most users
will feel comfortable typing at a latency between 20-150ms. As DSCP
value EF ultimately maps to the WMM AC_V0 access category (following the
reconciled IETF Diffserv to IEEE 802.11 mappings, RFC 8325), on modern
operating systems with modern WiFi, your SSH keypresses essentially are
given a tiny boost to reach the access point. How cool is that!

> This has been observed recently on this list with Oliver Freyermuth's
> posts starting from October of 2025. His provider eventually resolved
> this

Yes! Emphasis mine: "HIS PROVIDER EVENTUALLY RESOLVED THIS". I interpret
this anecdote as a success story: it could very well be that OpenSSH's
implementation choices nudged this particular provider to improve their
service offering, who knows. In any case, active use of codepoints is an
important way to ensure such codepoints remain usable over time.

Kind regards,

Job