If you run sipp in foreground mode, it will screen_init.

This fails with the following error if TERM is unset:

$ TERM= sipp some.host
Error opening terminal: unknown.

I think should retry screen_init with TERM=dummy if it fails. That way you can start a sipp command from a cron job / zabbix job / whatever without having to resort to -bg mode.

(That takes care of this addition https://code.osso.nl/projects/sipp/changeset/48b81a8bd29f/ )