Req: POST /rd/4521?lt=7200

Res: 4.01 Unauthorized
Echo: 0x0123

(EP tries again immediately.)

Req: POST /rd/4521?lt=7200
Echo: 0x0123

Res: 2.04 Changed
Echo: 0x0124

(Later, the EP regains its confidence in its long-term reachability.)

Req: POST /rd/4521?lt=90000
Echo: 0x0124

Res: 2.04 Changed
Echo: 0x0247