End User              dCDN            uCDN fallback          uCDN RR
    +                   +                   +                   +
    |                   |                   |                   |
(1) | Request sent s123.ucdn.example.com    |                   |
    +-------------------+-------------------+------------------->
    |                   |                   |                   |
(2) | Redirect to us-east1.dcdn.example.com |                   |
    <-------------------+-------------------+-------------------+
    |                   |                   |                   |
(3) | Request us-east1.dcdn.example.com     |                   |
    +------------------->                   |                   |
    |                   |                   |                   |
(4) | Redirect back to fallback-a.service123.ucdn.example       |
    <-------------------+                   |                   |
    |                   |                   |                   |
(5) | Request fallback-a.service123.ucdn.example                |
    +--------------------------------------->                   |
    |                   |                   |                   |
(6) | Response          |                   |                   | 
    <-------------------+-------------------+                   |
    |                   |                   |                   |
    +                   +                   +                   +