dCDN                                                    uCDN
    +                                                       +
    |                                                       |
(1) | MI:  host: s123.ucdn.example.com                      |
    |      host-metadata:                                   |
    |          < metadata objects >                         |
    |          < MI.FallbackTarget                          |
    |            host: fallback-a.service123.ucdn.example > |
    |          < metadata objects >                         |
    <-------------------------------------------------------+
    |                                                       |
(2) | FCI:  capability-type: FCI.RedirectTarget             |
    |       redirecting-hosts: s123.ucdn.example.com        |
    |       target host: us-east1.dcdn.example.com          |
    +------------------------------------------------------->
    |                                                       |
    |                                                       |
    +                                                       +