[pcp] PCP Draft 29 question about PREFER_FAILURE and Suggested External Address field of all zeroes

[Norman Sardella <[e-mail]>]

In draft-ietf-pcp-base-29,
  "Section 13.2.  PREFER_FAILURE Option for MAP Opcode",
The draft mentions that when PREFER_FAILURE is set,
   "all-zeros IPv4 or all-zeros IPv6 address in the Suggested External
    Address field, it is invalid."

Is this a valid case? Its unclear in a CGNAT/DS-LITE environment
where the CPE would be running a UPnP to PCP gateway and the CGNAT
equipment provides an External Address, how the CPE could provide an 
initial PCP MAP request with an External IP Address when the CGNAT 
device maintains them.  On a MAPPING update, I could see providing an
Suggested External Address


I understand that UPnP IGDv1 client allows to dictate
mapping a specific port, but its unclear in a CGNAT/DS-LITE environment
how to specify an Suggested External IP address.


On a MAPPING update, I could see providing an Suggested External
Address, but don't understand how a CPE provide the Suggested External
Address on an initial MAP request.

> 13.2.  PREFER_FAILURE Option for MAP Opcode
...
>
>    If a PCP request contains the PREFER_FAILURE option and has zero in
>    the Suggested External Port field, or has the all-zeros IPv4 or all-
>    zeros IPv6 address in the Suggested External Address field, it is
>    invalid.  The PCP server MUST reject such a message with the
>    MALFORMED_OPTION error code.