[pcp] A question: How can an internal client get the external IP and port of the inner NAT

[Chenguohai <[e-mail]>]

Hi all,
I am new to PCP and reading RFC6887.I have a question about section 8.1. On the upper part of page 22 writing following,
the PCP-controlled NAT creating pointless non-functional mappings.
   When such an intervening non-PCP-aware inner NAT is detected,
   mappings must first be created by some other means in the inner NAT,
   before mappings can be usefully created in the outer PCP-controlled
   NAT.  Having created mappings in the inner NAT by some other means,
   the PCP client should then use the inner NAT’s external address as
   the client IP address, to signal to the outer PCP-controlled NAT that
   the client is aware of the inner NAT, and has taken steps to create
   mappings in it by some other means, so that mappings created in the
   outer NAT will not be a pointless waste of resources.

My question is that how an interal client could get the external IP and port of the inner NAT.
Should there be server between the inner NAT and the external one? How could it be assured that the inner NAT does not change external port?

BR
G.Robert  Chen
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G.Robert  Chen (Chen Guohai  陈国海). Network Research Department, Huawei Technologies Co., Ltd. Telephone: [phone];   http://www.huawei.com
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