我正在阅读rfc 4787，作者提到了UNSAF方法。什么是UNSAF方法
正如selbie所说，UNSAF在RFC 3424中定义，意思是单边自地址固定。
当客户端在NAT后面时，它不知道它的外部地址。根据这个RFC:
As a result of the nature of Network Address Translation (NAT)
Middleboxes, communicating endpoints that are separated by one or
more NATs do not know how to refer to themselves using addresses that
are valid in the addressing realms of their (current and future)
peers. Various proposals have been made for "UNilateral Self-Address
Fixing (UNSAF)" processes. These are processes whereby some
originating endpoint attempts to determine or fix the address (and
port) by which it is known to another endpoint - e.g. to be able to
use address data in the protocol exchange, or to advertise a public
address from which it will receive connections.