About UDP Encapsulation of IPsec Packets

[Jerry Yao <jerryyao@mail.jl.cn>]

I read the IETF draft "UDP Encapsulation of IPsec Packets" and I have a question about it. 
    If I receive a packet from the communication peer who behind NAT, and the packet is Transport Mode ESP Encapsulation:

         -------------------------------------------------------------
   IPv4  |orig IP hdr  | UDP | Non-| ESP |     |      |   ESP   | ESP|
         |(any options)| Hdr | IKE | Hdr | TCP | Data | Trailer |Auth|
         -------------------------------------------------------------
                                         |<----- encrypted ---->|
                                   |<------ authenticated ----->|

   Now I don't know the original IP address of the communication peer, How can I locate the corresponding sa to decrypt or authenticate the ESP packet?