Encapsulation Security Payload

Protection Extent

                 BEFORE APPLYING ESP
            ----------------------------
      IPv4  |orig IP hdr  |     |      |
            |(any options)| TCP | Data |
            ----------------------------

Transport Mode:
                 AFTER APPLYING ESP
            -------------------------------------------------
      IPv4  |orig IP hdr  | ESP |     |      |   ESP   | ESP|
            |(any options)| Hdr | TCP | Data | Trailer |Auth|
            -------------------------------------------------
                                |<----- encrypted ---->|
                          |<------ authenticated ----->|

Tunnel Mode:
            -----------------------------------------------------------
      IPv4  | new IP hdr* |     | orig IP hdr*  |   |    | ESP   | ESP|
            |(any options)| ESP | (any options) |TCP|Data|Trailer|Auth|
            -----------------------------------------------------------
                                |<--------- encrypted ---------->|
                          |<----------- authenticated ---------->|