[strongSwan] IPSEC Processing on a Security Gateway

[Bharat S]

Hi all,
        I have a question regarding IPSec processing on Security Gateway (SEG). 
Consider a network as below.


Host A ----------------------Gateway 
--------------------Gateway--------------------Host B
                                      1                                 2

If suppose the IPSec tunnel is required to be initiated from Host A to Host B, I 
was wondering how will the IPSec packets be 
processed on route to Host B. Lets say its ESP in tunnel mode. The packet from 
Host A to Gateway 1  would appear as below


New IP | ESP | Orig IP | UDP

My question is, when this packet is received on Gateway 1, will the ESP header 
of this packet be decrypted to form another ESP
and the resulting packet going out would appear like

New IP | ESP | Orig IP | UDP


OR 

Or its the entire IP packet received is given as input to form another ESP 
packet.. And the resulting packet going out would appear like

New IP | ESP | IP | ESP | Orig IP |  UDP
                     |<-----------hashed---------> |


I hope you have got my question. Please correct me If am wrong at any place.. 
And would appreciate if you could guide me to  some 
specification that explains the IPSec Processing on Gateways.


Many Thanks,
Bharat


      
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