ESP and AH questions

["john ipsec" <ipsec123@hotmail.com>]

(Sorry, if this topic had been discussed before. Is there an FAQ?)

Questions:

1. In the tunnel mode, what is the value for the next-header field? The 
next-header seems to be the original IP header (unlike in the transport 
mode, the next-header is the "transport protocol" header).

2. The ESP header and trailer do not specify the size of the "Authentication 
Data," unlike the AH. It uses SA to deduce the size of the Authentication 
Data (if present). If so, why AH cannot use SA to deduce the size of the 
Authentication Data field?

Thanks,
John

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