nice explanation but their is one mistake.. Payload length field does not define the length of original payload/data; it defines the length of the authentication header in 4-byte multiples,
you are doing a wonderful job of explaining the things in local language.. Just a bit of side suggestion.. if you can speak in bit slow pace without hurrying.. it will be even more nicer way of explaining and teaching good stuff.. Thanks and Hope that helps..
it is based on which mode you are using Only in tunnel mode you use the Ip sec header so, AH is used after IPsec, not as an IPsecHeader here is the sequence: IPsec header, AH, IP header, TCP/others, Payload
This video was very helpful and informative, but with all due respect I'd like to point out that u missed about Hashing part and thr creation of a Message Digest (MD)