Let's look at the fields inside the header.

Next header: 8 bits wide. Identifies the next payload after the Authentication Payload.

Payload length: 8 bits wide. The length of the authentication data field in 32-bit words. Minimum value is 0 words, which is only used in the case of a "null" authentication algorithm. This should not happen in IPSec, which must have one specified.

Reserved: 16 bits wide. Reserved for future use. It must be set to all zeros when sent. The value is included in the authentication data calculation, but is otherwise ignored by the recipient.