Constant UNSPECIFIED_SECTION_SIZE 

pub const UNSPECIFIED_SECTION_SIZE: u64 = u64::MAX; // 18_446_744_073_709_551_615u64

According to pcapng RFC Section 4.1:

If the Section Length is -1 (0xFFFFFFFFFFFFFFFF), this means that the size of the section is not specified, and the only way to skip the section is to parse the blocks that it contains.