Constant LINK_HEADER_LEN 

pub const LINK_HEADER_LEN: usize = ETHERNET_HEADER_LEN; // 14usize

The headroom every egress packet reserves for the link-layer header below IP.

The Ethernet header in a build that drives Ethernet interfaces, since an IP packet may end up going out of one. Zero in a build that only drives Medium::Ip interfaces, which prepend nothing.