PIPEX_IPGRE_HDRLEN
reqlen = PIPEX_IPGRE_HDRLEN + (has_seq + has_ack) * 4;
if (m0->m_pkthdr.len < PIPEX_IPGRE_HDRLEN) {
KASSERT(m0->m_pkthdr.len >= PIPEX_IPGRE_HDRLEN);
hlen = PIPEX_IPGRE_HDRLEN + 4 * (has_seq + has_ack);