--- src/sys/net/pf/pf_norm.c	2006/12/22 23:44:57	1.7
+++ src/sys/net/pf/pf_norm.c	2007/08/11 18:52:31	1.7.4.1
@@ -495,9 +495,23 @@ pf_reassemble(struct mbuf **m0, struct p
 		m2 = frent->fr_m;
 		pool_put(&pf_frent_pl, frent);
 		pf_nfrents--;
+		m->m_pkthdr.csum_flags &= m2->m_pkthdr.csum_flags;
+		m->m_pkthdr.csum_data += m2->m_pkthdr.csum_data;
 		m_cat(m, m2);
 	}
 
+	/*
+	 * Note: this 1's complement optimization with <= 65535 fragments.
+	 *
+	 * Handle 1's complement carry for the 16 bit result.  This can
+	 * result in another carry which must also be handled.
+	 */
+	m->m_pkthdr.csum_data = (m->m_pkthdr.csum_data & 0xffff) +
+				(m->m_pkthdr.csum_data >> 16);
+	if (m->m_pkthdr.csum_data > 0xFFFF)
+		m->m_pkthdr.csum_data -= 0xFFFF;
+
+
 	ip->ip_src = (*frag)->fr_src;
 	ip->ip_dst = (*frag)->fr_dst;