diff --git a/sys/netinet/ip_input.c b/sys/netinet/ip_input.c
index 2250116c6ed..00667a529cb 100644
--- a/sys/netinet/ip_input.c
+++ b/sys/netinet/ip_input.c
@@ -988,6 +988,13 @@ found:
 		m->m_pkthdr.csum_data += q->m_pkthdr.csum_data;
 		m_cat(m, q);
 	}
+	/*
+	 * In order to do checksumming faster we do 'end-around carry' here
+	 * (and not in for{} loop), though it implies we are not going to
+	 * reassemble more than 64k fragments.
+	 */
+	m->m_pkthdr.csum_data =
+	    (m->m_pkthdr.csum_data & 0xffff) + (m->m_pkthdr.csum_data >> 16);
 #ifdef MAC
 	mac_create_datagram_from_ipq(fp, m);
 	mac_destroy_ipq(fp);