Index: sys/net/iflib.c
===================================================================
--- sys/net/iflib.c
+++ sys/net/iflib.c
@@ -3030,6 +3030,10 @@
 					if (__predict_false((m = m_pullup(m, pi->ipi_ehdrlen + sizeof(struct ip6_hdr) + sizeof(struct tcphdr))) == NULL))
 						return (ENOMEM);
 				}
+				/*
+				 * TSO always requires hardware checksum offload.
+				 */
+				pi->ipi_csum_flags |= CSUM_IP_TCP;
 				pi->ipi_tcp_hflags = th->th_flags;
 				pi->ipi_tcp_hlen = th->th_off << 2;
 				pi->ipi_tcp_seq = th->th_seq;
@@ -3038,9 +3042,7 @@
 				if (__predict_false(ip6->ip6_nxt != IPPROTO_TCP))
 					return (ENXIO);
 				/*
-				 * The corresponding flag is set by the stack in the IPv4
-				 * TSO case, but not in IPv6 (at least in FreeBSD 10.2).
-				 * So, set it here because the rest of the flow requires it.
+				 * TSO always requires hardware checksum offload.
 				 */
 				pi->ipi_csum_flags |= CSUM_IP6_TCP;
 				th->th_sum = in6_cksum_pseudo(ip6, 0, IPPROTO_TCP, 0);