diff --git a/app/test-pmd/csumonly.c b/app/test-pmd/csumonly.c
index 41711fd6d7..4b438d13a9 100644
--- a/app/test-pmd/csumonly.c
+++ b/app/test-pmd/csumonly.c
@@ -183,16 +183,15 @@ process_inner_cksums(void *l3_hdr, uint16_t ethertype, uint16_t l3_len,
 		ipv4_hdr = l3_hdr;
 		ipv4_hdr->hdr_checksum = 0;
 
+		ol_flags |= PKT_TX_IPV4;
 		if (tso_segsz != 0 && l4_proto == IPPROTO_TCP) {
 			ol_flags |= PKT_TX_IP_CKSUM;
 		} else {
 			if (testpmd_ol_flags & TESTPMD_TX_OFFLOAD_IP_CKSUM)
 				ol_flags |= PKT_TX_IP_CKSUM;
-			else {
+			else
 				ipv4_hdr->hdr_checksum =
 					rte_ipv4_cksum(ipv4_hdr);
-				ol_flags |= PKT_TX_IPV4;
-			}
 		}
 	} else if (ethertype == _htons(ETHER_TYPE_IPv6))
 		ol_flags |= PKT_TX_IPV6;
@@ -261,6 +260,7 @@ process_outer_cksums(void *outer_l3_hdr, uint16_t outer_ethertype,
 
 	if (outer_ethertype == _htons(ETHER_TYPE_IPv4)) {
 		ipv4_hdr->hdr_checksum = 0;
+		ol_flags |= PKT_TX_OUTER_IPV4;
 
 		if (testpmd_ol_flags & TESTPMD_TX_OFFLOAD_VXLAN_CKSUM)
 			ol_flags |= PKT_TX_OUTER_IP_CKSUM;
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index a554247d7f..cefb54a5e5 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -141,24 +141,55 @@ extern "C" {
 #define PKT_TX_UDP_CKSUM     (3ULL << 52) /**< UDP cksum of TX pkt. computed by NIC. */
 #define PKT_TX_L4_MASK       (3ULL << 52) /**< Mask for L4 cksum offload request. */
 
-#define PKT_TX_IP_CKSUM      (1ULL << 54) /**< IP cksum of TX pkt. computed by NIC. */
+/**
+ * Offload the IP checksum in the hardware. The flag PKT_TX_IPV4 should
+ * also be set by the application, although a PMD will only check
+ * PKT_TX_IP_CKSUM.
+ *  - set the IP checksum field in the packet to 0
+ *  - fill the mbuf offload information: l2_len, l3_len
+ */
+#define PKT_TX_IP_CKSUM      (1ULL << 54)
 
-/** Packet is IPv4 without requiring IP checksum offload. */
+/**
+ * Packet is IPv4. This flag must be set when using any offload feature
+ * (TSO, L3 or L4 checksum) to tell the NIC that the packet is an IPv4
+ * packet. If the packet is a tunneled packet, this flag is related to
+ * the inner headers.
+ */
 #define PKT_TX_IPV4          (1ULL << 55)
 
-/** Tell the NIC it's an IPv6 packet.*/
+/**
+ * Packet is IPv6. This flag must be set when using an offload feature
+ * (TSO or L4 checksum) to tell the NIC that the packet is an IPv6
+ * packet. If the packet is a tunneled packet, this flag is related to
+ * the inner headers.
+ */
 #define PKT_TX_IPV6          (1ULL << 56)
 
 #define PKT_TX_VLAN_PKT      (1ULL << 57) /**< TX packet is a 802.1q VLAN packet. */
 
-/** Outer IP checksum of TX packet, computed by NIC for tunneling packet.
- *  The tunnel type must also be specified, ex: PKT_TX_UDP_TUNNEL_PKT. */
+/**
+ * Offload the IP checksum of an external header in the hardware. The
+ * flag PKT_TX_OUTER_IPV4 should also be set by the application, alto ugh
+ * a PMD will only check PKT_TX_IP_CKSUM.  The IP checksum field in the
+ * packet must be set to 0.
+ *  - set the outer IP checksum field in the packet to 0
+ *  - fill the mbuf offload information: outer_l2_len, outer_l3_len
+ */
 #define PKT_TX_OUTER_IP_CKSUM   (1ULL << 58)
 
-/** Packet is outer IPv4 without requiring IP checksum offload for tunneling packet. */
+/**
+ * Packet outer header is IPv4. This flag must be set when using any
+ * outer offload feature (L3 or L4 checksum) to tell the NIC that the
+ * outer header of the tunneled packet is an IPv4 packet.
+ */
 #define PKT_TX_OUTER_IPV4   (1ULL << 59)
 
-/** Tell the NIC it's an outer IPv6 packet for tunneling packet */
+/**
+ * Packet outer header is IPv6. This flag must be set when using any
+ * outer offload feature (L4 checksum) to tell the NIC that the outer
+ * header of the tunneled packet is an IPv6 packet.
+ */
 #define PKT_TX_OUTER_IPV6    (1ULL << 60)
 
 /* Use final bit of flags to indicate a control mbuf */