diff --git a/net/checksum.c b/net/checksum.c
index 196aaa3459..39ad73f4b4 100644
--- a/net/checksum.c
+++ b/net/checksum.c
@@ -18,9 +18,7 @@
 #include "qemu/osdep.h"
 #include "qemu-common.h"
 #include "net/checksum.h"
-
-#define PROTO_TCP  6
-#define PROTO_UDP 17
+#include "net/eth.h"
 
 uint32_t net_checksum_add_cont(int len, uint8_t *buf, int seq)
 {
@@ -57,40 +55,82 @@ uint16_t net_checksum_tcpudp(uint16_t length, uint16_t proto,
 
 void net_checksum_calculate(uint8_t *data, int length)
 {
-    int hlen, plen, proto, csum_offset;
-    uint16_t csum;
+    int ip_len;
+    struct ip_header *ip;
+
+    /*
+     * Note: We cannot assume "data" is aligned, so the all code uses
+     * some macros that take care of possible unaligned access for
+     * struct members (just in case).
+     */
 
     /* Ensure data has complete L2 & L3 headers. */
-    if (length < 14 + 20) {
+    if (length < (sizeof(struct eth_header) + sizeof(struct ip_header))) {
         return;
     }
 
-    if ((data[14] & 0xf0) != 0x40)
-	return; /* not IPv4 */
-    hlen  = (data[14] & 0x0f) * 4;
-    plen  = (data[16] << 8 | data[17]) - hlen;
-    proto = data[23];
+    ip = (struct ip_header *)(data + sizeof(struct eth_header));
 
-    switch (proto) {
-    case PROTO_TCP:
-	csum_offset = 16;
-	break;
-    case PROTO_UDP:
-	csum_offset = 6;
-	break;
+    if (IP_HEADER_VERSION(ip) != IP_HEADER_VERSION_4) {
+        return; /* not IPv4 */
+    }
+
+    ip_len = lduw_be_p(&ip->ip_len);
+
+    /* Last, check that we have enough data for the all IP frame */
+    if (length < ip_len) {
+        return;
+    }
+
+    ip_len -= IP_HDR_GET_LEN(ip);
+
+    switch (ip->ip_p) {
+    case IP_PROTO_TCP:
+    {
+        uint16_t csum;
+        tcp_header *tcp = (tcp_header *)(ip + 1);
+
+        if (ip_len < sizeof(tcp_header)) {
+            return;
+        }
+
+        /* Set csum to 0 */
+        stw_he_p(&tcp->th_sum, 0);
+
+        csum = net_checksum_tcpudp(ip_len, ip->ip_p,
+                                   (uint8_t *)&ip->ip_src,
+                                   (uint8_t *)tcp);
+
+        /* Store computed csum */
+        stw_be_p(&tcp->th_sum, csum);
+
+        break;
+    }
+    case IP_PROTO_UDP:
+    {
+        uint16_t csum;
+        udp_header *udp = (udp_header *)(ip + 1);
+
+        if (ip_len < sizeof(udp_header)) {
+            return;
+        }
+
+        /* Set csum to 0 */
+        stw_he_p(&udp->uh_sum, 0);
+
+        csum = net_checksum_tcpudp(ip_len, ip->ip_p,
+                                   (uint8_t *)&ip->ip_src,
+                                   (uint8_t *)udp);
+
+        /* Store computed csum */
+        stw_be_p(&udp->uh_sum, csum);
+
+        break;
+    }
     default:
-	return;
+        /* Can't handle any other protocol */
+        break;
     }
-
-    if (plen < csum_offset + 2 || 14 + hlen + plen > length) {
-        return;
-    }
-
-    data[14+hlen+csum_offset]   = 0;
-    data[14+hlen+csum_offset+1] = 0;
-    csum = net_checksum_tcpudp(plen, proto, data+14+12, data+14+hlen);
-    data[14+hlen+csum_offset]   = csum >> 8;
-    data[14+hlen+csum_offset+1] = csum & 0xff;
 }
 
 uint32_t