diff --git a/P4D2_2017/exercises/arp/arp.p4 b/P4D2_2017/exercises/arp/arp.p4
index 0193407..c8ae980 100644
--- a/P4D2_2017/exercises/arp/arp.p4
+++ b/P4D2_2017/exercises/arp/arp.p4
@@ -152,7 +152,7 @@ parser MyParser(
  ************   C H E C K S U M    V E R I F I C A T I O N   *************
  *************************************************************************/
 control MyVerifyChecksum(
-    in    my_headers_t   hdr,
+    inout my_headers_t   hdr,
     inout my_metadata_t  meta)
 {
     apply {     }
diff --git a/P4D2_2017/exercises/arp/solution/arp.p4 b/P4D2_2017/exercises/arp/solution/arp.p4
index 69065e6..1fe7f14 100644
--- a/P4D2_2017/exercises/arp/solution/arp.p4
+++ b/P4D2_2017/exercises/arp/solution/arp.p4
@@ -150,7 +150,7 @@ parser MyParser(
  ************   C H E C K S U M    V E R I F I C A T I O N   *************
  *************************************************************************/
 control MyVerifyChecksum(
-    in    my_headers_t   hdr,
+    inout my_headers_t   hdr,
     inout my_metadata_t  meta)
 {
     apply {     }
diff --git a/P4D2_2017/exercises/calc/calc.p4 b/P4D2_2017/exercises/calc/calc.p4
index e01e47d..63e0dea 100644
--- a/P4D2_2017/exercises/calc/calc.p4
+++ b/P4D2_2017/exercises/calc/calc.p4
@@ -69,7 +69,7 @@ parser MyParser(
  ************   C H E C K S U M    V E R I F I C A T I O N   *************
  *************************************************************************/
 control MyVerifyChecksum(
-    in    my_headers_t   hdr,
+    inout my_headers_t   hdr,
     inout my_metadata_t  meta)
 {
     apply {
diff --git a/P4D2_2017/exercises/calc/solution/calc.p4 b/P4D2_2017/exercises/calc/solution/calc.p4
index 9115d7a..d03b129 100644
--- a/P4D2_2017/exercises/calc/solution/calc.p4
+++ b/P4D2_2017/exercises/calc/solution/calc.p4
@@ -134,7 +134,7 @@ parser MyParser(
  ************   C H E C K S U M    V E R I F I C A T I O N   *************
  *************************************************************************/
 control MyVerifyChecksum(
-    in    my_headers_t   hdr,
+    inout my_headers_t   hdr,
     inout my_metadata_t  meta)
 {
     apply {
diff --git a/P4D2_2017/exercises/ipv4_forward/ipv4_forward.p4 b/P4D2_2017/exercises/ipv4_forward/ipv4_forward.p4
index 414612c..f069145 100644
--- a/P4D2_2017/exercises/ipv4_forward/ipv4_forward.p4
+++ b/P4D2_2017/exercises/ipv4_forward/ipv4_forward.p4
@@ -72,7 +72,7 @@ parser ParserImpl(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control verifyChecksum(in headers hdr, inout metadata meta) {   
+control verifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
diff --git a/P4D2_2017/exercises/ipv4_forward/solution/ipv4_forward.p4 b/P4D2_2017/exercises/ipv4_forward/solution/ipv4_forward.p4
index 1130b81..c9c52b0 100644
--- a/P4D2_2017/exercises/ipv4_forward/solution/ipv4_forward.p4
+++ b/P4D2_2017/exercises/ipv4_forward/solution/ipv4_forward.p4
@@ -75,7 +75,7 @@ parser ParserImpl(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control verifyChecksum(in headers hdr, inout metadata meta) {   
+control verifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -132,25 +132,21 @@ control computeChecksum(
     inout headers  hdr,
     inout metadata meta)
 {
-    Checksum16() ipv4_checksum;
-    
     apply {
-        if (hdr.ipv4.isValid()) {
-            hdr.ipv4.hdrChecksum = ipv4_checksum.get(
-                {    
-                    hdr.ipv4.version,
-                    hdr.ipv4.ihl,
-                    hdr.ipv4.diffserv,
-                    hdr.ipv4.totalLen,
-                    hdr.ipv4.identification,
-                    hdr.ipv4.flags,
-                    hdr.ipv4.fragOffset,
-                    hdr.ipv4.ttl,
-                    hdr.ipv4.protocol,
-                    hdr.ipv4.srcAddr,
-                    hdr.ipv4.dstAddr
-                });
-        }
+        update_checksum(true,
+            { hdr.ipv4.version,
+                hdr.ipv4.ihl,
+                hdr.ipv4.diffserv,
+                hdr.ipv4.totalLen,
+                hdr.ipv4.identification,
+                hdr.ipv4.flags,
+                hdr.ipv4.fragOffset,
+                hdr.ipv4.ttl,
+                hdr.ipv4.protocol,
+                hdr.ipv4.srcAddr,
+                hdr.ipv4.dstAddr
+            },
+            hdr.ipv4.hdrChecksum, HashAlgorithm.csum16);
     }
 }
 
diff --git a/P4D2_2017/exercises/mri/mri.p4 b/P4D2_2017/exercises/mri/mri.p4
index d602260..11f8bfd 100644
--- a/P4D2_2017/exercises/mri/mri.p4
+++ b/P4D2_2017/exercises/mri/mri.p4
@@ -150,7 +150,7 @@ inout standard_metadata_t standard_metadata) {
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control verifyChecksum(in headers hdr, inout metadata meta) {   
+control verifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
diff --git a/P4D2_2017/exercises/mri/solution/mri.p4 b/P4D2_2017/exercises/mri/solution/mri.p4
index 16026af..2f0b1a2 100644
--- a/P4D2_2017/exercises/mri/solution/mri.p4
+++ b/P4D2_2017/exercises/mri/solution/mri.p4
@@ -138,7 +138,7 @@ inout standard_metadata_t standard_metadata) {
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control verifyChecksum(in headers hdr, inout metadata meta) {   
+control verifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -228,13 +228,9 @@ control computeChecksum(
 inout headers  hdr,
 inout metadata meta)
 {
-    Checksum16() ipv4_checksum;
-    
     apply {
-        if (hdr.ipv4.isValid()) {
-            hdr.ipv4.hdrChecksum = ipv4_checksum.get(
-            {    
-                hdr.ipv4.version,
+        update_checksum(true,
+            { hdr.ipv4.version,
                 hdr.ipv4.ihl,
                 hdr.ipv4.diffserv,
                 hdr.ipv4.totalLen,
@@ -245,8 +241,8 @@ inout metadata meta)
                 hdr.ipv4.protocol,
                 hdr.ipv4.srcAddr,
                 hdr.ipv4.dstAddr
-            });
-        }
+            },
+            hdr.ipv4.hdrChecksum, HashAlgorithm.csum16);
     }
 }
 
diff --git a/SIGCOMM_2017/exercises/basic/basic.p4 b/SIGCOMM_2017/exercises/basic/basic.p4
index 80f3676..c0e7d93 100644
--- a/SIGCOMM_2017/exercises/basic/basic.p4
+++ b/SIGCOMM_2017/exercises/basic/basic.p4
@@ -62,7 +62,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -157,4 +157,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/basic/solution/basic.p4 b/SIGCOMM_2017/exercises/basic/solution/basic.p4
index e51b285..7d491ea 100644
--- a/SIGCOMM_2017/exercises/basic/solution/basic.p4
+++ b/SIGCOMM_2017/exercises/basic/solution/basic.p4
@@ -74,7 +74,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -173,4 +173,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/calc/calc.p4 b/SIGCOMM_2017/exercises/calc/calc.p4
index 4b235d0..b4ed4b4 100644
--- a/SIGCOMM_2017/exercises/calc/calc.p4
+++ b/SIGCOMM_2017/exercises/calc/calc.p4
@@ -132,7 +132,7 @@ parser MyParser(packet_in packet,
 /*************************************************************************
  ************   C H E C K S U M    V E R I F I C A T I O N   *************
  *************************************************************************/
-control MyVerifyChecksum(in headers hdr,
+control MyVerifyChecksum(inout headers hdr,
                          inout metadata meta) {
     apply { }
 }
diff --git a/SIGCOMM_2017/exercises/calc/solution/calc.p4 b/SIGCOMM_2017/exercises/calc/solution/calc.p4
index 4cc2892..3c7e86c 100644
--- a/SIGCOMM_2017/exercises/calc/solution/calc.p4
+++ b/SIGCOMM_2017/exercises/calc/solution/calc.p4
@@ -132,7 +132,7 @@ parser MyParser(packet_in packet,
 /*************************************************************************
  ************   C H E C K S U M    V E R I F I C A T I O N   *************
  *************************************************************************/
-control MyVerifyChecksum(in headers hdr,
+control MyVerifyChecksum(inout headers hdr,
                          inout metadata meta) {
     apply { }
 }
diff --git a/SIGCOMM_2017/exercises/ecn/ecn.p4 b/SIGCOMM_2017/exercises/ecn/ecn.p4
index 6270e8b..576169f 100644
--- a/SIGCOMM_2017/exercises/ecn/ecn.p4
+++ b/SIGCOMM_2017/exercises/ecn/ecn.p4
@@ -78,7 +78,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -185,4 +185,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/ecn/solution/ecn.p4 b/SIGCOMM_2017/exercises/ecn/solution/ecn.p4
index 59b9756..2aebe20 100644
--- a/SIGCOMM_2017/exercises/ecn/solution/ecn.p4
+++ b/SIGCOMM_2017/exercises/ecn/solution/ecn.p4
@@ -76,7 +76,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -185,4 +185,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/hula/hula.p4 b/SIGCOMM_2017/exercises/hula/hula.p4
index 7ac4f49..8faa0ed 100644
--- a/SIGCOMM_2017/exercises/hula/hula.p4
+++ b/SIGCOMM_2017/exercises/hula/hula.p4
@@ -130,7 +130,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -430,4 +430,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/hula/solution/hula.p4 b/SIGCOMM_2017/exercises/hula/solution/hula.p4
index 06f4ae6..e8ea288 100644
--- a/SIGCOMM_2017/exercises/hula/solution/hula.p4
+++ b/SIGCOMM_2017/exercises/hula/solution/hula.p4
@@ -130,7 +130,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
diff --git a/SIGCOMM_2017/exercises/load_balance/load_balance.p4 b/SIGCOMM_2017/exercises/load_balance/load_balance.p4
index 5ded175..54dfed6 100644
--- a/SIGCOMM_2017/exercises/load_balance/load_balance.p4
+++ b/SIGCOMM_2017/exercises/load_balance/load_balance.p4
@@ -86,7 +86,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {
     apply { }
 }
 
diff --git a/SIGCOMM_2017/exercises/load_balance/solution/load_balance.p4 b/SIGCOMM_2017/exercises/load_balance/solution/load_balance.p4
index bfb5a07..64d9bcb 100644
--- a/SIGCOMM_2017/exercises/load_balance/solution/load_balance.p4
+++ b/SIGCOMM_2017/exercises/load_balance/solution/load_balance.p4
@@ -87,7 +87,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {
     apply { }
 }
 
diff --git a/SIGCOMM_2017/exercises/mri/mri.p4 b/SIGCOMM_2017/exercises/mri/mri.p4
index 5fcf1d3..1f7871b 100644
--- a/SIGCOMM_2017/exercises/mri/mri.p4
+++ b/SIGCOMM_2017/exercises/mri/mri.p4
@@ -148,7 +148,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -277,4 +277,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/mri/solution/mri.p4 b/SIGCOMM_2017/exercises/mri/solution/mri.p4
index ef369b5..1ce23be 100644
--- a/SIGCOMM_2017/exercises/mri/solution/mri.p4
+++ b/SIGCOMM_2017/exercises/mri/solution/mri.p4
@@ -140,7 +140,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -264,4 +264,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/scrambler/scrambler.p4 b/SIGCOMM_2017/exercises/scrambler/scrambler.p4
index 34ebabe..8d4e115 100644
--- a/SIGCOMM_2017/exercises/scrambler/scrambler.p4
+++ b/SIGCOMM_2017/exercises/scrambler/scrambler.p4
@@ -74,7 +74,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -177,4 +177,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/scrambler/solution/scrambler.p4 b/SIGCOMM_2017/exercises/scrambler/solution/scrambler.p4
index 6c79468..e508af6 100644
--- a/SIGCOMM_2017/exercises/scrambler/solution/scrambler.p4
+++ b/SIGCOMM_2017/exercises/scrambler/solution/scrambler.p4
@@ -74,7 +74,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -188,4 +188,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/source_routing/solution/source_routing.p4 b/SIGCOMM_2017/exercises/source_routing/solution/source_routing.p4
index 33f01e8..30c497b 100644
--- a/SIGCOMM_2017/exercises/source_routing/solution/source_routing.p4
+++ b/SIGCOMM_2017/exercises/source_routing/solution/source_routing.p4
@@ -92,7 +92,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -178,4 +178,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;
diff --git a/SIGCOMM_2017/exercises/source_routing/source_routing.p4 b/SIGCOMM_2017/exercises/source_routing/source_routing.p4
index 6c85b99..a84479e 100644
--- a/SIGCOMM_2017/exercises/source_routing/source_routing.p4
+++ b/SIGCOMM_2017/exercises/source_routing/source_routing.p4
@@ -96,7 +96,7 @@ parser MyParser(packet_in packet,
 ************   C H E C K S U M    V E R I F I C A T I O N   *************
 *************************************************************************/
 
-control MyVerifyChecksum(in headers hdr, inout metadata meta) {   
+control MyVerifyChecksum(inout headers hdr, inout metadata meta) {   
     apply {  }
 }
 
@@ -188,4 +188,4 @@ MyIngress(),
 MyEgress(),
 MyComputeChecksum(),
 MyDeparser()
-) main;
\ No newline at end of file
+) main;