diff --git a/payload_generator/ab_generator.cc b/payload_generator/ab_generator.cc
index 3b0d012..f727c71 100644
--- a/payload_generator/ab_generator.cc
+++ b/payload_generator/ab_generator.cc
@@ -17,6 +17,7 @@
 #include "update_engine/payload_generator/ab_generator.h"
 
 #include <algorithm>
+#include <utility>
 
 #include <base/strings/stringprintf.h>
 
@@ -173,7 +174,6 @@
     InstallOperation new_op;
     *(new_op.add_dst_extents()) = dst_ext;
     uint32_t data_size = dst_ext.num_blocks() * kBlockSize;
-    new_op.set_dst_length(data_size);
     // If this is a REPLACE, attempt to reuse portions of the existing blob.
     if (is_replace) {
       new_op.set_type(InstallOperation::REPLACE);
@@ -238,15 +238,9 @@
       if (is_delta_op) {
         ExtendExtents(last_aop.op.mutable_src_extents(),
                       curr_aop.op.src_extents());
-        if (curr_aop.op.src_length() > 0)
-          last_aop.op.set_src_length(last_aop.op.src_length() +
-                                     curr_aop.op.src_length());
       }
       ExtendExtents(last_aop.op.mutable_dst_extents(),
                     curr_aop.op.dst_extents());
-      if (curr_aop.op.dst_length() > 0)
-        last_aop.op.set_dst_length(last_aop.op.dst_length() +
-                                   curr_aop.op.dst_length());
       // Set the data length to zero so we know to add the blob later.
       if (is_a_replace)
         last_aop.op.set_data_length(0);
@@ -276,9 +270,9 @@
                                     BlobFileWriter* blob_file) {
   TEST_AND_RETURN_FALSE(IsAReplaceOperation(aop->op.type()));
 
-  brillo::Blob data(aop->op.dst_length());
   vector<Extent> dst_extents;
   ExtentsToVector(aop->op.dst_extents(), &dst_extents);
+  brillo::Blob data(BlocksInExtents(dst_extents) * kBlockSize);
   TEST_AND_RETURN_FALSE(utils::ReadExtents(target_part_path,
                                            dst_extents,
                                            &data,
diff --git a/payload_generator/ab_generator_unittest.cc b/payload_generator/ab_generator_unittest.cc
index ab4b164..25609c7 100644
--- a/payload_generator/ab_generator_unittest.cc
+++ b/payload_generator/ab_generator_unittest.cc
@@ -42,7 +42,9 @@
 
 namespace {
 
-bool ExtentEquals(const Extent& ext, uint64_t start_block, uint64_t num_blocks) {
+bool ExtentEquals(const Extent& ext,
+                  uint64_t start_block,
+                  uint64_t num_blocks) {
   return ext.start_block() == start_block && ext.num_blocks() == num_blocks;
 }
 
@@ -85,7 +87,6 @@
                                            op_ex1_num_blocks);
   *(op.add_dst_extents()) = ExtentForRange(op_ex2_start_block,
                                            op_ex2_num_blocks);
-  op.set_dst_length(op_ex1_num_blocks + op_ex2_num_blocks);
 
   brillo::Blob op_data;
   op_data.insert(op_data.end(),
@@ -136,7 +137,8 @@
   EXPECT_EQ("SplitTestOp:0", result_ops[0].name);
   InstallOperation first_op = result_ops[0].op;
   EXPECT_EQ(expected_type, first_op.type());
-  EXPECT_EQ(op_ex1_size, first_op.dst_length());
+  EXPECT_FALSE(first_op.has_src_length());
+  EXPECT_FALSE(first_op.has_dst_length());
   EXPECT_EQ(1, first_op.dst_extents().size());
   EXPECT_TRUE(ExtentEquals(first_op.dst_extents(0), op_ex1_start_block,
                            op_ex1_num_blocks));
@@ -165,7 +167,8 @@
   EXPECT_EQ("SplitTestOp:1", result_ops[1].name);
   InstallOperation second_op = result_ops[1].op;
   EXPECT_EQ(expected_type, second_op.type());
-  EXPECT_EQ(op_ex2_size, second_op.dst_length());
+  EXPECT_FALSE(second_op.has_src_length());
+  EXPECT_FALSE(second_op.has_dst_length());
   EXPECT_EQ(1, second_op.dst_extents().size());
   EXPECT_TRUE(ExtentEquals(second_op.dst_extents(0), op_ex2_start_block,
                            op_ex2_num_blocks));
@@ -235,7 +238,6 @@
   InstallOperation first_op;
   first_op.set_type(orig_type);
   const size_t first_op_size = first_op_num_blocks * kBlockSize;
-  first_op.set_dst_length(first_op_size);
   *(first_op.add_dst_extents()) = ExtentForRange(0, first_op_num_blocks);
   brillo::Blob first_op_data(part_data.begin(),
                                part_data.begin() + first_op_size);
@@ -255,8 +257,6 @@
 
   InstallOperation second_op;
   second_op.set_type(orig_type);
-  const size_t second_op_size = second_op_num_blocks * kBlockSize;
-  second_op.set_dst_length(second_op_size);
   *(second_op.add_dst_extents()) = ExtentForRange(first_op_num_blocks,
                                                   second_op_num_blocks);
   brillo::Blob second_op_data(part_data.begin() + first_op_size,
@@ -302,7 +302,7 @@
   InstallOperation new_op = aops[0].op;
   EXPECT_EQ(expected_op_type, new_op.type());
   EXPECT_FALSE(new_op.has_src_length());
-  EXPECT_EQ(total_op_num_blocks * kBlockSize, new_op.dst_length());
+  EXPECT_FALSE(new_op.has_dst_length());
   EXPECT_EQ(1, new_op.dst_extents().size());
   EXPECT_TRUE(ExtentEquals(new_op.dst_extents(0), 0, total_op_num_blocks));
   EXPECT_EQ("first,second", aops[0].name);
diff --git a/payload_generator/delta_diff_utils.cc b/payload_generator/delta_diff_utils.cc
index fb72d48..39438dd 100644
--- a/payload_generator/delta_diff_utils.cc
+++ b/payload_generator/delta_diff_utils.cc
@@ -737,26 +737,30 @@
     }
   }
 
-  size_t removed_bytes = 0;
   // Remove identical src/dst block ranges in MOVE operations.
   if (operation.type() == InstallOperation::MOVE) {
-    removed_bytes = RemoveIdenticalBlockRanges(
+    auto removed_bytes = RemoveIdenticalBlockRanges(
         &src_extents, &dst_extents, new_data.size());
+    operation.set_src_length(old_data.size() - removed_bytes);
+    operation.set_dst_length(new_data.size() - removed_bytes);
   }
-  // Set legacy src_length and dst_length fields.
-  operation.set_src_length(old_data.size() - removed_bytes);
-  operation.set_dst_length(new_data.size() - removed_bytes);
 
-  // Embed extents in the operation.
-  StoreExtents(src_extents, operation.mutable_src_extents());
+  // Set legacy |src_length| and |dst_length| fields for both BSDIFF and
+  // SOURCE_BSDIFF as only these two use these parameters.
+  if (operation.type() == InstallOperation::BSDIFF ||
+      operation.type() == InstallOperation::SOURCE_BSDIFF) {
+    operation.set_src_length(old_data.size());
+    operation.set_dst_length(new_data.size());
+  }
+
+  // Embed extents in the operation. Replace (all variants), zero and discard
+  // operations should not have source extents.
+  if (!IsNoSourceOperation(operation.type())) {
+    StoreExtents(src_extents, operation.mutable_src_extents());
+  }
+  // All operations have dst_extents.
   StoreExtents(dst_extents, operation.mutable_dst_extents());
 
-  // Replace operations should not have source extents.
-  if (IsAReplaceOperation(operation.type())) {
-    operation.clear_src_extents();
-    operation.clear_src_length();
-  }
-
   *out_data = std::move(data_blob);
   *out_op = operation;
   return true;
@@ -768,6 +772,12 @@
           op_type == InstallOperation::REPLACE_XZ);
 }
 
+bool IsNoSourceOperation(InstallOperation_Type op_type) {
+  return (IsAReplaceOperation(op_type) ||
+          op_type == InstallOperation::ZERO ||
+          op_type == InstallOperation::DISCARD);
+}
+
 // Returns true if |op| is a no-op operation that doesn't do any useful work
 // (e.g., a move operation that copies blocks onto themselves).
 bool IsNoopOperation(const InstallOperation& op) {
diff --git a/payload_generator/delta_diff_utils.h b/payload_generator/delta_diff_utils.h
index 2d49459..d21468f 100644
--- a/payload_generator/delta_diff_utils.h
+++ b/payload_generator/delta_diff_utils.h
@@ -112,9 +112,12 @@
                                brillo::Blob* out_blob,
                                InstallOperation_Type* out_type);
 
-// Returns whether op_type is one of the REPLACE full operations.
+// Returns whether |op_type| is one of the REPLACE full operations.
 bool IsAReplaceOperation(InstallOperation_Type op_type);
 
+// Returns true if an operation with type |op_type| has no |src_extents|.
+bool IsNoSourceOperation(InstallOperation_Type op_type);
+
 // Returns true if |op| is a no-op operation that doesn't do any useful work
 // (e.g., a move operation that copies blocks onto themselves).
 bool IsNoopOperation(const InstallOperation& op);
diff --git a/payload_generator/delta_diff_utils_unittest.cc b/payload_generator/delta_diff_utils_unittest.cc
index bb83942..34e33ad 100644
--- a/payload_generator/delta_diff_utils_unittest.cc
+++ b/payload_generator/delta_diff_utils_unittest.cc
@@ -364,7 +364,7 @@
     EXPECT_EQ(0, op.src_extents_size());
     EXPECT_FALSE(op.has_src_length());
     EXPECT_EQ(1, op.dst_extents_size());
-    EXPECT_EQ(data_to_test.size(), op.dst_length());
+    EXPECT_FALSE(op.has_dst_length());
     EXPECT_EQ(1U, BlocksInExtents(op.dst_extents()));
   }
 }
diff --git a/update_metadata.proto b/update_metadata.proto
index 1120bd1..f91f673 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -178,14 +178,15 @@
   // Ordered list of extents that are read from (if any) and written to.
   repeated Extent src_extents = 4;
   // Byte length of src, equal to the number of blocks in src_extents *
-  // block_size. It is used for BSDIFF, because we need to pass that
-  // external program the number of bytes to read from the blocks we pass it.
-  // This is not used in any other operation.
+  // block_size. It is used for BSDIFF and SOURCE_BSDIFF, because we need to
+  // pass that external program the number of bytes to read from the blocks we
+  // pass it.  This is not used in any other operation.
   optional uint64 src_length = 5;
 
   repeated Extent dst_extents = 6;
   // Byte length of dst, equal to the number of blocks in dst_extents *
-  // block_size. Used for BSDIFF, but not in any other operation.
+  // block_size. Used for BSDIFF and SOURCE_BSDIFF, but not in any other
+  // operation.
   optional uint64 dst_length = 7;
 
   // Optional SHA 256 hash of the blob associated with this operation.