diff --git a/src/shared/config.h b/src/shared/config.h
index e6a525d..213cb93 100644
--- a/src/shared/config.h
+++ b/src/shared/config.h
@@ -44,7 +44,7 @@ typedef struct Config {
   bool partial;
 } Config;
 
-#define PROTOCOL_VERSION "1.3.0"
+#define PROTOCOL_VERSION "2.0.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(char* version, char* send_directory, char* receive_directory,
diff --git a/src/shared/file.c b/src/shared/file.c
index 5a412cd..8134a6c 100644
--- a/src/shared/file.c
+++ b/src/shared/file.c
@@ -1,4 +1,5 @@
 #include <dirent.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <libgen.h>
 #include <stddef.h>
diff --git a/src/shared/metadata.c b/src/shared/metadata.c
index b4de62e..4134c13 100644
--- a/src/shared/metadata.c
+++ b/src/shared/metadata.c
@@ -11,6 +11,19 @@
 #include <time.h>
 #include <unistd.h>
 
+/*
+ * Wire format serialization (protocol version 2.0.0+):
+ * All metadata fields are serialized as fixed-width integers (int32_t / int64_t)
+ * to ensure cross-platform binary compatiblity.  See metadata.h for the
+ * exact wire layout.
+ *
+ * Compile-time assertions verify that the native platform types fit within
+ * the chosen fixed-width representations.
+ */
+typedef char static_assert_mode_t_fits[(sizeof(mode_t) <= sizeof(int32_t)) ? 1 : -1];
+typedef char static_assert_uid_t_fits[(sizeof(uid_t) <= sizeof(int32_t)) ? 1 : -1];
+typedef char static_assert_gid_t_fits[(sizeof(gid_t) <= sizeof(int32_t)) ? 1 : -1];
+
 void metadata_to_buf(char** buf, const FileMetadata* m) {
   int32_t present = (m != NULL) ? 1 : 0;
   memcpy(*buf, &present, sizeof(present));
diff --git a/src/shared/metadata.h b/src/shared/metadata.h
index 518077b..f15570f 100644
--- a/src/shared/metadata.h
+++ b/src/shared/metadata.h
@@ -6,6 +6,20 @@
 #include <stdint.h>
 #include <sys/stat.h>
 
+/*
+ * Wire format (introduced in protocol version 2.0.0):
+ *   int32_t present
+ *   int32_t mode    (was mode_t, platform-dependent)
+ *   int32_t uid     (was uid_t,  platform-dependent)
+ *   int32_t gid     (was gid_t,  platform-dependent)
+ *   int64_t mtime_sec (was time_t, platform-dependent)
+ *   int64_t mtime_nsec (was long,  platform-dependent)
+ *
+ * Prior to 2.0.0 the wire format used the raw platform-dependent types,
+ * which broke compatiblity across different systems.  All fields are now
+ * serialized as fixed-width integers.
+ */
+
 #define FILE_METADATA_WIRE_SIZE (sizeof(int32_t) * 3 + sizeof(int64_t) * 2)
 
 void metadata_to_buf(char** buf, const FileMetadata* m);