Index: src/cgi.c
==================================================================
--- src/cgi.c
+++ src/cgi.c
@@ -313,11 +313,11 @@
               cgi_rfc822_datestamp(etag_mtime()));
     }
   }else if( g.isConst ){
     /* isConst means that the reply is guaranteed to be invariant, even
     ** after configuration changes and/or Fossil binary recompiles. */
-    fprintf(g.httpOut, "Cache-Control: max-age=31536000\r\n");
+    fprintf(g.httpOut, "Cache-Control: max-age=315360000, immutable\r\n");
   }else{
     fprintf(g.httpOut, "Cache-control: no-cache\r\n");
   }
 
   if( blob_size(&extraHeader)>0 ){

Index: src/etag.c
==================================================================
--- src/etag.c
+++ src/etag.c
@@ -173,10 +173,26 @@
   cgi_set_status(304, "Not Modified");
   cgi_reply();
   db_close(0);
   fossil_exit(0);
 }
+
+/*
+** If the output is determined purely by hash parameter and the hash
+** is long enough to be invariant, then set the g.isConst flag, indicating
+** that the output will never change.
+*/
+void etag_check_for_invariant_name(const char *zHash){
+  size_t nHash = strlen(zHash);
+  if( nHash<HNAME_MIN ){
+    return;  /* Name is too short */
+  }
+  if( !validate16(zHash, (int)nHash) ){
+    return;  /* Name is not pure hex */
+  }
+  g.isConst = 1;  /* A long hex identifier must be a unique hash */
+}
 
 /*
 ** Accept a new Last-Modified time.  This routine should be called by
 ** page generators that know a valid last-modified time.  This routine
 ** might generate a 304 Not Modified reply and exit(), never returning.
@@ -212,11 +228,11 @@
 }
 
 /* Return the ETag, if there is one.
 */
 const char *etag_tag(void){
-  return zETag;
+  return g.isConst ? "" : zETag;
 }
 
 /* Return the recommended max-age
 */
 int etag_maxage(void){

Index: src/tar.c
==================================================================
--- src/tar.c
+++ src/tar.c
@@ -774,10 +774,13 @@
   nRid = strlen(zRid);
   zInclude = P("in");
   if( zInclude ) pInclude = glob_create(zInclude);
   zExclude = P("ex");
   if( zExclude ) pExclude = glob_create(zExclude);
+  if( zInclude==0 && zExclude==0 ){
+    etag_check_for_invariant_name(z);
+  }
   nName = strlen(zName);
   if( nName>7 && fossil_strcmp(&zName[nName-7], ".tar.gz")==0 ){
     /* Special case:  Remove the ".tar.gz" suffix.  */
     nName -= 7;
     zName[nName] = 0;

Index: src/zip.c
==================================================================
--- src/zip.c
+++ src/zip.c
@@ -940,10 +940,13 @@
   nRid = strlen(zRid);
   zInclude = P("in");
   if( zInclude ) pInclude = glob_create(zInclude);
   zExclude = P("ex");
   if( zExclude ) pExclude = glob_create(zExclude);
+  if( zInclude==0 && zExclude==0 ){
+    etag_check_for_invariant_name(z);
+  }
   if( eType==ARCHIVE_ZIP 
    && nName>4
    && fossil_strcmp(&zName[nName-4], ".zip")==0
   ){
     /* Special case:  Remove the ".zip" suffix.  */