+PERFORMANCE ----------------------------------------------------------
+
+MD4 file_sum
+
+ If we're doing a local transfer, or using -W, then perhaps don't
+ send the file checksum. If we're doing a local transfer, then
+ calculating MD4 checksums uses 90% of CPU and is unlikely to be
+ useful.
+
+ Indeed for transfers over zlib or ssh we can also rely on the
+ transport to have quite strong protection against corruption.
+
+ Perhaps we should have an option to disable this, analogous to
+ --whole-file, although it would default to disabled. The file
+ checksum takes up a definite space in the protocol -- we can either
+ set it to 0, or perhaps just leave it out.
+
+MD4
+
+ Perhaps borrow an assembler MD4 from someone?
+
+ Make sure we call MD4 with properly-sized blocks whenever possible
+ to avoid copying into the residue region?
+