Implement the compression V2 data format for stub and lz4.

Implement the compression V2 data format for stub and lz4.

Patch V2: Fix minor issues found by Steffan
Patch V3: split wire codes and compression flags
Patch V4: Fix further issues reported by Gert
Patch V5: really fix the issues that should be fixed in v2
Patch V6: fix more minor things

It has been tested against v3 server and again itself. From James Mail:

 Compression V2

   I have observed that compression in many cases, even when
   enabled, often does not produce packet size reduction
   because much of the packet data typically generated by web
   sessions is already compressed.  Further, the single byte that
   precedes the packet and indicates whether or not compression
   occurred has the unfortunate side effect of misaligning the IP
   packet in cases where compression did not occur.  To remedy this,
   I propose a Compression V2 header that is optimized for the
   case where compression does not occur.

   a. No compression occurred and first byte of IP/Ethernet packet
      is NOT 0x50 (0 bytes of overhead and maintains alignment):

        [ uncompressed IP/Ethernet packet ]

   b. No compression occurred and first byte of IP/Ethernet packet
      is 0x50 (2 bytes of overhead but unlikely since no known
      IP packet can begin with 0x50):

        [ 0x50 ] [ 0x00 ] [ uncompressed IP/Ethernet packet ]

   c. Compression occurred (2 bytes of overhead):

        [ 0x50 ] [ compression Alg ID ] [ compressed IP/Ethernet packet ]

      Compression Alg ID is one-byte algorithm identifier
      for LZ4 (0x1), LZO (0x2), or Snappy (0x3).

   This approach has several beneficial effects:

   1. In the common case where compression does not occur, no
      compression op is required, therefore there is zero overhead.

   2. When compression does not occur, the IP/Ethernet packet
      alignment is retained.

   3. This technique does not require any byte swapping with
      the tail of the packet which can potentially incur an
      expensive cache miss.

Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1451842066-13475-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10925

Signed-off-by: Gert Doering <gert@greenie.muc.de>