Link Compression Extension

From Gnutella Developers

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Source - [Latest draft. (http://rfc-gnutella.sourceforge.net/src/rfc-0_6-draft.html)]

Compression of the message traffic over a Gnutella connection is OPTIONAL. The following assumes the "deflate" scheme is used, but any compression algorithm MAY be used.

Compression is meant to be done on a per-connection basis. The "deflate" scheme is handled by the zlib (http://www.zlib.org/) library (the deflate/ inflate routines). This means there will be a compression dictionary and history per connection on both ends, meaning a good compression ability (compared to compressing each message individually).

However, this stream compression of the traffic means that we need to individually compress the same packet on each connection, using a dedicated compressing state maintained per connection.

Because compression algorithms don't necessarily produce output when fed input (e.g. if you feed them with "aaaaa", they'll wait for the next char, and will do so until it's a "b" for instance), it is necessary to periodically direct them to flush output. However, this comes at a cost, because once flushed, data is inflatable by the decompressor, and this is done at the expense of sending the necessary dictionary information.

To negotiate compression, we're making full use of the 3-way handshaking. The idea is that decompression is fast, so it's OK to be sent compressed data, but the compressing side must decide based on the resources it has available whether it will compress or not.

Basically, the side supporting decompression will say:

  Accept-Encoding: deflate

Note that although we only specify "deflate" here, the peer MAY advertise the set of various compression algorithms it knows, subsequent items being separated by a ",".

And to accept compression, the other side acknowledges by sending:

  Content-Encoding: deflate

The peer just picks the compression scheme it supports amongst the ones advertised by the remote end in the Accept-Encoding line. The Content-Encoding MUST contain only one value.

This also means that compression settings is asymmetric: a node can send compressed data but receive uncompressed data.

Here's an example where both nodes support compression, comments starting with "--", and ending removed for clarity:

  GNUTELLA CONNECT/0.6
  Accept-Encoding: deflate       -- OK for reception of compressed data

      GNUTELLA/0.6 200 OK
      Accept-Encoding: deflate   -- I can also receive compressed data
      Content-Encoding: deflate  -- And I will send compressed data

  GNUTELLA/0.6 200 OK
  Content-Encoding: deflate      -- OK, will also compress data

Here's an example where compression will only be made on the transmission side of the first node (A is the node initiating the handshake, B is the node replying):

  GNUTELLA CONNECT/0.6<CR><LF>
  User-Agent: SomeServent/1.0<CR><LF>
  (Other-Headers: Values<CR><LF>)...
  Accept-Encoding: deflate<CR><LF>      -- OK for reception of compressed data
  <CR><LF>

      GNUTELLA/0.6 200 OK<CR><LF>
      User-Agent: OtherServent/1.0<CR><LF>
      (Other-Headers: Values<CR><LF>...)
      Accept-Encoding: deflate<CR><LF>  -- I can also receive compressed data
      <CR><LF>                          -- I refuse to compress data, sorry

  GNUTELLA/0.6 200 OK<CR><LF>
  Content-Encoding: deflate<CR><LF>     -- OK, I will compress data sent
  <CR><LF>                              -- But I will receive uncompressed data

  <flow from A->B is compressed, flow from B->A is not>

Even though GGEP payloads (see Appendix 1) can be compressed, and this information is visible in the GGEP header, it is not advisable to decompress those payloads before sending them to the compressing layer. The deflate algorithm does not expand already-compressed data by a large factor and emits them as clearly marked non-compressible data (the overhead is limited to roughly 0.1%). If connection compression is widely used on the Gnutella network, individual GGEP extensions SHOULD NOT be compressed.

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