6.2. Compression

6.2. Compression

If compression has been negotiated, the payload field (and only it) will be compressed using the negotiated algorithm. The packet_length field and mac will be computed from the compressed payload. Encryption will be done after compression.

Compression MAY be stateful, depending on the method. Compression MUST be independent for each direction, and implementations MUST allow independent choosing of the algorithm for each direction. In practice however, it is RECOMMENDED that the compression method be the same in both directions.

The following compression methods are currently defined:

none     REQUIRED        no compression
zlib     OPTIONAL        ZLIB (LZ77) compression

The "zlib" compression is described in [RFC1950] and in [RFC1951]. The compression context is initialized after each key exchange, and is passed from one packet to the next, with only a partial flush being performed at the end of each packet. A partial flush means that the current compressed block is ended and all data will be output. If the current block is not a stored block, one or more empty blocks are added after the current block to ensure that there are at least 8 bits, counting from the start of the end-of-block code of the current block to the end of the packet payload.

Additional methods may be defined as specified in [SSH-ARCH] and [SSH-NUMBERS].