3.1.8.2.1 Setting the Compression Flags

The sender MUST always specify the compression flags associated with a compressed payload. These flags MUST be set in the header field appropriate to the type of data payload, such as fast-path output data (section 2.2.9.1.2.1), virtual channel data (section 2.2.6.1), or slow-path output data (section 2.2.9.1.1).

The compression flags are produced by performing a logical OR operation of the compression type with one or more of the following flags.

Compression flag

Meaning

PACKET_COMPRESSED

0x20

Used to indicate that the data is compressed. This flag is equivalent to MPPC bit C (for more information see [RFC2118] section 3.1). This flag MUST be set when compression of the data was successful.

PACKET_AT_FRONT

0x40

Used to indicate that the decompressed data MUST be placed at the beginning of the local history buffer. This flag is equivalent to MPPC bit B (for more information see [RFC2118] section 3.1). This flag MUST be set in conjunction with the PACKET_COMPRESSED (0x20) flag.

There are two conditions on the "compressor-side" that generate this scenario: (1) this is the first packet to be compressed, and (2) the data to be compressed will not fit at the end of the history buffer but instead needs to be placed at the start of the history buffer.

PACKET_FLUSHED

0x80

Used to indicate that the decompressor MUST reinitialized the history buffer (by filling it with zeros) and reset the HistoryOffset to zero. After it has been reinitialized, the entire history buffer is immediately regarded as valid. This flag is equivalent to MPPC bit A (for more information see [RFC2118] section 3.1).

If the PACKET_COMPRESSED (0x20) flag is also present, then the PACKET_FLUSHED flag MUST be processed first.

Data that is tagged as compressed (using the PACKET_COMPRESSED flag) MUST NOT be larger in size than the original data. This implies that in a minority of cases it is possible for compressed data to be the same size as the original data, and still be regarded as compressed. In effect, the statement that "data is compressed" simply implies that the data is encoded using a particular scheme, and that a decoder (or decompressor) is required to obtain the original data.

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