[−][src]Module httlib_hpack::encoder

Provides an implementation of the HPACK encoder.

The encoder performs the task of data compression. It converts the data from its original readable form into an optimized byte sequence by applying the rules defined in the HPACK specification.

The HPACK encoding has specific rules for representing integer and string primitive types.

  • Integer representation defines the rules for encoding integer numbers. Integers are used to represent name indexes, header field indexes, or character string lengths.

  • String literal representation defines the rules for encoding string literals. With these, we encode the header name and value literals. The content of these rules can be written in plain text format or encoded with the Huffman algorithm.

With these basic rules, HPACK defines the binary formats for the representation of the actual headers.

  • Indexed header field representation represents fully indexed headers. These are the headers that are stored in the indexing table under specific index numbers. Since both the header name and value are stored in the indexing table, only this index number is encoded. Such headers are really minimal and therefore optimal in terms of performance.

  • Literal header field representation defines headers that are not or only partially indexed. If the header field name matches the header field name of an entry stored in the static or dynamic table, the header field name can be displayed using the index of this entry. Otherwise, the header field name is displayed as a string literal. Header values are always displayed as a string literal. Such headers can be marked as "index", "do not index" or "never index". The latter tells us that the data is sensitive and that the entity should handle it with some restrictions (e.g.: protect it with a password).

HPACK is designed as a single-standing mechanism that can also be used outside the HTTP/2 protocol. For this reason, the specification provides a rule for signaling changes related to the allowed size of the dynamic table.

  • Dynamic table size update defines the rule for signaling changes in the size of the dynamic table. Such a change is signaled by the encoder, while the limit must be less than or equal to the limit determined by the protocol using HPACK. In HTTP/2 this limit is the last value of the SETTINGS_HEADER_TABLE_SIZE received by the decoder and acknowledged by the encoder. Encoder and decoder use the HTTP/2 protocol to communicate the change in table size and if the change is accepted at both ends, the encoder applies the change and reports it to the decoder using the HPACK mechanism.

These five rules, with some additional conditional rules described by the HPACK specification, define the HPACK encoder.

Structs

Encoder

Provides the encoding engine for HTTP/2 headers.

Enums

EncoderError

Contains error options that can be encountered while performing the encoding of an HPACK header set.

EncoderInput

Provides encoder input format options.