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2. Compression Process Overview

This specification does not describe a specific algorithm for an encoder. Instead, it defines precisely how a decoder is expected to operate, allowing encoders to produce any encoding that this definition permits.

2.1. Header List Ordering​

HPACK preserves the ordering of header fields inside the header list. An encoder MUST order header field representations in the header block according to their ordering in the original header list. A decoder MUST order header fields in the decoded header list according to their ordering in the header block.

2.2. Encoding and Decoding Contexts​

To decompress header blocks, a decoder only needs to maintain a dynamic table (see Section 2.3.2) as a decoding context. No other dynamic state is needed.

When used for bidirectional communication, such as in HTTP, the encoding and decoding dynamic tables maintained by an endpoint are completely independent, i.e., the request and response dynamic tables are separate.

2.3. Indexing Tables​

HPACK uses two tables for associating header fields to indexes. The static table (see Section 2.3.1) is predefined and contains common header fields (most of them with an empty value). The dynamic table (see Section 2.3.2) is dynamic and can be used by the encoder to index header fields repeated in the encoded header lists.

These two tables are combined into a single address space for defining index values (see Section 2.3.3).

2.3.1. Static Table​

The static table consists of a predefined static list of header fields. Its entries are defined in Appendix A.

2.3.2. Dynamic Table​

The dynamic table consists of a list of header fields maintained in first-in, first-out order. The first and newest entry in a dynamic table is at the lowest index, and the oldest entry of a dynamic table is at the highest index.

The dynamic table is initially empty. Entries are added as each header block is decompressed.

The dynamic table can contain duplicate entries (i.e., entries with the same name and same value). Therefore, duplicate entries MUST NOT be treated as an error by a decoder.

The encoder decides how to update the dynamic table and as such can control how much memory is used by the dynamic table. To limit the memory requirements of the decoder, the dynamic table size is strictly bounded (see Section 4.2).

The decoder updates the dynamic table during the processing of a list of header field representations (see Section 3.2).

2.3.3. Index Address Space​

The static table and the dynamic table are combined into a single index address space.

Indices between 1 and the length of the static table (inclusive) refer to elements in the static table (see Section 2.3.1).

Indices strictly greater than the length of the static table refer to elements in the dynamic table (see Section 2.3.2). The length of the static table is subtracted to find the index into the dynamic table.

Indices strictly greater than the sum of the lengths of both tables MUST be treated as a decoding error.

For a static table size of s and a dynamic table size of k, the following diagram shows the entire valid index address space.

        `<----------  Index Address Space ---------->`
        `<-- Static  Table -->`  `<-- Dynamic Table -->`
        +---+-----------+---+  +---+-----------+---+
        | 1 |    ...    | s |  |s+1|    ...    |s+k|
        +---+-----------+---+  +---+-----------+---+
                               ^                   |
                               |                   V
                        Insertion Point      Dropping Point

                    Figure 1: Index Address Space

2.4. Header Field Representation​

An encoded header field can be represented either as an index or as a literal.

An indexed representation defines a header field as a reference to an entry in either the static table or the dynamic table (see Section 6.1).

A literal representation defines a header field by specifying its name and value. The header field name can be represented literally or as a reference to an entry in either the static table or the dynamic table. The header field value is represented literally.

Three different literal representations are defined:

  • A literal representation that adds the header field as a new entry at the beginning of the dynamic table (see Section 6.2.1).

  • A literal representation that does not add the header field to the dynamic table (see Section 6.2.2).

  • A literal representation that does not add the header field to the dynamic table, with the additional stipulation that this header field always use a literal representation, in particular when re-encoded by an intermediary (see Section 6.2.3). This representation is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1.3 for more details).

The selection of one of these literal representations can be guided by security considerations, in order to protect sensitive header field values (see Section 7.1).

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