The qpack_static_table_version TLS extension

Abstract

This document specifies a new "qpack_static_table_version" TLS extension to enable clients and servers to agree upon the version of the HTTP/3 QPACK Static Table to use for communications between them.¶

1. Introduction

HTTP/3 uses QPACK [RFC9204] for compression of the header and trailer sections.¶

QPACK reuses core concepts from the HPACK algorithm used for HTTP/2, but as noted in its design "[...]is redesigned to allow correctness in the presence of out-of-order delivery, with flexibility for implementations to balance between resilience against head-of-line blocking and optimal compression ratio."¶

A core feature of QPACK is the use of a 'static table' which contains a predefined list of frequently-used field lines, each of which is identified by a unique, unchanging index in the table, enabling excellent compression of these field lines. Each entry in the static table may contain a field name or a field name/value combination. Less frequently-used fields or field name/value combinations are passed in the QPACK 'dynamic table'. Because of the optimal compression, defining field lines in the static table is significantly desirable over passing them in the dynamic table.¶

When the static table was originally defined, the field line entries it contained were calculated by determining the relative frequency with which each field line was found in a representative sampling of web traffic. However, as new HTTP fields become more frequently-used over time, it makes sense that they be added as additional entries to the static table to retain the high level of compression.¶

When clients and servers communicate using HTTP/3, they each need to be aware of how many static table entries the other is using, so there is no mismatch where one refers to a static table entry that the other does not recognize. As a result, clients and servers must agree upon the size of the static table they will both use. Because the size of the static table must be known prior to any HTTP requests passing between the client and server, this information must be passed during the TLS handshake.¶

This specification defines a new TLS extension which can be passed by both client and server to identify the version (size) of the static table that they will both use.¶

2. QPACK Static Table Background

The original QPACK static table [RFC9204] contains 99 entries (starting at entry 0, ending at entry 98), as follows:¶

The table was generated by analyzing actual Internet traffic in 2018 and including the most common header fields, after filtering out some unsupported and non-standard values. Due to this methodology, some of the entries may be inconsistent or appear multiple times with similar but not identical values. The order of the entries is optimized to encode the most common header fields with the smallest number of bytes.¶

3. QPACK Static Table Additions

Over time, the IANA will re-sample actual internet traffic and as new field names or field name/value combinations become more prevalent, they will be appended as new entries to the static table and a new version of the static table will be published by IANA.¶

To maintain backwards-compatibility, new static table entries MUST be appended to the end of the current published version of the static table. Once added, entries MUST NOT be removed from a published version of the static table. As new versions of the static table are published, they can be implemented by both clients and servers. It is assumed that clients and servers will support only a single version of the static table.¶

4. The qpack_static_table_version TLS extension

The qpack_static_table_version TLS extension is an integer value which represents the number of entries in the static table supported by the sender.¶

The extension_data for the qpack_static_table_version extension is StaticTableLength, described below using the syntax defined in [RFC8446]:¶

struct {
    uint8 StaticTableLength;
} QSTVExtension;

The default value of the qpack_static_table_version extension is 99 - the size of the original QPACK static table.¶

The qpack_static_table_version extension has a minimum allowed value equal to the default value. Any value lower than the minimum allowed value, including a negative value, is invalid.¶

The qpack_static_table_version extension is OPTIONAL and MAY be passed by a client to a server in the ClientHello, to indicate the version of the static table that the client supports - that is, the number of static table entries that the client supports. If the qpack_static_table_version extension is not present in the ClientHello, the server MUST assume that the client is using the default value.¶

If the ClientHello includes the qpack_static_table_version extension, the server MAY respond in the ServerHello with its own version of the static table.¶

If either the ClientHello or the ServerHello do not include the qpack_static_table_version extension, or if an invalid value is specified, then both client and server must fall back to use the default value.¶

If both the client and the server pass the qpack_static_table_version extension, the lower version MUST be used by both client and server, thus ensuring that both client and server only reference entries in their static table up to the shared lowest version number.¶

Sample pseudocode processing:¶

Initialize variable C to supported StaticTableLength value
Include qpack_static_table_version with value C in ClientHello
if (ServerHello does not contain qpack_static_table_version) {
    set C to default value
} else {
    save qpack_static_table_version from ServerHello as S
    if (S is invalid) {
        set C to default value
    } else if (S < C) {
        set C to value of S
    }
}
Use C for static table index processing

Initialize variable S to supported StaticTableLength value
if (ClientHello does not contain qpack_static_table_version) {
    set S to default value
} else {
    save qpack_static_table_version from ClientHello as C
    if (C is invalid) {
        set S to default value
    } else if (C < S) {
        set S to value of C
    }
    Include qpack_static_table_version with value S in ServerHello
}
Use S for static table index processing

Examples (assuming that multiple versions, including versions 114 and 126, have been published by IANA):¶

7. References