The qpack_static_table_version TLS extension

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Table of Contents

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 central feature of QPACK is the use of a 'static table' which contains a predefined list of frequently-used field names or field name/value combinations (generically "field strings"), each of which is identified by a unique, unchanging index in the table, enabling excellent compression of the HTTP fields. Less frequently-used field strings are defined as ASCII text in the QPACK 'dynamic table'. Because of the optimal compression, defining field strings in the static table is significantly desirable over passing them in the dynamic table.¶

When the static table was originally defined, the entries it contained were calculated by determining the relative frequency with which each field string 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.¶

This specification defines a new TLS extension which can be passed by both client and server to identify and agree on the version (size) of the static table that 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 also defines a new IANA registry where the entries in the QPACK static table will be published and updated. Additional related registries will be created over time.¶

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

A registry called QPACK Static Table Version 1 will be created and maintained by IANA in which the initial QPACK static table will be published. See Section 7 for more details.¶

Over time, subsequent related registries using a naming convention of QPACK Static Table Version N (where N is an integer incremented by 1 from the prior registry) will be created to contain a re-ordered copy of the table that is defined in the prior registry.¶

Each registry will have two 'attributes' that are used by clients and servers during TLS negotiation - a Version (part of the registry name) and a Length (the number of entries in the registry). In this document, a registry may be referred to using the shorthand of "Version, Length" - for example "1,99".¶

The initial registry (Version 1) will have 99 entries (Length 99), shorthandedly known as 1,99. These values match the initial value of the QPACK static table as defined in the QPACK RFC [RFC9204]. Every subsequent registry will have a Length value greater than or equal to 99.¶

Periodically (see Section 7 for proposed periods), IESG designated experts will re-sample actual internet traffic and as new field strings are found to have become more prevalent than the current last entry in the registry with the highest Version, the new field strings will be ordered by prevalence with one another (if more than one field string) and appended as new entries to that registry.¶

Periodically (see Section 7 for proposed periods), a new QPACK Static Table Version N registry will be created (where N is incremented by 1 from the prior registry Version). This registry will contain all the entries in the prior registry Version, but they will all be re-ordered, to ensure that the most commonly-found header lines are near the start of the table. When a new QPACK Static Table Version N registry is published, all prior registries are 'locked' and will not be changed; new entries will not be appended to them.¶

Thus at any time, there may be multiple concurrently-published QPACK Static Table registries, each with a different Version.¶

4. QPACK Static Table support

HTTP/3-supporting clients and servers MUST always support QPACK Static Table Version 1. They MAY also support other registry Versions as long as they support all intermediate Versions between Version 1 and the Version for which they advertise support in qpack_static_table_version. This allows both client and server to downgrade to a shared supported Version without the client needing to advertise multiple supported Versions.¶

HTTP/3-supporting clients and servers MUST encode into their software a maximum Length (number of entries) in each supported registry Version. This can be any value from the number of entries when that registry Version was created up to the number of entries when it was locked due to the creation of a new registry, inclusive. This ensures that if a sender (maliciously or otherwise) specifies that it supports a registry Version prior to that supported by the receiver, but also specifies a number of entries for that registry Version that is higher than the maximum value for that Version as stored by the receiver (for instance, if the sender announces support for Version 1, with Length 105), the receiver can correctly identify that this is an invalid combination. In this scenario, the receiver MUST ignore the invalid Length field passed by the sender and use the lower of the maximum Length supported by the receiver or 99. See Example 10 in Section 6 for an example of this.¶

5. The qpack_static_table_version TLS extension

The qpack_static_table_version TLS extension is a structure which represents the Version and Length of the static table supported by the sender. It is OPTIONAL and MAY be sent by either client or server or both or neither.¶

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

struct {
    uint8 StaticTableSupportedVersion;
    uint8 StaticTableSupportedLength;
} qpack_static_table_version;

The StaticTableSupportedVersion field must be a positive integer and indicates the highest registry Version supported. A negative value or a value of 0 is invalid and the receiver MUST treat this as if a value of 1 was passed by the sender.¶

The StaticTableSupportedLength field must be a positive integer. A negative value or a value of 0 is invalid and the receiver MUST treat this as if a value of 99 was passed by the sender. A value that is lower than the current length of the registry identified by the StaticTableSupportedVersion field is valid; this allows a client or server which may have very limited memory and/or which expects to send/receive only a few known headers to specify a low Length e.g. 1,30, indicating that they support only the first 30 entries in the QPACK Static Table Version 1 registry and any other header lines must be specified in the dynamic table.¶

The qpack_static_table_version extension MAY be passed by a client to a server in the ClientHello, to indicate the Version/Length of the registry that the client supports. If the qpack_static_table_version extension is not present in the ClientHello, the server MUST assume that the client implicitly supports QPACK Static Table Version 1 up to 99 entries.¶

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

If either the ClientHello or the ServerHello do not include the qpack_static_table_version extension, or if an invalid value is specified for either of its component fields, then both client and server must fall back to use QPACK Static Table Version 1 up to 99 entries.¶

If both the client and the server pass the qpack_static_table_version extension with valid values, the lower Version and Length 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 maximum length of the registry. Note that the chosen Version and Length may be separately derived from either client or server. See example 9 in Section 6 for an example of this.¶

If either client or server passes a StaticTableSupportedLength field which is higher than the number of entries in the registry indicated by the StaticTableSupportedVersion field, the receiver MUST fall back to using 99 entries (or the maximum Length supported by the receiver if lower than 99) in the registry indicated by the StaticTableSupportedVersion field (or the highest Version registry supported by the receiver if lower). As noted in Section 4, this means that clients and servers must encode into their software a maximum allowed Length for each Version.¶

Sample pseudocode processing for client and server:¶

Initialize variable CV to supported Version value
Initialize variable CL to supported Length value
Include qpack_static_table_version with values CV,CL in ClientHello
if (ServerHello does not contain qpack_static_table_version or if qpack_static_table_version contains invalid values) {
    set CV to 1
    set CL to lesser of 99 and CL
} else {
    save qpack_static_table_version from ServerHello as SV,SL
    set SL_max to maximum valid entries for specified SV
    if (SV < CV) {
        set CV to SV
    }
    if (SL < CL) {
        if (SL > SL_max) {
            set CL to lesser of 99 and CL
        } else {
            set CL to SL
        }
    }
}
use CV,CL for static table index processing

Initialize variable SV to supported Version value
Initialize variable SL to supported Length value
if (ClientHello does not contain qpack_static_table_version or if qpack_static_table_version contains invalid values) {
    set SV to 1
    set SL to lesser of 99 and SL
} else {
    save qpack_static_table_version from ClientHello as CV,CL
    set CL_max to maximum valid entries for specified CV
    if (CV < SV) {
        set SV to CV
    }
    if (CL < SL) {
        if (CL > CL_max) {
            set SL to lesser of 99 and SL
        } else {
            set SL to CL
        }
    }
    Include qpack_static_table_version with values SV,SL in ServerHello
}
Use SV,SL for static table index processing

6. Processing Examples

Example published registries (after some time has passed and multiple registries have been created):¶

Example values passed for the qpack_static_table_version:¶

7. IANA Considerations

This memo requests that IANA creates a registry for the QPACK static table, to be periodically updated and supserseded by IESG-appointed Designated Expert(s).¶

The registry name is QPACK Static Table Version 1 and is part of the Transport Layer Security (TLS) registry grouping. The registration policy is Expert Review.¶

The registry will contain a copy of the QPACK static table ("table"). Each entry in the table consists of the following three fields:¶

The initial set of entries in the registry will be copied directly from the entries in the QPACK static table defined in Appendix A in [RFC9204].¶

As per the technical information in this document, on a periodic basis, the Designated Expert(s) will sample HTTP/3 web traffic and may, as a result, append new entries to the registry. It is suggested that the sampling process occurs approximately once every year, but the decision of whether to append new entries to the registry may occur at a slower rate, since it depends entirely on the rate at which new fields become sufficiently prevalent in HTTP/3 web traffic.¶

On a separate periodic basis, a new registry will be created using the same format as the above registry, but will be named QPACK Static Table Version N (where N is an integer incremented by 1 from the prior registry). This new registry will be a re-ordered copy of the prior registry. When a new QPACK Static Table registry is created, all prior registry versions must be locked and no new entries can be appended to them. It is suggested that this process happens approximately once every three years, but again, it depends on the results of the HTTP/3 web traffic sampling.¶

The intention is that any appending of new entries to the registry and/or the creation of a new QPACK Static Table registry will be accompanied by a published RFC. But in order to allow for the allocation of values prior to the RFC being approved for publication, the Designated Expert can undertake either action once it seems clear that an RFC will be published. The Designated expert will post a request to the HTTP WG mailing list (or a successor designated by the Area Director) for comment and review, including an Internet-Draft.¶

8. Security Considerations

This document should not affect the security of the Internet.¶

Acknowledgements

Many thanks to Rich Salz and Mike Bishop at Akamai for their invaluable help.¶

Author's Address