RFC 9204: QPACK: Field Compression for HTTP/3 (original) (raw)

RFC 9204 QPACK June 2022
Krasic, et al. Standards Track [Page]

QPACK: Field Compression for HTTP/3

Abstract

This specification defines QPACK: a compression format for efficiently representing HTTP fields that is to be used in HTTP/3. This is a variation of HPACK compression that seeks to reduce head-of-line blocking.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc9204.

Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

▲

Table of Contents

1. Introduction

The QUIC transport protocol ([QUIC-TRANSPORT]) is designed to support HTTP semantics, and its design subsumes many of the features of HTTP/2 ([HTTP/2]). HTTP/2 uses HPACK ([RFC7541]) for compression of the header and trailer sections. If HPACK were used for HTTP/3 ([HTTP/3]), it would induce head-of-line blocking for field sections due to built-in assumptions of a total ordering across frames on all streams.

QPACK reuses core concepts from HPACK, but 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. The design goals are to closely approach the compression ratio of HPACK with substantially less head-of-line blocking under the same loss conditions.

1.1. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174]when, and only when, they appear in all capitals, as shown here.

The following terms are used in this document:

HTTP fields:

Metadata sent as part of an HTTP message. The term encompasses both header and trailer fields. Colloquially, the term "headers" has often been used to refer to HTTP header fields and trailer fields; this document uses "fields" for generality.

HTTP field line:

A name-value pair sent as part of an HTTP field section. See Sections 6.3 and 6.5 of [HTTP].

HTTP field value:

Data associated with a field name, composed from all field line values with that field name in that section, concatenated together with comma separators.

Field section:

An ordered collection of HTTP field lines associated with an HTTP message. A field section can contain multiple field lines with the same name. It can also contain duplicate field lines. An HTTP message can include both header and trailer sections.

Representation:

An instruction that represents a field line, possibly by reference to the dynamic and static tables.

Encoder:

An implementation that encodes field sections.

Decoder:

An implementation that decodes encoded field sections.

Absolute Index:

A unique index for each entry in the dynamic table.

Base:

A reference point for relative and post-Base indices. Representations that reference dynamic table entries are relative to a Base.

Insert Count:

The total number of entries inserted in the dynamic table.

Note that QPACK is a name, not an abbreviation.

1.2. Notational Conventions

Diagrams in this document use the format described in Section 3.1 of [RFC2360], with the following additional conventions:

x (A)

Indicates that x is A bits long.

x (A+)

Indicates that x uses the prefixed integer encoding defined inSection 4.1.1, beginning with an A-bit prefix.

x ...

Indicates that x is variable length and extends to the end of the region.

2. Compression Process Overview

Like HPACK, QPACK uses two tables for associating field lines ("headers") to indices. The static table (Section 3.1) is predefined and contains common header field lines (some of them with an empty value). The dynamic table (Section 3.2) is built up over the course of the connection and can be used by the encoder to index both header and trailer field lines in the encoded field sections.

QPACK defines unidirectional streams for sending instructions from encoder to decoder and vice versa.

2.1. Encoder

An encoder converts a header or trailer section into a series of representations by emitting either an indexed or a literal representation for each field line in the list; see Section 4.5. Indexed representations achieve high compression by replacing the literal name and possibly the value with an index to either the static or dynamic table. References to the static table and literal representations do not require any dynamic state and never risk head-of-line blocking. References to the dynamic table risk head-of-line blocking if the encoder has not received an acknowledgment indicating the entry is available at the decoder.

An encoder MAY insert any entry in the dynamic table it chooses; it is not limited to field lines it is compressing.

QPACK preserves the ordering of field lines within each field section. An encoder MUST emit field representations in the order they appear in the input field section.

QPACK is designed to place the burden of optional state tracking on the encoder, resulting in relatively simple decoders.

2.1.1. Limits on Dynamic Table Insertions

Inserting entries into the dynamic table might not be possible if the table contains entries that cannot be evicted.

A dynamic table entry cannot be evicted immediately after insertion, even if it has never been referenced. Once the insertion of a dynamic table entry has been acknowledged and there are no outstanding references to the entry in unacknowledged representations, the entry becomes evictable. Note that references on the encoder stream never preclude the eviction of an entry, because those references are guaranteed to be processed before the instruction evicting the entry.

If the dynamic table does not contain enough room for a new entry without evicting other entries, and the entries that would be evicted are not evictable, the encoder MUST NOT insert that entry into the dynamic table (including duplicates of existing entries). In order to avoid this, an encoder that uses the dynamic table has to keep track of each dynamic table entry referenced by each field section until those representations are acknowledged by the decoder; see Section 4.4.1.

2.1.1.1. Avoiding Prohibited Insertions

To ensure that the encoder is not prevented from adding new entries, the encoder can avoid referencing entries that are close to eviction. Rather than reference such an entry, the encoder can emit a Duplicate instruction (Section 4.3.4) and reference the duplicate instead.

Determining which entries are too close to eviction to reference is an encoder preference. One heuristic is to target a fixed amount of available space in the dynamic table: either unused space or space that can be reclaimed by evicting non-blocking entries. To achieve this, the encoder can maintain a draining index, which is the smallest absolute index (Section 3.2.4) in the dynamic table that it will emit a reference for. As new entries are inserted, the encoder increases the draining index to maintain the section of the table that it will not reference. If the encoder does not create new references to entries with an absolute index lower than the draining index, the number of unacknowledged references to those entries will eventually become zero, allowing them to be evicted.

         <-- Newer Entries          Older Entries -->
           (Larger Indices)       (Smaller Indices)

+--------+---------------------------------+----------+ | Unused | Referenceable | Draining | | Space | Entries | Entries | +--------+---------------------------------+----------+ ^ ^ ^ | | | Insertion Point Draining Index Dropping Point

Figure 1:Draining Dynamic Table Entries

2.1.2. Blocked Streams

Because QUIC does not guarantee order between data on different streams, a decoder might encounter a representation that references a dynamic table entry that it has not yet received.

Each encoded field section contains a Required Insert Count (Section 4.5.1), the lowest possible value for the Insert Count with which the field section can be decoded. For a field section encoded using references to the dynamic table, the Required Insert Count is one larger than the largest absolute index of all referenced dynamic table entries. For a field section encoded with no references to the dynamic table, the Required Insert Count is zero.

When the decoder receives an encoded field section with a Required Insert Count greater than its own Insert Count, the stream cannot be processed immediately and is considered "blocked"; see Section 2.2.1.

The decoder specifies an upper bound on the number of streams that can be blocked using the SETTINGS_QPACK_BLOCKED_STREAMS setting; see Section 5. An encoder MUST limit the number of streams that could become blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. If a decoder encounters more blocked streams than it promised to support, it MUST treat this as a connection error of type QPACK_DECOMPRESSION_FAILED.

Note that the decoder might not become blocked on every stream that risks becoming blocked.

An encoder can decide whether to risk having a stream become blocked. If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS, compression efficiency can often be improved by referencing dynamic table entries that are still in transit, but if there is loss or reordering, the stream can become blocked at the decoder. An encoder can avoid the risk of blocking by only referencing dynamic table entries that have been acknowledged, but this could mean using literals. Since literals make the encoded field section larger, this can result in the encoder becoming blocked on congestion or flow-control limits.

2.1.3. Avoiding Flow-Control Deadlocks

Writing instructions on streams that are limited by flow control can produce deadlocks.

A decoder might stop issuing flow-control credit on the stream that carries an encoded field section until the necessary updates are received on the encoder stream. If the granting of flow-control credit on the encoder stream (or the connection as a whole) depends on the consumption and release of data on the stream carrying the encoded field section, a deadlock might result.

More generally, a stream containing a large instruction can become deadlocked if the decoder withholds flow-control credit until the instruction is completely received.

To avoid these deadlocks, an encoder SHOULD NOT write an instruction unless sufficient stream and connection flow-control credit is available for the entire instruction.

2.1.4. Known Received Count

The Known Received Count is the total number of dynamic table insertions and duplications acknowledged by the decoder. The encoder tracks the Known Received Count in order to identify which dynamic table entries can be referenced without potentially blocking a stream. The decoder tracks the Known Received Count in order to be able to send Insert Count Increment instructions.

A Section Acknowledgment instruction (Section 4.4.1) implies that the decoder has received all dynamic table state necessary to decode the field section. If the Required Insert Count of the acknowledged field section is greater than the current Known Received Count, the Known Received Count is updated to that Required Insert Count value.

An Insert Count Increment instruction (Section 4.4.3) increases the Known Received Count by its Increment parameter. See Section 2.2.2.3 for guidance.

2.2. Decoder

As in HPACK, the decoder processes a series of representations and emits the corresponding field sections. It also processes instructions received on the encoder stream that modify the dynamic table. Note that encoded field sections and encoder stream instructions arrive on separate streams. This is unlike HPACK, where encoded field sections (header blocks) can contain instructions that modify the dynamic table, and there is no dedicated stream of HPACK instructions.

The decoder MUST emit field lines in the order their representations appear in the encoded field section.

2.2.1. Blocked Decoding

Upon receipt of an encoded field section, the decoder examines the Required Insert Count. When the Required Insert Count is less than or equal to the decoder's Insert Count, the field section can be processed immediately. Otherwise, the stream on which the field section was received becomes blocked.

While blocked, encoded field section data SHOULD remain in the blocked stream's flow-control window. This data is unusable until the stream becomes unblocked, and releasing the flow control prematurely makes the decoder vulnerable to memory exhaustion attacks. A stream becomes unblocked when the Insert Count becomes greater than or equal to the Required Insert Count for all encoded field sections the decoder has started reading from the stream.

When processing encoded field sections, the decoder expects the Required Insert Count to equal the lowest possible value for the Insert Count with which the field section can be decoded, as prescribed in Section 2.1.2. If it encounters a Required Insert Count smaller than expected, it MUST treat this as a connection error of type QPACK_DECOMPRESSION_FAILED; seeSection 2.2.3. If it encounters a Required Insert Count larger than expected, it MAY treat this as a connection error of type QPACK_DECOMPRESSION_FAILED.

2.2.2. State Synchronization

The decoder signals the following events by emitting decoder instructions (Section 4.4) on the decoder stream.

2.2.2.1. Completed Processing of a Field Section

After the decoder finishes decoding a field section encoded using representations containing dynamic table references, it MUST emit a Section Acknowledgment instruction (Section 4.4.1). A stream may carry multiple field sections in the case of intermediate responses, trailers, and pushed requests. The encoder interprets each Section Acknowledgment instruction as acknowledging the earliest unacknowledged field section containing dynamic table references sent on the given stream.

2.2.2.2. Abandonment of a Stream

When an endpoint receives a stream reset before the end of a stream or before all encoded field sections are processed on that stream, or when it abandons reading of a stream, it generates a Stream Cancellation instruction; seeSection 4.4.2. This signals to the encoder that all references to the dynamic table on that stream are no longer outstanding. A decoder with a maximum dynamic table capacity (Section 3.2.3) equal to zero MAY omit sending Stream Cancellations, because the encoder cannot have any dynamic table references. An encoder cannot infer from this instruction that any updates to the dynamic table have been received.

The Section Acknowledgment and Stream Cancellation instructions permit the encoder to remove references to entries in the dynamic table. When an entry with an absolute index lower than the Known Received Count has zero references, then it is considered evictable; see Section 2.1.1.

2.2.2.3. New Table Entries

After receiving new table entries on the encoder stream, the decoder chooses when to emit Insert Count Increment instructions; seeSection 4.4.3. Emitting this instruction after adding each new dynamic table entry will provide the timeliest feedback to the encoder, but could be redundant with other decoder feedback. By delaying an Insert Count Increment instruction, the decoder might be able to coalesce multiple Insert Count Increment instructions or replace them entirely with Section Acknowledgments; see Section 4.4.1. However, delaying too long may lead to compression inefficiencies if the encoder waits for an entry to be acknowledged before using it.

2.2.3. Invalid References

If the decoder encounters a reference in a field line representation to a dynamic table entry that has already been evicted or that has an absolute index greater than or equal to the declared Required Insert Count (Section 4.5.1), it MUST treat this as a connection error of type QPACK_DECOMPRESSION_FAILED.

If the decoder encounters a reference in an encoder instruction to a dynamic table entry that has already been evicted, it MUST treat this as a connection error of type QPACK_ENCODER_STREAM_ERROR.

3. Reference Tables

Unlike in HPACK, entries in the QPACK static and dynamic tables are addressed separately. The following sections describe how entries in each table are addressed.

4. Wire Format

4.1. Primitives

4.1.1. Prefixed Integers

The prefixed integer from Section 5.1 of [RFC7541] is used heavily throughout this document. The format from [RFC7541] is used unmodified. Note, however, that QPACK uses some prefix sizes not actually used in HPACK.

QPACK implementations MUST be able to decode integers up to and including 62 bits long.

4.1.2. String Literals

The string literal defined by Section 5.2 of [RFC7541] is also used throughout. This string format includes optional Huffman encoding.

HPACK defines string literals to begin on a byte boundary. They begin with a single bit flag, denoted as 'H' in this document (indicating whether the string is Huffman encoded), followed by the string length encoded as a 7-bit prefix integer, and finally the indicated number of bytes of data. When Huffman encoding is enabled, the Huffman table from Appendix B of [RFC7541] is used without modification and the indicated length is the size of the string after encoding.

This document expands the definition of string literals by permitting them to begin other than on a byte boundary. An "N-bit prefix string literal" begins mid-byte, with the first (8-N) bits allocated to a previous field. The string uses one bit for the Huffman flag, followed by the length of the encoded string as a (N-1)-bit prefix integer. The prefix size, N, can have a value between 2 and 8, inclusive. The remainder of the string literal is unmodified.

A string literal without a prefix length noted is an 8-bit prefix string literal and follows the definitions in [RFC7541] without modification.

4.2. Encoder and Decoder Streams

QPACK defines two unidirectional stream types:

HTTP/3 endpoints contain a QPACK encoder and decoder. Each endpoint MUSTinitiate, at most, one encoder stream and, at most, one decoder stream. Receipt of a second instance of either stream type MUST be treated as a connection error of type H3_STREAM_CREATION_ERROR.

The sender MUST NOT close either of these streams, and the receiver MUST NOTrequest that the sender close either of these streams. Closure of either unidirectional stream type MUST be treated as a connection error of type H3_CLOSED_CRITICAL_STREAM.

An endpoint MAY avoid creating an encoder stream if it will not be used (for example, if its encoder does not wish to use the dynamic table or if the maximum size of the dynamic table permitted by the peer is zero).

An endpoint MAY avoid creating a decoder stream if its decoder sets the maximum capacity of the dynamic table to zero.

An endpoint MUST allow its peer to create an encoder stream and a decoder stream even if the connection's settings prevent their use.

4.3. Encoder Instructions

An encoder sends encoder instructions on the encoder stream to set the capacity of the dynamic table and add dynamic table entries. Instructions adding table entries can use existing entries to avoid transmitting redundant information. The name can be transmitted as a reference to an existing entry in the static or the dynamic table or as a string literal. For entries that already exist in the dynamic table, the full entry can also be used by reference, creating a duplicate entry.

4.3.1. Set Dynamic Table Capacity

An encoder informs the decoder of a change to the dynamic table capacity using an instruction that starts with the '001' 3-bit pattern. This is followed by the new dynamic table capacity represented as an integer with a 5-bit prefix; see Section 4.1.1.

0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | Capacity (5+) | +---+---+---+-------------------+

Figure 5:Set Dynamic Table Capacity

The new capacity MUST be lower than or equal to the limit described inSection 3.2.3. In HTTP/3, this limit is the value of the SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter (Section 5) received from the decoder. The decoder MUST treat a new dynamic table capacity value that exceeds this limit as a connection error of type QPACK_ENCODER_STREAM_ERROR.

Reducing the dynamic table capacity can cause entries to be evicted; seeSection 3.2.2. This MUST NOT cause the eviction of entries that are not evictable; see Section 2.1.1. Changing the capacity of the dynamic table is not acknowledged as this instruction does not insert an entry.

4.3.2. Insert with Name Reference

An encoder adds an entry to the dynamic table where the field name matches the field name of an entry stored in the static or the dynamic table using an instruction that starts with the '1' 1-bit pattern. The second ('T') bit indicates whether the reference is to the static or dynamic table. The 6-bit prefix integer (Section 4.1.1) that follows is used to locate the table entry for the field name. When T=1, the number represents the static table index; when T=0, the number is the relative index of the entry in the dynamic table.

The field name reference is followed by the field value represented as a string literal; see Section 4.1.2.

 0   1   2   3   4   5   6   7

+---+---+---+---+---+---+---+---+ | 1 | T | Name Index (6+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+

Figure 6:Insert Field Line -- Indexed Name

4.3.3. Insert with Literal Name

An encoder adds an entry to the dynamic table where both the field name and the field value are represented as string literals using an instruction that starts with the '01' 2-bit pattern.

This is followed by the name represented as a 6-bit prefix string literal and the value represented as an 8-bit prefix string literal; seeSection 4.1.2.

 0   1   2   3   4   5   6   7

+---+---+---+---+---+---+---+---+ | 0 | 1 | H | Name Length (5+) | +---+---+---+-------------------+ | Name String (Length bytes) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+

Figure 7:Insert Field Line -- New Name

4.3.4. Duplicate

An encoder duplicates an existing entry in the dynamic table using an instruction that starts with the '000' 3-bit pattern. This is followed by the relative index of the existing entry represented as an integer with a 5-bit prefix; see Section 4.1.1.

 0   1   2   3   4   5   6   7

+---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | Index (5+) | +---+---+---+-------------------+

Figure 8:Duplicate

The existing entry is reinserted into the dynamic table without resending either the name or the value. This is useful to avoid adding a reference to an older entry, which might block inserting new entries.

4.4. Decoder Instructions

A decoder sends decoder instructions on the decoder stream to inform the encoder about the processing of field sections and table updates to ensure consistency of the dynamic table.

4.4.2. Stream Cancellation

When a stream is reset or reading is abandoned, the decoder emits a Stream Cancellation instruction. The instruction starts with the '01' 2-bit pattern, followed by the stream ID of the affected stream encoded as a 6-bit prefix integer.

This instruction is used as described in Section 2.2.2.

0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | Stream ID (6+) | +---+---+-----------------------+

Figure 10:Stream Cancellation

4.4.3. Insert Count Increment

The Insert Count Increment instruction starts with the '00' 2-bit pattern, followed by the Increment encoded as a 6-bit prefix integer. This instruction increases the Known Received Count (Section 2.1.4) by the value of the Increment parameter. The decoder should send an Increment value that increases the Known Received Count to the total number of dynamic table insertions and duplications processed so far.

0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | Increment (6+) | +---+---+-----------------------+

Figure 11:Insert Count Increment

An encoder that receives an Increment field equal to zero, or one that increases the Known Received Count beyond what the encoder has sent, MUST treat this as a connection error of type QPACK_DECODER_STREAM_ERROR.

4.5. Field Line Representations

An encoded field section consists of a prefix and a possibly empty sequence of representations defined in this section. Each representation corresponds to a single field line. These representations reference the static table or the dynamic table in a particular state, but they do not modify that state.

Encoded field sections are carried in frames on streams defined by the enclosing protocol.

4.5.2. Indexed Field Line

An indexed field line representation identifies an entry in the static table or an entry in the dynamic table with an absolute index less than the value of the Base.

0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | T | Index (6+) | +---+---+-----------------------+

Figure 13:Indexed Field Line

This representation starts with the '1' 1-bit pattern, followed by the 'T' bit, indicating whether the reference is into the static or dynamic table. The 6-bit prefix integer (Section 4.1.1) that follows is used to locate the table entry for the field line. When T=1, the number represents the static table index; when T=0, the number is the relative index of the entry in the dynamic table.

4.5.3. Indexed Field Line with Post-Base Index

An indexed field line with post-Base index representation identifies an entry in the dynamic table with an absolute index greater than or equal to the value of the Base.

0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 1 | Index (4+) | +---+---+---+---+---------------+

Figure 14:Indexed Field Line with Post-Base Index

This representation starts with the '0001' 4-bit pattern. This is followed by the post-Base index (Section 3.2.6) of the matching field line, represented as an integer with a 4-bit prefix; see Section 4.1.1.

4.5.4. Literal Field Line with Name Reference

A literal field line with name reference representation encodes a field line where the field name matches the field name of an entry in the static table or the field name of an entry in the dynamic table with an absolute index less than the value of the Base.

 0   1   2   3   4   5   6   7

+---+---+---+---+---+---+---+---+ | 0 | 1 | N | T |Name Index (4+)| +---+---+---+---+---------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+

Figure 15:Literal Field Line with Name Reference

This representation starts with the '01' 2-bit pattern. The following bit, 'N', indicates whether an intermediary is permitted to add this field line to the dynamic table on subsequent hops. When the 'N' bit is set, the encoded field line MUST always be encoded with a literal representation. In particular, when a peer sends a field line that it received represented as a literal field line with the 'N' bit set, it MUST use a literal representation to forward this field line. This bit is intended for protecting field values that are not to be put at risk by compressing them; see Section 7.1 for more details.

The fourth ('T') bit indicates whether the reference is to the static or dynamic table. The 4-bit prefix integer (Section 4.1.1) that follows is used to locate the table entry for the field name. When T=1, the number represents the static table index; when T=0, the number is the relative index of the entry in the dynamic table.

Only the field name is taken from the dynamic table entry; the field value is encoded as an 8-bit prefix string literal; see Section 4.1.2.

4.5.5. Literal Field Line with Post-Base Name Reference

A literal field line with post-Base name reference representation encodes a field line where the field name matches the field name of a dynamic table entry with an absolute index greater than or equal to the value of the Base.

 0   1   2   3   4   5   6   7

+---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | N |NameIdx(3+)| +---+---+---+---+---+-----------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+

Figure 16:Literal Field Line with Post-Base Name Reference

This representation starts with the '0000' 4-bit pattern. The fifth bit is the 'N' bit as described in Section 4.5.4. This is followed by a post-Base index of the dynamic table entry (Section 3.2.6) encoded as an integer with a 3-bit prefix; see Section 4.1.1.

Only the field name is taken from the dynamic table entry; the field value is encoded as an 8-bit prefix string literal; see Section 4.1.2.

4.5.6. Literal Field Line with Literal Name

The literal field line with literal name representation encodes a field name and a field value as string literals.

 0   1   2   3   4   5   6   7

+---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | N | H |NameLen(3+)| +---+---+---+---+---+-----------+ | Name String (Length bytes) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+

Figure 17:Literal Field Line with Literal Name

This representation starts with the '001' 3-bit pattern. The fourth bit is the 'N' bit as described in Section 4.5.4. The name follows, represented as a 4-bit prefix string literal, then the value, represented as an 8-bit prefix string literal; see Section 4.1.2.

5. Configuration

QPACK defines two settings for the HTTP/3 SETTINGS frame:

SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x01):

The default value is zero. See Section 3.2 for usage. This is the equivalent of the SETTINGS_HEADER_TABLE_SIZE from HTTP/2.

SETTINGS_QPACK_BLOCKED_STREAMS (0x07):

The default value is zero. See Section 2.1.2.

6. Error Handling

The following error codes are defined for HTTP/3 to indicate failures of QPACK that prevent the stream or connection from continuing:

QPACK_DECOMPRESSION_FAILED (0x0200):

The decoder failed to interpret an encoded field section and is not able to continue decoding that field section.

QPACK_ENCODER_STREAM_ERROR (0x0201):

The decoder failed to interpret an encoder instruction received on the encoder stream.

QPACK_DECODER_STREAM_ERROR (0x0202):

The encoder failed to interpret a decoder instruction received on the decoder stream.

7. Security Considerations

This section describes potential areas of security concern with QPACK:

7.1. Probing Dynamic Table State

QPACK reduces the encoded size of field sections by exploiting the redundancy inherent in protocols like HTTP. The ultimate goal of this is to reduce the amount of data that is required to send HTTP requests or responses.

The compression context used to encode header and trailer fields can be probed by an attacker who can both define fields to be encoded and transmitted and observe the length of those fields once they are encoded. When an attacker can do both, they can adaptively modify requests in order to confirm guesses about the dynamic table state. If a guess is compressed into a shorter length, the attacker can observe the encoded length and infer that the guess was correct.

This is possible even over the Transport Layer Security Protocol ([TLS]) and the QUIC Transport Protocol ([QUIC-TRANSPORT]), because while TLS and QUIC provide confidentiality protection for content, they only provide a limited amount of protection for the length of that content.

Attacks like CRIME ([CRIME]) demonstrated the existence of these general attacker capabilities. The specific attack exploited the fact that DEFLATE ([RFC1951]) removes redundancy based on prefix matching. This permitted the attacker to confirm guesses a character at a time, reducing an exponential-time attack into a linear-time attack.

7.1.1. Applicability to QPACK and HTTP

QPACK mitigates, but does not completely prevent, attacks modeled on CRIME ([CRIME]) by forcing a guess to match an entire field line rather than individual characters. An attacker can only learn whether a guess is correct or not, so the attacker is reduced to a brute-force guess for the field values associated with a given field name.

Therefore, the viability of recovering specific field values depends on the entropy of values. As a result, values with high entropy are unlikely to be recovered successfully. However, values with low entropy remain vulnerable.

Attacks of this nature are possible any time that two mutually distrustful entities control requests or responses that are placed onto a single HTTP/3 connection. If the shared QPACK compressor permits one entity to add entries to the dynamic table, and the other to refer to those entries while encoding chosen field lines, then the attacker (the second entity) can learn the state of the table by observing the length of the encoded output.

For example, requests or responses from mutually distrustful entities can occur when an intermediary either:

Web browsers also need to assume that requests made on the same connection by different web origins ([RFC6454]) are made by mutually distrustful entities. Other scenarios involving mutually distrustful entities are also possible.

7.1.2. Mitigation

Users of HTTP that require confidentiality for header or trailer fields can use values with entropy sufficient to make guessing infeasible. However, this is impractical as a general solution because it forces all users of HTTP to take steps to mitigate attacks. It would impose new constraints on how HTTP is used.

Rather than impose constraints on users of HTTP, an implementation of QPACK can instead constrain how compression is applied in order to limit the potential for dynamic table probing.

An ideal solution segregates access to the dynamic table based on the entity that is constructing the message. Field values that are added to the table are attributed to an entity, and only the entity that created a particular value can extract that value.

To improve compression performance of this option, certain entries might be tagged as being public. For example, a web browser might make the values of the Accept-Encoding header field available in all requests.

An encoder without good knowledge of the provenance of field values might instead introduce a penalty for many field lines with the same field name and different values. This penalty could cause a large number of attempts to guess a field value to result in the field not being compared to the dynamic table entries in future messages, effectively preventing further guesses.

This response might be made inversely proportional to the length of the field value. Disabling access to the dynamic table for a given field name might occur for shorter values more quickly or with higher probability than for longer values.

This mitigation is most effective between two endpoints. If messages are re-encoded by an intermediary without knowledge of which entity constructed a given message, the intermediary could inadvertently merge compression contexts that the original encoder had specifically kept separate.

7.1.3. Never-Indexed Literals

Implementations can also choose to protect sensitive fields by not compressing them and instead encoding their value as literals.

Refusing to insert a field line into the dynamic table is only effective if doing so is avoided on all hops. The never-indexed literal bit (seeSection 4.5.4) can be used to signal to intermediaries that a particular value was intentionally sent as a literal.

An intermediary MUST NOT re-encode a value that uses a literal representation with the 'N' bit set with another representation that would index it. If QPACK is used for re-encoding, a literal representation with the 'N' bit set MUST be used. If HPACK is used for re-encoding, the never-indexed literal representation (see Section 6.2.3 of [RFC7541]) MUST be used.

The choice to mark that a field value should never be indexed depends on several factors. Since QPACK does not protect against guessing an entire field value, short or low-entropy values are more readily recovered by an adversary. Therefore, an encoder might choose not to index values with low entropy.

An encoder might also choose not to index values for fields that are considered to be highly valuable or sensitive to recovery, such as the Cookie or Authorization header fields.

On the contrary, an encoder might prefer indexing values for fields that have little or no value if they were exposed. For instance, a User-Agent header field does not commonly vary between requests and is sent to any server. In that case, confirmation that a particular User-Agent value has been used provides little value.

Note that these criteria for deciding to use a never-indexed literal representation will evolve over time as new attacks are discovered.

7.2. Static Huffman Encoding

There is no currently known attack against a static Huffman encoding. A study has shown that using a static Huffman encoding table created an information leakage; however, this same study concluded that an attacker could not take advantage of this information leakage to recover any meaningful amount of information (see [PETAL]).

7.3. Memory Consumption

An attacker can try to cause an endpoint to exhaust its memory. QPACK is designed to limit both the peak and stable amounts of memory allocated by an endpoint.

QPACK uses the definition of the maximum size of the dynamic table and the maximum number of blocking streams to limit the amount of memory the encoder can cause the decoder to consume. In HTTP/3, these values are controlled by the decoder through the settings parameters SETTINGS_QPACK_MAX_TABLE_CAPACITY and SETTINGS_QPACK_BLOCKED_STREAMS, respectively (seeSection 3.2.3 and Section 2.1.2). The limit on the size of the dynamic table takes into account the size of the data stored in the dynamic table, plus a small allowance for overhead. The limit on the number of blocked streams is only a proxy for the maximum amount of memory required by the decoder. The actual maximum amount of memory will depend on how much memory the decoder uses to track each blocked stream.

A decoder can limit the amount of state memory used for the dynamic table by setting an appropriate value for the maximum size of the dynamic table. In HTTP/3, this is realized by setting an appropriate value for the SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter. An encoder can limit the amount of state memory it uses by choosing a smaller dynamic table size than the decoder allows and signaling this to the decoder (see Section 4.3.1).

A decoder can limit the amount of state memory used for blocked streams by setting an appropriate value for the maximum number of blocked streams. In HTTP/3, this is realized by setting an appropriate value for the SETTINGS_QPACK_BLOCKED_STREAMS parameter. Streams that risk becoming blocked consume no additional state memory on the encoder.

An encoder allocates memory to track all dynamic table references in unacknowledged field sections. An implementation can directly limit the amount of state memory by only using as many references to the dynamic table as it wishes to track; no signaling to the decoder is required. However, limiting references to the dynamic table will reduce compression effectiveness.

The amount of temporary memory consumed by an encoder or decoder can be limited by processing field lines sequentially. A decoder implementation does not need to retain a complete list of field lines while decoding a field section. An encoder implementation does not need to retain a complete list of field lines while encoding a field section if it is using a single-pass algorithm. Note that it might be necessary for an application to retain a complete list of field lines for other reasons; even if QPACK does not force this to occur, application constraints might make this necessary.

While the negotiated limit on the dynamic table size accounts for much of the memory that can be consumed by a QPACK implementation, data that cannot be immediately sent due to flow control is not affected by this limit. Implementations should limit the size of unsent data, especially on the decoder stream where flexibility to choose what to send is limited. Possible responses to an excess of unsent data might include limiting the ability of the peer to open new streams, reading only from the encoder stream, or closing the connection.

7.4. Implementation Limits

An implementation of QPACK needs to ensure that large values for integers, long encoding for integers, or long string literals do not create security weaknesses.

An implementation has to set a limit for the values it accepts for integers, as well as for the encoded length; see Section 4.1.1. In the same way, it has to set a limit to the length it accepts for string literals; seeSection 4.1.2. These limits SHOULD be large enough to process the largest individual field the HTTP implementation can be configured to accept.

If an implementation encounters a value larger than it is able to decode, thisMUST be treated as a stream error of type QPACK_DECOMPRESSION_FAILED if on a request stream or a connection error of the appropriate type if on the encoder or decoder stream.

8. IANA Considerations

This document makes multiple registrations in the registries defined by[HTTP/3]. The allocations created by this document are all assigned permanent status and list a change controller of the IETF and a contact of the HTTP working group (ietf-http-wg@w3.org).

8.1. Settings Registration

This document specifies two settings. The entries in the following table are registered in the "HTTP/3 Settings" registry established in [HTTP/3].

Table 1:Additions to the HTTP/3 Settings Registry

Setting Name Code Specification Default
QPACK_MAX_TABLE_CAPACITY 0x01 Section 5 0
QPACK_BLOCKED_STREAMS 0x07 Section 5 0

For formatting reasons, the setting names here are abbreviated by removing the 'SETTINGS_' prefix.

8.2. Stream Type Registration

This document specifies two stream types. The entries in the following table are registered in the "HTTP/3 Stream Types" registry established in [HTTP/3].

Table 2:Additions to the HTTP/3 Stream Types Registry

Stream Type Code Specification Sender
QPACK Encoder Stream 0x02 Section 4.2 Both
QPACK Decoder Stream 0x03 Section 4.2 Both

8.3. Error Code Registration

This document specifies three error codes. The entries in the following table are registered in the "HTTP/3 Error Codes" registry established in [HTTP/3].

Table 3:Additions to the HTTP/3 Error Codes Registry

Name Code Description Specification
QPACK_DECOMPRESSION_FAILED 0x0200 Decoding of a field section failed Section 6
QPACK_ENCODER_STREAM_ERROR 0x0201 Error on the encoder stream Section 6
QPACK_DECODER_STREAM_ERROR 0x0202 Error on the decoder stream Section 6

9. References

9.1. Normative References

[HTTP]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/info/rfc9110>.

[HTTP/3]

Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114, June 2022, <https://www.rfc-editor.org/info/rfc9114>.

[QUIC-TRANSPORT]

Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/info/rfc9000>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC2360]

Scott, G., "Guide for Internet Standards Writers", BCP 22, RFC 2360, DOI 10.17487/RFC2360, June 1998, <https://www.rfc-editor.org/info/rfc2360>.

[RFC7541]

Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, <https://www.rfc-editor.org/info/rfc7541>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[CRIME]

Wikipedia, "CRIME", May 2015, <http://en.wikipedia.org/w/index.php?title=CRIME&oldid=660948120>.

[HTTP/2]

Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, June 2022, <https://www.rfc-editor.org/info/rfc9113>.

[PETAL]

Tan, J. and J. Nahata, "PETAL: Preset Encoding Table Information Leakage", April 2013, <http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-13-106.pdf>.

[RFC1951]

Deutsch, P., "DEFLATE Compressed Data Format Specification version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996, <https://www.rfc-editor.org/info/rfc1951>.

[RFC6454]

Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011, <https://www.rfc-editor.org/info/rfc6454>.

[TLS]

Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.

Appendix A. Static Table

This 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.

Table 4:Static Table

Index Name Value
0 :authority
1 :path /
2 age 0
3 content-disposition
4 content-length 0
5 cookie
6 date
7 etag
8 if-modified-since
9 if-none-match
10 last-modified
11 link
12 location
13 referer
14 set-cookie
15 :method CONNECT
16 :method DELETE
17 :method GET
18 :method HEAD
19 :method OPTIONS
20 :method POST
21 :method PUT
22 :scheme http
23 :scheme https
24 :status 103
25 :status 200
26 :status 304
27 :status 404
28 :status 503
29 accept */*
30 accept application/dns-message
31 accept-encoding gzip, deflate, br
32 accept-ranges bytes
33 access-control-allow-headers cache-control
34 access-control-allow-headers content-type
35 access-control-allow-origin *
36 cache-control max-age=0
37 cache-control max-age=2592000
38 cache-control max-age=604800
39 cache-control no-cache
40 cache-control no-store
41 cache-control public, max-age=31536000
42 content-encoding br
43 content-encoding gzip
44 content-type application/dns-message
45 content-type application/javascript
46 content-type application/json
47 content-type application/x-www-form-urlencoded
48 content-type image/gif
49 content-type image/jpeg
50 content-type image/png
51 content-type text/css
52 content-type text/html; charset=utf-8
53 content-type text/plain
54 content-type text/plain;charset=utf-8
55 range bytes=0-
56 strict-transport-security max-age=31536000
57 strict-transport-security max-age=31536000; includesubdomains
58 strict-transport-security max-age=31536000; includesubdomains; preload
59 vary accept-encoding
60 vary origin
61 x-content-type-options nosniff
62 x-xss-protection 1; mode=block
63 :status 100
64 :status 204
65 :status 206
66 :status 302
67 :status 400
68 :status 403
69 :status 421
70 :status 425
71 :status 500
72 accept-language
73 access-control-allow-credentials FALSE
74 access-control-allow-credentials TRUE
75 access-control-allow-headers *
76 access-control-allow-methods get
77 access-control-allow-methods get, post, options
78 access-control-allow-methods options
79 access-control-expose-headers content-length
80 access-control-request-headers content-type
81 access-control-request-method get
82 access-control-request-method post
83 alt-svc clear
84 authorization
85 content-security-policy script-src 'none'; object-src 'none'; base-uri 'none'
86 early-data 1
87 expect-ct
88 forwarded
89 if-range
90 origin
91 purpose prefetch
92 server
93 timing-allow-origin *
94 upgrade-insecure-requests 1
95 user-agent
96 x-forwarded-for
97 x-frame-options deny
98 x-frame-options sameorigin

Any line breaks that appear within field names or values are due to formatting.

Appendix B. Encoding and Decoding Examples

The following examples represent a series of exchanges between an encoder and a decoder. The exchanges are designed to exercise most QPACK instructions and highlight potentially common patterns and their impact on dynamic table state. The encoder sends three encoded field sections containing one field line each, as well as two speculative inserts that are not referenced.

The state of the encoder's dynamic table is shown, along with its current size. Each entry is shown with the Absolute Index of the entry (Abs), the current number of outstanding encoded field sections with references to that entry (Ref), along with the name and value. Entries above the 'acknowledged' line have been acknowledged by the decoder.

B.1. Literal Field Line with Name Reference

The encoder sends an encoded field section containing a literal representation of a field with a static name reference.

Data | Interpretation | Encoder's Dynamic Table

Stream: 0 0000 | Required Insert Count = 0, Base = 0 510b 2f69 6e64 6578 | Literal Field Line with Name Reference 2e68 746d 6c | Static Table, Index=1 | (:path=/index.html)

                          Abs Ref Name        Value
                          ^-- acknowledged --^
                          Size=0

B.2. Dynamic Table

The encoder sets the dynamic table capacity, inserts a header with a dynamic name reference, then sends a potentially blocking, encoded field section referencing this new entry. The decoder acknowledges processing the encoded field section, which implicitly acknowledges all dynamic table insertions up to the Required Insert Count.

Stream: Encoder 3fbd01 | Set Dynamic Table Capacity=220 c00f 7777 772e 6578 | Insert With Name Reference 616d 706c 652e 636f | Static Table, Index=0 6d | (:authority=www.example.com) c10c 2f73 616d 706c | Insert With Name Reference 652f 7061 7468 | Static Table, Index=1 | (:path=/sample/path)

                          Abs Ref Name        Value
                          ^-- acknowledged --^
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                          Size=106

Stream: 4 0381 | Required Insert Count = 2, Base = 0 10 | Indexed Field Line With Post-Base Index | Absolute Index = Base(0) + Index(0) = 0 | (:authority=www.example.com) 11 | Indexed Field Line With Post-Base Index | Absolute Index = Base(0) + Index(1) = 1 | (:path=/sample/path)

                          Abs Ref Name        Value
                          ^-- acknowledged --^
                           0   1  :authority  www.example.com
                           1   1  :path       /sample/path
                          Size=106

Stream: Decoder 84 | Section Acknowledgment (stream=4)

                          Abs Ref Name        Value
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                          ^-- acknowledged --^
                          Size=106

B.3. Speculative Insert

The encoder inserts a header into the dynamic table with a literal name. The decoder acknowledges receipt of the entry. The encoder does not send any encoded field sections.

Stream: Encoder 4a63 7573 746f 6d2d | Insert With Literal Name 6b65 790c 6375 7374 | (custom-key=custom-value) 6f6d 2d76 616c 7565 |

                          Abs Ref Name        Value
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                          ^-- acknowledged --^
                           2   0  custom-key  custom-value
                          Size=160

Stream: Decoder 01 | Insert Count Increment (1)

                          Abs Ref Name        Value
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                           2   0  custom-key  custom-value
                          ^-- acknowledged --^
                          Size=160

B.4. Duplicate Instruction, Stream Cancellation

The encoder duplicates an existing entry in the dynamic table, then sends an encoded field section referencing the dynamic table entries including the duplicated entry. The packet containing the encoder stream data is delayed. Before the packet arrives, the decoder cancels the stream and notifies the encoder that the encoded field section was not processed.

Stream: Encoder 02 | Duplicate (Relative Index = 2) | Absolute Index = | Insert Count(3) - Index(2) - 1 = 0

                          Abs Ref Name        Value
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                           2   0  custom-key  custom-value
                          ^-- acknowledged --^
                           3   0  :authority  www.example.com
                          Size=217

Stream: 8 0500 | Required Insert Count = 4, Base = 4 80 | Indexed Field Line, Dynamic Table | Absolute Index = Base(4) - Index(0) - 1 = 3 | (:authority=www.example.com) c1 | Indexed Field Line, Static Table Index = 1 | (:path=/) 81 | Indexed Field Line, Dynamic Table | Absolute Index = Base(4) - Index(1) - 1 = 2 | (custom-key=custom-value)

                          Abs Ref Name        Value
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                           2   1  custom-key  custom-value
                          ^-- acknowledged --^
                           3   1  :authority  www.example.com
                          Size=217

Stream: Decoder 48 | Stream Cancellation (Stream=8)

                          Abs Ref Name        Value
                           0   0  :authority  www.example.com
                           1   0  :path       /sample/path
                           2   0  custom-key  custom-value
                          ^-- acknowledged --^
                           3   0  :authority  www.example.com
                          Size=217

B.5. Dynamic Table Insert, Eviction

The encoder inserts another header into the dynamic table, which evicts the oldest entry. The encoder does not send any encoded field sections.

Stream: Encoder 810d 6375 7374 6f6d | Insert With Name Reference 2d76 616c 7565 32 | Dynamic Table, Relative Index = 1 | Absolute Index = | Insert Count(4) - Index(1) - 1 = 2 | (custom-key=custom-value2)

                          Abs Ref Name        Value
                           1   0  :path       /sample/path
                           2   0  custom-key  custom-value
                          ^-- acknowledged --^
                           3   0  :authority  www.example.com
                           4   0  custom-key  custom-value2
                          Size=215

Appendix C. Sample Single-Pass Encoding Algorithm

Pseudocode for single-pass encoding, excluding handling of duplicates, non-blocking mode, available encoder stream flow control and reference tracking.

Helper functions:

====

Encode an integer with the specified prefix and length

encodeInteger(buffer, prefix, value, prefixLength)

Encode a dynamic table insert instruction with optional static

or dynamic name index (but not both)

encodeInsert(buffer, staticNameIndex, dynamicNameIndex, fieldLine)

Encode a static index reference

encodeStaticIndexReference(buffer, staticIndex)

Encode a dynamic index reference relative to Base

encodeDynamicIndexReference(buffer, dynamicIndex, base)

Encode a literal with an optional static name index

encodeLiteral(buffer, staticNameIndex, fieldLine)

Encode a literal with a dynamic name index relative to Base

encodeDynamicLiteral(buffer, dynamicNameIndex, base, fieldLine)

Encoding Algorithm

====

base = dynamicTable.getInsertCount() requiredInsertCount = 0 for line in fieldLines: staticIndex = staticTable.findIndex(line) if staticIndex is not None: encodeStaticIndexReference(streamBuffer, staticIndex) continue

dynamicIndex = dynamicTable.findIndex(line) if dynamicIndex is None: # No matching entry. Either insert+index or encode literal staticNameIndex = staticTable.findName(line.name) if staticNameIndex is None: dynamicNameIndex = dynamicTable.findName(line.name)

if shouldIndex(line) and dynamicTable.canIndex(line):
  encodeInsert(encoderBuffer, staticNameIndex,
               dynamicNameIndex, line)
  dynamicIndex = dynamicTable.add(line)

if dynamicIndex is None: # Could not index it, literal if dynamicNameIndex is not None: # Encode literal with dynamic name, possibly above Base encodeDynamicLiteral(streamBuffer, dynamicNameIndex, base, line) requiredInsertCount = max(requiredInsertCount, dynamicNameIndex) else: # Encodes a literal with a static name or literal name encodeLiteral(streamBuffer, staticNameIndex, line) else: # Dynamic index reference assert(dynamicIndex is not None) requiredInsertCount = max(requiredInsertCount, dynamicIndex) # Encode dynamicIndex, possibly above Base encodeDynamicIndexReference(streamBuffer, dynamicIndex, base)

encode the prefix

if requiredInsertCount == 0: encodeInteger(prefixBuffer, 0x00, 0, 8) encodeInteger(prefixBuffer, 0x00, 0, 7) else: wireRIC = ( requiredInsertCount % (2 * getMaxEntries(maxTableCapacity)) ) + 1; encodeInteger(prefixBuffer, 0x00, wireRIC, 8) if base >= requiredInsertCount: encodeInteger(prefixBuffer, 0x00, base - requiredInsertCount, 7) else: encodeInteger(prefixBuffer, 0x80, requiredInsertCount - base - 1, 7)

return encoderBuffer, prefixBuffer + streamBuffer

Acknowledgments

The IETF QUIC Working Group received an enormous amount of support from many people.

The compression design team did substantial work exploring the problem space and influencing the initial draft version of this document. The contributions of design team members Roberto Peon, Martin Thomson, andDmitri Tikhonov are gratefully acknowledged.

The following people also provided substantial contributions to this document:

This document draws heavily on the text of [RFC7541]. The indirect input of those authors is also gratefully acknowledged.

Buck Krasic's contribution was supported by Google during his employment there.

A portion of Mike Bishop's contribution was supported by Microsoft during his employment there.

Authors' Addresses

Charles 'Buck' Krasic

Mike Bishop

Akamai Technologies

Alan Frindell (editor)

Facebook