QUIC Working GroupC. Krasic
Internet-DraftNetflix
Intended status: Standards TrackM. Bishop
Expires: June 8, 2019Akamai Technologies
A. Frindell, Editor
Facebook
December 5, 2018

QPACK: Header Compression for HTTP over QUIC

Abstract

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

Note to Readers

Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic.

Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-qpack.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

This Internet-Draft will expire on June 8, 2019.

Copyright Notice

Copyright © 2018 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 (http://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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.



1. Introduction

The QUIC transport protocol was designed from the outset to support HTTP semantics, and its design subsumes many of the features of HTTP/2. HTTP/2 uses HPACK ([RFC7541]) for header compression, but QUIC’s stream multiplexing comes into some conflict with HPACK. A key goal of the design of QUIC is to improve stream multiplexing relative to HTTP/2 by reducing head-of-line blocking. If HPACK were used for HTTP/3, it would induce head-of-line blocking due to built-in assumptions of a total ordering across frames on all streams.

QUIC is described in [QUIC-TRANSPORT]. The HTTP/3 mapping is described in [HTTP3]. For a full description of HTTP/2, see [RFC7540]. The description of HPACK is [RFC7541].

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.

Definitions of terms that are used in this document:

Header field:
A name-value pair sent as part of an HTTP message.
Header list:
An ordered collection of header fields associated with an HTTP message. A header list can contain multiple header fields with the same name. It can also contain duplicate header fields.
Header block:
The compressed representation of a header list.
Encoder:
An implementation which transforms a header list into a header block.
Decoder:
An implementation which transforms a header block into a header list.
Absolute Index:
A unique index for each entry in the dynamic table.
Base Index:
An absolute index in a header block from which relative indices are made.
Largest Reference:
The largest absolute index of an entry referenced in a header block.

QPACK is a name, not an acronym.

1.2. Notational Conventions

Diagrams 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 in Section 5.1 of [RFC7541], 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 header fields to indices. The static table (see Section 3.1) is predefined and contains common header fields (some of them with an empty value). The dynamic table (see Section 3.2) is built up over the course of the connection and can be used by the encoder to index header fields in the encoded header lists.

QPACK instructions appear in three different types of streams:

2.1. Encoder

An encoder compresses a header list by emitting either an indexed or a literal representation for each header field in the list. 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 acknowledgement 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 header fields it is compressing.

QPACK preserves the ordering of header fields within each header list. An encoder MUST emit header field representations in the order they appear in the input header list.

QPACK is designed to contain the more complex state tracking to the encoder, while the decoder is relatively simple.

2.1.1. Reference Tracking

An encoder MUST ensure that a header block which references a dynamic table entry is not received by the decoder after the referenced entry has been evicted. Hence the encoder needs to track information about each compressed header block that references the dynamic table until that header block is acknowledged by the decoder.

2.1.2. Blocked Dynamic Table Insertions

An encoder MUST NOT insert an entry into the dynamic table (or duplicate an existing entry) if doing so would evict an entry with unacknowledged references. For header blocks that might rely on the newly added entry, the encoder can use a literal representation and maybe insert the entry later.

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 (see Section 4.3.3), 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 unreferenced entries. To achieve this, the encoder can maintain a draining index, which is the smallest absolute index 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.

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

Figure 1: Draining Dynamic Table Entries

2.1.3. Avoiding Head-of-Line Blocking

Because QUIC does not guarantee order between data on different streams, a header block might reference an entry in the dynamic table that has not yet been received.

Each header block contains a Largest Reference (Section 4.5.1) which identifies the table state necessary for decoding. If the greatest absolute index in the dynamic table is less than the value of the Largest Reference, the stream is considered “blocked.” While blocked, header field data SHOULD remain in the blocked stream’s flow control window. When the Largest Reference is zero, the frame contains no references to the dynamic table and can always be processed immediately. A stream becomes unblocked when the greatest absolute index in the dynamic table becomes greater than or equal to the Largest Reference for all header blocks the decoder has started reading from the stream. If the decoder encounters a header block where the actual largest reference is not equal to the Largest Reference declared in the prefix, it MAY treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.

The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies an upper bound on the number of streams which can be blocked. An encoder MUST limit the number of streams which could become blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note that the decoder might not actually become blocked on every stream which risks becoming blocked. If the decoder encounters more blocked streams than it promised to support, it MUST treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.

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 avoids the risk of blocking by only referencing dynamic table entries which have been acknowledged, but this could mean using literals. Since literals make the header block larger, this can result in the encoder becoming blocked on congestion or flow control limits.

2.1.4. Largest Known Received

In order to identify which dynamic table entries can be safely used without a stream becoming blocked, the encoder tracks the absolute index of the decoder’s Largest Known Received entry.

When blocking references are permitted, the encoder uses header block acknowledgement to identify the Largest Known Received index, as described in Section 4.4.2.

To acknowledge dynamic table entries which are not referenced by header blocks, for example because the encoder or the decoder have chosen not to risk blocked streams, the decoder sends a Table State Synchronize instruction (see Section 4.4.1).

2.2. Decoder

As in HPACK, the decoder processes header blocks and emits the corresponding header lists. It also processes dynamic table modifications from instructions on the encoder stream.

The decoder MUST emit header fields in the order their representations appear in the input header block.

2.2.1. State Synchronization

The decoder stream (Section 4.4) signals key events at the decoder that permit the encoder to track the decoder’s state. These events are:

  • Complete processing of a header block
  • Abandonment of a stream which might have remaining header blocks
  • Receipt of new dynamic table entries

Knowledge that a header block with references to the dynamic table has been processed permits the encoder to evict entries to which no unacknowledged references remain, regardless of whether those references were potentially blocking (see Section 2.1.2). When a stream is reset or abandoned, the indication that these header blocks will never be processed serves a similar function; see Section 4.4.3.

The decoder chooses when to emit Table State Synchronize instructions (see Section 4.4.1). Emitting an instruction after adding each new dynamic table entry will provide the most timely feedback to the encoder, but could be redundant with other decoder feedback. By delaying a Table State Synchronize instruction, the decoder might be able to coalesce multiple Table State Synchronize instructions, or replace them entirely with Header Acknowledgements (see Section 4.4.2). However, delaying too long may lead to compression inefficiencies if the encoder waits for an entry to be acknowledged before using it.

2.2.2. Blocked Decoding

To track blocked streams, the necessary Largest Reference value for each stream can be used. Whenever the decoder processes a table update, it can begin decoding any blocked streams that now have their dependencies satisfied.


3. Header 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.

3.1. Static Table

The static table consists of a predefined static list of header fields, each of which has a fixed index over time. Its entries are defined in Appendix A.

Note the QPACK static table is indexed from 0, whereas the HPACK static table is indexed from 1.

When the decoder encounters an invalid static table index on a request stream or push stream it MUST treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED. If this index is received on the encoder stream, this MUST be treated as a connection error of type HTTP_QPACK_ENCODER_STREAM_ERROR.

3.2. Dynamic Table

The dynamic table consists of a list of header fields maintained in first-in, first-out order. The dynamic table is initially empty. Entries are added by instructions on the encoder stream (see Section 4.3).

The maximum size of the dynamic table can be modified by the encoder, subject to a decoder-controlled limit (see Section 5 and Section 4.3.4). The initial maximum size is determined by the corresponding setting when HTTP requests or responses are first permitted to be sent. For clients using 0-RTT data in HTTP/3, the table size is the remembered value of the setting, even if the server later specifies a larger maximum in its SETTINGS frame. For HTTP/3 servers and HTTP/3 clients when 0-RTT is not attempted or is rejected, the initial maximum table size is the value of the setting in the peer’s SETTINGS frame.

Before a new entry is added to the dynamic table, entries are evicted from the end of the dynamic table until the size of the dynamic table is less than or equal to (maximum size - new entry size) or until the table is empty. The encoder MUST NOT evict a dynamic table entry unless it has first been acknowledged by the decoder.

If the size of the new entry is less than or equal to the maximum size, that entry is added to the table. It is an error to attempt to add an entry that is larger than the maximum size; this MUST be treated as a connection error of type HTTP_QPACK_ENCODER_STREAM_ERROR.

A new entry can reference an entry in the dynamic table that will be evicted when adding this new entry into the dynamic table. Implementations are cautioned to avoid deleting the referenced name if the referenced entry is evicted from the dynamic table prior to inserting the new entry.

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 the decoder.

3.2.1. Maximum Table Size

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.

The decoder determines the maximum size that the encoder is permitted to use for the dynamic table. In HTTP/3, this value is determined by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 5).

An encoder can choose to use less capacity than this maximum size (see Section 4.3.4), but the chosen size MUST stay lower than or equal to the maximum set by the decoder. Whenever the maximum size for the dynamic table is reduced, entries are evicted from the end of the dynamic table until the size of the dynamic table is less than or equal to the maximum size.

This mechanism can be used to completely clear entries from the dynamic table by setting a maximum size of 0, which can subsequently be restored.

3.2.2. Calculating Table Size

The size of the dynamic table is the sum of the size of its entries.

The size of an entry is the sum of its name’s length in bytes (as defined in Section 4.1.2), its value’s length in bytes, and 32.

The size of an entry is calculated using the length of its name and value without any Huffman encoding applied.

MaxEntries is the maximum number of entries that the dynamic table can have. The smallest entry has empty name and value strings and has the size of 32. The MaxEntries is calculated as

   MaxEntries = floor( MaxTableSize / 32 )

MaxTableSize is the maximum size of the dynamic table as specified by the decoder (see Section 3.2.1).

3.2.3. Absolute Indexing

Each entry possesses both an absolute index which is fixed for the lifetime of that entry and a relative index which changes based on the context of the reference. The first entry inserted has an absolute index of “1”; indices increase sequentially with each insertion.

3.2.4. Relative Indexing

The relative index begins at zero and increases in the opposite direction from the absolute index. Determining which entry has a relative index of “0” depends on the context of the reference.

On the encoder stream, a relative index of “0” always refers to the most recently inserted value in the dynamic table. Note that this means the entry referenced by a given relative index will change while interpreting instructions on the encoder stream.

    +---+---------------+-----------+
    | n |      ...      |   d + 1   |  Absolute Index
    + - +---------------+ - - - - - +
    | 0 |      ...      | n - d - 1 |  Relative Index
    +---+---------------+-----------+
      ^                       |
      |                       V
Insertion Point         Dropping Point

n = count of entries inserted
d = count of entries dropped

Example Dynamic Table Indexing - Control Stream

Because frames from request streams can be delivered out of order with instructions on the encoder stream, relative indices are relative to the Base Index at the beginning of the header block (see Section 4.5.1). The Base Index is an absolute index. When interpreting the rest of the frame, the entry identified by Base Index has a relative index of zero. The relative indices of entries do not change while interpreting headers on a request or push stream.

             Base Index
                 |
                 V
    +---+-----+-----+-----+-------+
    | n | n-1 | n-2 | ... |  d+1  |  Absolute Index
    +---+-----+  -  +-----+   -   +
              |  0  | ... | n-d-3 |  Relative Index
              +-----+-----+-------+

n = count of entries inserted
d = count of entries dropped

Example Dynamic Table Indexing - Relative Index on Request Stream

3.2.5. Post-Base Indexing

A header block on the request stream can reference entries added after the entry identified by the Base Index. This allows an encoder to process a header block in a single pass and include references to entries added while processing this (or other) header blocks. Newly added entries are referenced using Post-Base instructions. Indices for Post-Base instructions increase in the same direction as absolute indices, but the zero value is one higher than the Base Index.

             Base Index
                 |
                 V
    +---+-----+-----+-----+-----+
    | n | n-1 | n-2 | ... | d+1 |  Absolute Index
    +---+-----+-----+-----+-----+
    | 1 |  0  |                    Post-Base Index
    +---+-----+

n = count of entries inserted
d = count of entries dropped

Example Dynamic Table Indexing - Post-Base Index on Request Stream

3.2.6. Invalid References

If the decoder encounters a reference on a request or push stream to a dynamic table entry which has already been evicted or which has an absolute index greater than the declared Largest Reference (see Section 4.5.1), it MUST treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.

If the decoder encounters a reference on the encoder stream to a dynamic table entry which has already been dropped, it MUST treat this as a connection error of type HTTP_QPACK_ENCODER_STREAM_ERROR.


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. QPACK implementations MUST be able to decode integers up to 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 flag (indicating whether the string is Huffman-coded), followed by the Length encoded as a 7-bit prefix integer, and finally Length bytes of data. When Huffman encoding is enabled, the Huffman table from Appendix B of [RFC7541] is used without modification.

This document expands the definition of string literals and permits them to begin other than on a byte boundary. An “N-bit prefix string literal” begins with the same Huffman flag, followed by the length encoded as an (N-1)-bit prefix integer. 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. Stream Types

QPACK instructions occur in three locations, each of which uses a separate instruction space:

  • The encoder stream is a unidirectional stream of type 0x48 (ASCII ‘H’) which carries table updates from encoder to decoder.
  • The decoder stream is a unidirectional stream of type 0x68 (ASCII ‘h’) which carries acknowledgements of table modifications and header processing from decoder to encoder.
  • Finally, the contents of HEADERS and PUSH_PROMISE frames on request streams and push streams reference the QPACK table state.

There MUST be exactly one of each unidirectional stream type in each direction. Receipt of a second instance of either stream type MUST be treated as a connection error of HTTP_WRONG_STREAM_COUNT. Closure of either unidirectional stream MUST be treated as a connection error of type HTTP_CLOSED_CRITICAL_STREAM.

This section describes the instructions which are possible on each stream type.

4.3. Encoder Stream

Table updates can add a table entry, possibly using 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 which already exist in the dynamic table, the full entry can also be used by reference, creating a duplicate entry.

The contents of the encoder stream are an unframed sequence of the following instructions.

4.3.1. Insert With Name Reference

An addition to the header table where the header field name matches the header field name of an entry stored in the static table or the dynamic table starts with the ‘1’ one-bit pattern. The S bit indicates whether the reference is to the static (S=1) or dynamic (S=0) table. The 6-bit prefix integer (see Section 5.1 of [RFC7541]) that follows is used to locate the table entry for the header name. When S=1, the number represents the static table index; when S=0, the number is the relative index of the entry in the dynamic table.

The header name reference is followed by the header field value represented as a string literal (see Section 5.2 of [RFC7541]).

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 | S |    Name Index (6+)    |
   +---+---+-----------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   |  Value String (Length bytes)  |
   +-------------------------------+

Insert Header Field -- Indexed Name

4.3.2. Insert Without Name Reference

An addition to the header table where both the header field name and the header field value are represented as string literals (see Section 4.1) starts with the ‘01’ two-bit pattern.

The name is represented as a 6-bit prefix string literal, while the value is represented as an 8-bit prefix string literal.

     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)  |
   +-------------------------------+

Insert Header Field -- New Name

4.3.3. Duplicate

Duplication of an existing entry in the dynamic table starts with the ‘000’ three-bit pattern. The relative index of the existing entry is represented as an integer with a 5-bit prefix.

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

Figure 2: Duplicate

The existing entry is re-inserted into the dynamic table without resending either the name or the value. This is useful to mitigate the eviction of older entries which are frequently referenced, both to avoid the need to resend the header and to avoid the entry in the table blocking the ability to insert new headers.

4.3.4. Dynamic Table Size Update

An encoder informs the decoder of a change to the size of the dynamic table using an instruction which begins with the ‘001’ three-bit pattern. The new maximum table size is represented as an integer with a 5-bit prefix (see Section 5.1 of [RFC7541]).

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

Figure 3: Maximum Dynamic Table Size Change

The new maximum size MUST be lower than or equal to the limit determined by the protocol using QPACK. A value that exceeds this limit MUST be treated as a connection error of type HTTP_QPACK_ENCODER_STREAM_ERROR. In HTTP/3, this limit is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 5) received from the decoder.

Reducing the maximum size of the dynamic table can cause entries to be evicted (see Section 4.3 of [RFC7541]). This MUST NOT cause the eviction of entries with outstanding references (see Section 2.1.1). Changing the size of the dynamic table is not acknowledged as this instruction does not insert an entry.

4.4. Decoder Stream

The decoder stream carries information used to ensure consistency of the dynamic table. Information is sent from the decoder to the encoder; that is, the server informs the client about the processing of the client’s header blocks and table updates, and the client informs the server about the processing of the server’s header blocks and table updates.

The contents of the decoder stream are an unframed sequence of the following instructions.

4.4.1. Table State Synchronize

The Table State Synchronize instruction begins with the ‘00’ two-bit pattern. The instruction specifies the total number of dynamic table inserts and duplications since the last Table State Synchronize or Header Acknowledgement that increased the Largest Known Received dynamic table entry (see Section 2.1.4). This is encoded as a 6-bit prefix integer. The encoder uses this value to determine which table entries might cause a stream to become blocked, as described in Section 2.2.1.

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

Figure 4: Table State Synchronize

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

4.4.2. Header Acknowledgement

After processing a header block whose declared Largest Reference is not zero, the decoder emits a Header Acknowledgement instruction on the decoder stream. The instruction begins with the ‘1’ one-bit pattern and includes the request stream’s stream ID, encoded as a 7-bit prefix integer. It is used by the peer’s encoder to know when it is safe to evict an entry, and possibly update Largest Known Received.

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

Figure 5: Header Acknowledgement

The same Stream ID can be identified multiple times, as multiple header blocks can be sent on a single stream in the case of intermediate responses, trailers, and pushed requests. Since header frames on each stream are received and processed in order, this gives the encoder precise feedback on which header blocks within a stream have been fully processed.

If an encoder receives a Header Acknowledgement instruction referring to a stream on which every header block with a non-zero Largest Reference has already been acknowledged, that MUST be treated as a connection error of type HTTP_QPACK_DECODER_STREAM_ERROR.

When blocking references are permitted, the encoder uses acknowledgement of header blocks to update the Largest Known Received index. If a header block was potentially blocking, the acknowledgement implies that the decoder has received all dynamic table state necessary to process the header block. If the Largest Reference of an acknowledged header block was greater than the encoder’s current Largest Known Received index, the block’s Largest Reference becomes the new Largest Known Received.

4.4.3. Stream Cancellation

The instruction begins with the ‘01’ two-bit pattern. The instruction includes the stream ID of the affected stream - a request or push stream - encoded as a 6-bit prefix integer.

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

Figure 6: Stream Cancellation

A stream that is reset might have multiple outstanding header blocks with dynamic table references. When an endpoint receives a stream reset before the end of a stream, it generates a Stream Cancellation instruction on the decoder stream. Similarly, when an endpoint abandons reading of a stream it needs to signal this using the Stream Cancellation instruction. 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 size 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.

4.5. Request and Push Streams

HEADERS and PUSH_PROMISE frames on request and push streams reference the dynamic table in a particular state without modifying it. Frames on these streams emit the headers for an HTTP request or response.

4.5.1. Header Data Prefix

Header data is prefixed with two integers, Largest Reference and Base Index.

  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
|     Largest Reference (8+)    |
+---+---------------------------+
| S |   Delta Base Index (7+)   |
+---+---------------------------+
|      Compressed Headers     ...
+-------------------------------+

Figure 7: Frame Payload

4.5.1.1. Largest Reference

Largest Reference identifies the largest absolute dynamic index referenced in the block. Blocking decoders use the Largest Reference to determine when it is safe to process the rest of the block. If Largest Reference is greater than zero, the encoder transforms it as follows before encoding:

   LargestReference = (LargestReference mod (2 * MaxEntries)) + 1

The decoder reconstructs the Largest Reference using the following algorithm:

   if LargestReference > 0:
      LargestReference -= 1
      CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries)

      if CurrentWrapped >= LargestReference + MaxEntries:
         # Largest Reference wrapped around 1 extra time
         LargestReference += 2 * MaxEntries
      else if CurrentWrapped + MaxEntries < LargestReference
         # Decoder wrapped around 1 extra time
         CurrentWrapped += 2 * MaxEntries

      LargestReference += TotalNumberOfInserts - CurrentWrapped

TotalNumberOfInserts is the total number of inserts into the decoder’s dynamic table. This encoding limits the length of the prefix on long-lived connections.

4.5.1.2. Base Index

Base Index is used to resolve references in the dynamic table as described in Section 3.2.4.

To save space, Base Index is encoded relative to Largest Reference using a one-bit sign and the Delta Base Index value. A sign bit of 0 indicates that the Base Index has an absolute index that is greater than or equal to the Largest Reference; the value of Delta Base Index is added to the Largest Reference to determine the absolute value of the Base Index. A sign bit of 1 indicates that the Base Index is less than the Largest Reference. That is:

   if sign == 0:
      baseIndex = largestReference + deltaBaseIndex
   else:
      baseIndex = largestReference - deltaBaseIndex - 1

A single-pass encoder determines the absolute value of Base Index before encoding a header block. If the encoder inserted entries in the dynamic table while encoding the header block, Largest Reference will be greater than Base Index, so the encoded difference is negative and the sign bit is set to 1. If the header block did not reference the most recent entry in the table and did not insert any new entries, Base Index will be greater than the Largest Reference, so the delta will be positive and the sign bit is set to 0.

An encoder that produces table updates before encoding a header block might set Largest Reference and Base Index to the same value. In such case, both the sign bit and the Delta Base Index will be set to zero.

A header block that does not reference the dynamic table can use any value for Base Index; setting both Largest Reference and Base Index to zero is the most efficient encoding.

4.5.2. Indexed Header Field

An indexed header field representation identifies an entry in either the static table or the dynamic table and causes that header field to be added to the decoded header list, as described in Section 3.2 of [RFC7541].

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

Indexed Header Field

If the entry is in the static table, or in the dynamic table with an absolute index less than or equal to Base Index, this representation starts with the ‘1’ 1-bit pattern, followed by the S bit indicating whether the reference is into the static (S=1) or dynamic (S=0) table. Finally, the relative index of the matching header field is represented as an integer with a 6-bit prefix (see Section 5.1 of [RFC7541]).

4.5.3. Indexed Header Field With Post-Base Index

If the entry is in the dynamic table with an absolute index greater than Base Index, the representation starts with the ‘0001’ 4-bit pattern, followed by the post-base index (see Section 3.2.5) of the matching header field, represented as an integer with a 4-bit prefix (see Section 5.1 of [RFC7541]).

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

Indexed Header Field with Post-Base Index

4.5.4. Literal Header Field With Name Reference

A literal header field with a name reference represents a header where the header field name matches the header field name of an entry stored in the static table or the dynamic table.

If the entry is in the static table, or in the dynamic table with an absolute index less than or equal to Base Index, this representation starts with the ‘01’ two-bit pattern. If the entry is in the dynamic table with an absolute index greater than Base Index, the representation starts with the ‘0000’ four-bit pattern.

The following bit, ‘N’, indicates whether an intermediary is permitted to add this header to the dynamic header table on subsequent hops. When the ‘N’ bit is set, the encoded header MUST always be encoded with a literal representation. In particular, when a peer sends a header field that it received represented as a literal header field with the ‘N’ bit set, it MUST use a literal representation to forward this header field. This bit is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1 of [RFC7541] for more details).

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 | N | S |Name Index (4+)|
   +---+---+---+---+---------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   |  Value String (Length bytes)  |
   +-------------------------------+

Literal Header Field With Name Reference

For entries in the static table or in the dynamic table with an absolute index less than or equal to Base Index, the header field name is represented using the relative index of that entry, which is represented as an integer with a 4-bit prefix (see Section 5.1 of [RFC7541]). The S bit indicates whether the reference is to the static (S=1) or dynamic (S=0) table.

4.5.5. Literal Header Field With Post-Base Name Reference

For entries in the dynamic table with an absolute index greater than Base Index, the header field name is represented using the post-base index of that entry (see Section 3.2.5) encoded as an integer with a 3-bit prefix.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 | 0 | N |NameIdx(3+)|
   +---+---+---+---+---+-----------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   |  Value String (Length bytes)  |
   +-------------------------------+

Literal Header Field With Post-Base Name Reference

4.5.6. Literal Header Field Without Name Reference

An addition to the header table where both the header field name and the header field value are represented as string literals (see Section 4.1) starts with the ‘001’ three-bit pattern.

The fourth bit, ‘N’, indicates whether an intermediary is permitted to add this header to the dynamic header table on subsequent hops. When the ‘N’ bit is set, the encoded header MUST always be encoded with a literal representation. In particular, when a peer sends a header field that it received represented as a literal header field with the ‘N’ bit set, it MUST use a literal representation to forward this header field. This bit is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1 of [RFC7541] for more details).

The name is represented as a 4-bit prefix string literal, while the value is represented as an 8-bit prefix string literal.

     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)  |
   +-------------------------------+

Literal Header Field Without Name Reference


5. Configuration

QPACK defines two settings which are included in the HTTP/3 SETTINGS frame.

SETTINGS_HEADER_TABLE_SIZE (0x1):
An integer with a maximum value of 2^30 - 1. The default value is zero bytes. See Section 3.2 for usage.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7):
An integer with a maximum value of 2^16 - 1. The default value is zero. See Section 2.1.3.

6. Error Handling

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

HTTP_QPACK_DECOMPRESSION_FAILED (TBD):
The decoder failed to interpret an instruction on a request or push stream and is not able to continue decoding that header block.
HTTP_QPACK_ENCODER_STREAM_ERROR (TBD):
The decoder failed to interpret an instruction on the encoder stream.
HTTP_QPACK_DECODER_STREAM_ERROR (TBD):
The encoder failed to interpret an instruction on the decoder stream.

Upon encountering an error, an implementation MAY elect to treat it as a connection error even if this document prescribes that it MUST be treated as a stream error.


7. Security Considerations

TBD.


8. IANA Considerations

8.1. Settings Registration

This document creates two new settings in the “HTTP/3 Settings” registry established in [HTTP3].

The entries in the following table are registered by this document.

Setting NameCodeSpecification
HEADER_TABLE_SIZE0x1Section 5
QPACK_BLOCKED_STREAMS0x7Section 5

8.2. Stream Type Registration

This document creates two new settings in the “HTTP/3 Stream Type” registry established in [HTTP3].

The entries in the following table are registered by this document.

Stream TypeCodeSpecificationSender
QPACK Encoder Stream0x48Section 4 Both
QPACK Decoder Stream0x68Section 4 Both

8.3. Error Code Registration

This document establishes the following new error codes in the “HTTP/3 Error Code” registry established in [HTTP3].

NameCodeDescriptionSpecification
HTTP_QPACK_DECOMPRESSION_FAILEDTBDDecompression of a header block failedSection 6
HTTP_QPACK_ENCODER_STREAM_ERRORTBDError on the encoder streamSection 6
HTTP_QPACK_DECODER_STREAM_ERRORTBDError on the decoder streamSection 6

9. References

9.1. Normative References

[HTTP3]
Bishop, M., Ed., “Hypertext Transfer Protocol Version 3 (HTTP/3)”, Internet-Draft draft-ietf-quic-http-latest (work in progress), December 2018.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., “QUIC: A UDP-Based Multiplexed and Secure Transport”, Internet-Draft draft-ietf-quic-transport-latest (work in progress), December 2018.
[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>.
[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

[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>.
[RFC7540]
Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <https://www.rfc-editor.org/info/rfc7540>.

A. Static Table

IndexNameValue
0:authority 
1:path/
2age0
3content-disposition 
4content-length0
5cookie 
6date 
7etag 
8if-modified-since 
9if-none-match 
10last-modified 
11link 
12location 
13referer 
14set-cookie 
15:methodCONNECT
16:methodDELETE
17:methodGET
18:methodHEAD
19:methodOPTIONS
20:methodPOST
21:methodPUT
22:schemehttp
23:schemehttps
24:status103
25:status200
26:status304
27:status404
28:status503
29accept*/*
30acceptapplication/dns-message
31accept-encodinggzip, deflate, br
32accept-rangesbytes
33access-control-allow-headerscache-control
34access-control-allow-headerscontent-type
35access-control-allow-origin*
36cache-controlmax-age=0
37cache-controlmax-age=2592000
38cache-controlmax-age=604800
39cache-controlno-cache
40cache-controlno-store
41cache-controlpublic, max-age=31536000
42content-encodingbr
43content-encodinggzip
44content-typeapplication/dns-message
45content-typeapplication/javascript
46content-typeapplication/json
47content-typeapplication/x-www-form-urlencoded
48content-typeimage/gif
49content-typeimage/jpeg
50content-typeimage/png
51content-typetext/css
52content-typetext/html; charset=utf-8
53content-typetext/plain
54content-typetext/plain;charset=utf-8
55rangebytes=0-
56strict-transport-securitymax-age=31536000
57strict-transport-securitymax-age=31536000; includesubdomains
58strict-transport-securitymax-age=31536000; includesubdomains; preload
59varyaccept-encoding
60varyorigin
61x-content-type-optionsnosniff
62x-xss-protection1; mode=block
63:status100
64:status204
65:status206
66:status302
67:status400
68:status403
69:status421
70:status425
71:status500
72accept-language 
73access-control-allow-credentialsFALSE
74access-control-allow-credentialsTRUE
75access-control-allow-headers*
76access-control-allow-methodsget
77access-control-allow-methodsget, post, options
78access-control-allow-methodsoptions
79access-control-expose-headerscontent-length
80access-control-request-headerscontent-type
81access-control-request-methodget
82access-control-request-methodpost
83alt-svcclear
84authorization 
85content-security-policyscript-src 'none'; object-src 'none'; base-uri 'none'
86early-data1
87expect-ct 
88forwarded 
89if-range 
90origin 
91purposeprefetch
92server 
93timing-allow-origin*
94upgrade-insecure-requests1
95user-agent 
96x-forwarded-for 
97x-frame-optionsdeny
98x-frame-optionssameorigin

B. Sample One Pass Encoding Algorithm

Pseudo-code for single pass encoding, excluding handling of duplicates, non-blocking mode, and reference tracking.

baseIndex = dynamicTable.baseIndex
largestReference = 0
for header in headers:
  staticIdx = staticTable.getIndex(header)
  if staticIdx:
    encodeIndexReference(streamBuffer, staticIdx)
    continue

  dynamicIdx = dynamicTable.getIndex(header)
  if !dynamicIdx:
    # No matching entry.  Either insert+index or encode literal
    nameIdx = getNameIndex(header)
    if shouldIndex(header) and dynamicTable.canIndex(header):
      encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
                                        header)
      dynamicTable.add(header)
      dynamicIdx = dynamicTable.baseIndex

  if !dynamicIdx:
    # Couldn't index it, literal
    if nameIdx <= staticTable.size:
      encodeLiteral(streamBuffer, nameIndex, header)
    else:
      # encode literal, possibly with nameIdx above baseIndex
      encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
                           header)
      largestReference = max(largestReference,
                             dynamicTable.toAbsolute(nameIdx))
  else:
    # Dynamic index reference
    assert(dynamicIdx)
    largestReference = max(largestReference, dynamicIdx)
    # Encode dynamicIdx, possibly with dynamicIdx above baseIndex
    encodeDynamicIndexReference(streamBuffer, dynamicIdx,
                                baseIndex)

# encode the prefix
encodeInteger(prefixBuffer, 0x00, largestReference, 8)
if baseIndex >= largestReference:
  encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
else:
  encodeInteger(prefixBuffer, 0x80,
                largestReference  - baseIndex, 7)

return controlBuffer, prefixBuffer + streamBuffer

C. Change Log

C.1. Since draft-ietf-quic-qpack-03 📄 🔍

Substantial editorial reorganization; no technical changes.

C.2. Since draft-ietf-quic-qpack-02 📄 🔍

  • Largest Reference encoded modulo MaxEntries (#1763)
  • New Static Table (#1355)
  • Table Size Update with Insert Count=0 is a connection error (#1762)
  • Stream Cancellations are optional when SETTINGS_HEADER_TABLE_SIZE=0 (#1761)
  • Implementations must handle 62 bit integers (#1760)
  • Different error types for each QPACK stream, other changes to error handling (#1726)
  • Preserve header field order (#1725)
  • Initial table size is the maximum permitted when table is first usable (#1642)

C.3. Since draft-ietf-quic-qpack-01 📄 🔍

  • Only header blocks that reference the dynamic table are acknowledged (#1603, #1605)

C.4. Since draft-ietf-quic-qpack-00 📄 🔍

  • Renumbered instructions for consistency (#1471, #1472)
  • Decoder is allowed to validate largest reference (#1404, #1469)
  • Header block acknowledgments also acknowledge the associated largest reference (#1370, #1400)
  • Added an acknowledgment for unread streams (#1371, #1400)
  • Removed framing from encoder stream (#1361,#1467)
  • Control streams use typed unidirectional streams rather than fixed stream IDs (#910,#1359)

C.5. Since draft-ietf-quic-qcram-00 📄

  • Separate instruction sets for table updates and header blocks (#1235, #1142, #1141)
  • Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125, #1314)
  • Added mechanisms that support one-pass encoding (#1138, #1320)
  • Added a setting to control the number of blocked decoders (#238, #1140, #1143)
  • Moved table updates and acknowledgments to dedicated streams (#1121, #1122, #1238)

Acknowledgments

This draft draws heavily on the text of [RFC7541]. The indirect input of those authors is gratefully acknowledged, as well as ideas from:

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

A substantial portion of Mike’s contribution was supported by Microsoft during his employment there.


Authors' Addresses

Charles 'Buck' Krasic
Netflix
EMail: ckrasic@netflix.com
Mike Bishop
Akamai Technologies
EMail: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
EMail: afrind@fb.com