QPACK: Header Compression for HTTP over QUIC

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

2.2. 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 5.2).¶

The maximum size of the dynamic table can be modified by the encoder, subject to a decoder-controlled limit (see Section 4 and Section 5.2.4). 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/QUIC, 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/QUIC servers and HTTP/QUIC 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 a decoder.¶

2.2.1. 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/QUIC, this value is determined by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 4).¶

An encoder can choose to use less capacity than this maximum size (see Section 5.2.4), 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.¶

2.2.2. 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 octets (as defined in Section 5.1.2), its value’s length in octets, 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 2.2.1).¶

2.2.3. Absolute Indexing

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

2.2.4. 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

Figure 1: 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 5.4.1). 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

Figure 2: Example Dynamic Table Indexing - Relative Index on Request Stream

2.2.5. 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

Figure 3: Example Dynamic Table Indexing - Post-Base Index on Request Stream

If the decoder encounters a reference on a request or push stream to a dynamic table entry which has already been dropped or which has an absolute index greater than the declared Largest Reference (see Section 5.4.1), 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.¶

5. Wire Format

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. Instructions on this stream modify the dynamic table state without generating output to any particular request.
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.¶

All table updates occur on the encoder stream. Request streams and push streams only carry header blocks that do not modify the state of the table.¶

5.1. Primitives

5.1.1. 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.¶

5.1.2. 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 octets 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.¶

5.2. QPACK 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.¶

5.2.1. 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 octets)  |
   +-------------------------------+

Figure 4: Insert Header Field -- Indexed Name

5.2.2. 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 5.1) 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 octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

Figure 5: Insert Header Field -- New Name

5.2.3. 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 6: 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.¶

5.2.4. 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 7: 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/QUIC, this limit is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 4) 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 7.3). Changing the size of the dynamic table is not acknowledged as this instruction does not insert an entry.¶

5.3. QPACK Decoder Stream

The decoder stream carries information used to ensure consistency of the dynamic table. Information is sent from the QPACK decoder to the QPACK 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.¶

5.3.1. 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. 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 2.3.1.¶

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

Figure 8: 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.¶

A decoder chooses when to emit Table State Synchronize instructions. Emitting a Table State Synchronize 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, a decoder might be able to coalesce multiple Table State Synchronize instructions, or replace them entirely with Header Acknowledgements. However, delaying too long may lead to compression inefficiencies if the encoder waits for an entry to be acknowledged before using it.¶

5.3.2. 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 QPACK encoder to know when it is safe to evict an entry.¶

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

Figure 9: 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.¶

5.3.3. Stream Cancellation

A stream that is reset might have multiple outstanding header blocks with dynamic table references. A decoder that receives a stream reset before the end of a stream generates a Stream Cancellation instruction on the decoder stream. Similarly, a decoder that abandons reading of a stream 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.¶

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 10: Stream Cancellation

5.4. 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.¶

5.4.1. 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 11: Frame Payload

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 = TableLargestAbsoluteIndex 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 +=
         (TableLargestAbsoluteIndex - CurrentWrapped)

TableLargestAbsoluteIndex is the Absolute Index of the most recently inserted item in the decoder’s dynamic table. This encoding limits the length of the prefix on long-lived connections.¶

Base Index is used to resolve references in the dynamic table as described in Section 2.2.4.¶

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

A single-pass encoder is expected to determine 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. When Largest Reference and Base Index are equal, the Delta Base Index is encoded with a zero sign bit. A sign bit set to 1 when the Delta Base Index is 0 MUST be treated as a decoder error.¶

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.¶

5.4.2. Instructions

5.4.2.1. 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+)       |
+---+---+-----------------------+

Figure 12: 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]).¶

5.4.2.2. 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 2.2.5) 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+)   |
+---+---+---+---+---------------+

Figure 13: Indexed Header Field with Post-Base Index

5.4.2.3. 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 octets)  |
   +-------------------------------+

Figure 14: 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.¶

5.4.2.4. 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 2.2.5) 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 octets)  |
   +-------------------------------+

Figure 15: Literal Header Field With Post-Base Name Reference

5.4.2.5. 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 5.1) 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 octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

Figure 16: Literal Header Field Without Name Reference

7. Encoding Strategies

7.1. Single Pass Encoding

An encoder making a single pass over a list of headers must choose Base Index before knowing Largest Reference. When trying to reference a header inserted to the table after encoding has begun, the entry is encoded with different instructions that tell the decoder to use an absolute index greater than the Base Index.¶

7.2. Preventing Eviction Races

Due to out-of-order arrival, QPACK’s eviction algorithm requires changes (relative to HPACK) to avoid the possibility that an indexed representation is decoded after the referenced entry has already been evicted. QPACK employs a two-phase eviction algorithm, in which the encoder will not evict entries that have outstanding (unacknowledged) references.¶

7.3. 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 already been evicted. An encoder also respects the limit set by the decoder on the number of streams that are allowed to become blocked. Even if the decoder is willing to tolerate blocked streams, the encoder might choose to avoid them in certain cases.¶

In order to enable this, the encoder will need to track outstanding (unacknowledged) header blocks and table updates using feedback received from the decoder.¶

7.3.1. 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 will be evicted soonest. Rather than reference such an entry, the encoder SHOULD emit a Duplicate instruction (see Section 5.2.3), 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. Draining entries - entries with an absolute index lower than the draining index - will not accumulate new references. The number of unacknowledged references to draining entries will eventually become zero, making the entry available for eviction.¶

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

Figure 17: Draining Dynamic Table Entries

7.3.2. Blocked Decoding

For header blocks encoded in non-blocking mode, the encoder needs to forego indexed representations that refer to table updates which have not yet been acknowledged (see Section 5.3). Since all table updates are processed in sequence on the control stream, an index into the dynamic table is sufficient to track which entries have been acknowledged.¶

To track blocked streams, the necessary Base Index 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.¶

7.4. Speculative table updates

Implementations can speculatively send instructions on the encoder stream which are not needed for any current HTTP request or response. Such headers could be used strategically to improve performance. For instance, the encoder might decide to refresh by sending Duplicate representations for popular header fields (Section 5.2.3), ensuring they have small indices and hence minimal size on the wire.¶

7.5. 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

Setting Name	Code	Specification
HEADER_TABLE_SIZE	0x1	Section 4
QPACK_BLOCKED_STREAMS	0x7	Section 4

Name	Code	Description	Specification
HTTP_QPACK_DECOMPRESSION_FAILED	TBD	Decompression of a header block failed	Section 6
HTTP_QPACK_ENCODER_STREAM_ERROR	TBD	Error on the encoder stream	Section 6
HTTP_QPACK_DECODER_STREAM_ERROR	TBD	Error on the decoder stream	Section 6

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

Stream Type	Code	Specification	Sender
QPACK Encoder Stream	0x48	Section 5	Both
QPACK Decoder Stream	0x68	Section 5	Both

10. References

10.1. Normative References

10.2. Informative References