Internet Engineering Task Force (IETF) C. Krasic
Request for Comments: 9204
Category: Standards Track M. Bishop
ISSN: 2070-1721 Akamai Technologies
A. Frindell, Ed.
Facebook
June 2022
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 at
https://www.rfc-editor.org/info/rfc9204.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction
1.1. Conventions and Definitions
1.2. Notational Conventions
2. Compression Process Overview
2.1. Encoder
2.1.1. Limits on Dynamic Table Insertions
2.1.2. Blocked Streams
2.1.3. Avoiding Flow-Control Deadlocks
2.1.4. Known Received Count
2.2. Decoder
2.2.1. Blocked Decoding
2.2.2. State Synchronization
2.2.3. Invalid References
3. Reference Tables
3.1. Static Table
3.2. Dynamic Table
3.2.1. Dynamic Table Size
3.2.2. Dynamic Table Capacity and Eviction
3.2.3. Maximum Dynamic Table Capacity
3.2.4. Absolute Indexing
3.2.5. Relative Indexing
3.2.6. Post-Base Indexing
4. Wire Format
4.1. Primitives
4.1.1. Prefixed Integers
4.1.2. String Literals
4.2. Encoder and Decoder Streams
4.3. Encoder Instructions
4.3.1. Set Dynamic Table Capacity
4.3.2. Insert with Name Reference
4.3.3. Insert with Literal Name
4.3.4. Duplicate
4.4. Decoder Instructions
4.4.1. Section Acknowledgment
4.4.2. Stream Cancellation
4.4.3. Insert Count Increment
4.5. Field Line Representations
4.5.1. Encoded Field Section Prefix
4.5.2. Indexed Field Line
4.5.3. Indexed Field Line with Post-Base Index
4.5.4. Literal Field Line with Name Reference
4.5.5. Literal Field Line with Post-Base Name Reference
4.5.6. Literal Field Line with Literal Name
5. Configuration
6. Error Handling
7. Security Considerations
7.1. Probing Dynamic Table State
7.1.1. Applicability to QPACK and HTTP
7.1.2. Mitigation
7.1.3. Never-Indexed Literals
7.2. Static Huffman Encoding
7.3. Memory Consumption
7.4. Implementation Limits
8. IANA Considerations
8.1. Settings Registration
8.2. Stream Type Registration
8.3. Error Code Registration
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Static Table
Appendix B. Encoding and Decoding Examples
B.1. Literal Field Line with Name Reference
B.2. Dynamic Table
B.3. Speculative Insert
B.4. Duplicate Instruction, Stream Cancellation
B.5. Dynamic Table Insert, Eviction
Appendix C. Sample Single-Pass Encoding Algorithm
Acknowledgments
Authors' Addresses
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
in Section 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; see Section 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; see Section 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; see
Section 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.
3.1. Static Table
The static table consists of a predefined list of field lines, each
of which has a fixed index over time. Its entries are defined in
Appendix A.
All entries in the static table have a name and a value. However,
values can be empty (that is, have a length of 0). Each entry is
identified by a unique index.
Note that 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 in a field
line representation, it MUST treat this as a connection error of type
QPACK_DECOMPRESSION_FAILED. If this index is received on the encoder
stream, this MUST be treated as a connection error of type
QPACK_ENCODER_STREAM_ERROR.
3.2. Dynamic Table
The dynamic table consists of a list of field lines maintained in
first-in, first-out order. A QPACK encoder and decoder share a
dynamic table that is initially empty. The encoder adds entries to
the dynamic table and sends them to the decoder via instructions on
the encoder stream; see Section 4.3.
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.
Dynamic table entries can have empty values.
3.2.1. Dynamic 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, its
value's length in bytes, and 32 additional bytes. The size of an
entry is calculated using the length of its name and value without
Huffman encoding applied.
3.2.2. Dynamic Table Capacity and Eviction
The encoder sets the capacity of the dynamic table, which serves as
the upper limit on its size. The initial capacity of the dynamic
table is zero. The encoder sends a Set Dynamic Table Capacity
instruction (Section 4.3.1) with a non-zero capacity to begin using
the dynamic table.
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 (table capacity - size of new entry). The
encoder MUST NOT cause a dynamic table entry to be evicted unless
that entry is evictable; see Section 2.1.1. The new entry is then
added to the table. It is an error if the encoder attempts to add an
entry that is larger than the dynamic table capacity; the decoder
MUST treat this as a connection error of type
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
or value if the referenced entry is evicted from the dynamic table
prior to inserting the new entry.
Whenever the dynamic table capacity is reduced by the encoder
(Section 4.3.1), entries are evicted from the end of the dynamic
table until the size of the dynamic table is less than or equal to
the new table capacity. This mechanism can be used to completely
clear entries from the dynamic table by setting a capacity of 0,
which can subsequently be restored.
3.2.3. Maximum Dynamic Table Capacity
To bound the memory requirements of the decoder, the decoder limits
the maximum value the encoder is permitted to set for the dynamic
table capacity. In HTTP/3, this limit is determined by the value of
SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder; see Section 5.
The encoder MUST NOT set a dynamic table capacity that exceeds this
maximum, but it can choose to use a lower dynamic table capacity; see
Section 4.3.1.
For clients using 0-RTT data in HTTP/3, the server's maximum table
capacity is the remembered value of the setting or zero if the value
was not previously sent. When the client's 0-RTT value of the
SETTING is zero, the server MAY set it to a non-zero value in its
SETTINGS frame. If the remembered value is non-zero, the server MUST
send the same non-zero value in its SETTINGS frame. If it specifies
any other value, or omits SETTINGS_QPACK_MAX_TABLE_CAPACITY from
SETTINGS, the encoder must treat this as a connection error of type
QPACK_DECODER_STREAM_ERROR.
For clients not using 0-RTT data (whether 0-RTT is not attempted or
is rejected) and for all HTTP/3 servers, the maximum table capacity
is 0 until the encoder processes a SETTINGS frame with a non-zero
value of SETTINGS_QPACK_MAX_TABLE_CAPACITY.
When the maximum table capacity is zero, the encoder MUST NOT insert
entries into the dynamic table and MUST NOT send any encoder
instructions on the encoder stream.
3.2.4. Absolute Indexing
Each entry possesses an absolute index that is fixed for the lifetime
of that entry. The first entry inserted has an absolute index of 0;
indices increase by one with each insertion.
3.2.5. Relative Indexing
Relative indices begin at zero and increase in the opposite direction
from the absolute index. Determining which entry has a relative
index of 0 depends on the context of the reference.
In encoder instructions (Section 4.3), a relative index of 0 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-1 | ... | d | Absolute Index
+ - - +---------------+ - - - +
| 0 | ... | n-d-1 | Relative Index
+-----+---------------+-------+
^ |
| V
Insertion Point Dropping Point
n = count of entries inserted
d = count of entries dropped
Figure 2: Example Dynamic Table Indexing - Encoder Stream
Unlike in encoder instructions, relative indices in field line
representations are relative to the Base at the beginning of the
encoded field section; see Section 4.5.1. This ensures that
references are stable even if encoded field sections and dynamic
table updates are processed out of order.
In a field line representation, a relative index of 0 refers to the
entry with absolute index equal to Base - 1.
Base
|
V
+-----+-----+-----+-----+-------+
| n-1 | n-2 | n-3 | ... | d | Absolute Index
+-----+-----+ - +-----+ - +
| 0 | ... | n-d-3 | Relative Index
+-----+-----+-------+
n = count of entries inserted
d = count of entries dropped
In this example, Base = n - 2
Figure 3: Example Dynamic Table Indexing - Relative Index in
Representation
3.2.6. Post-Base Indexing
Post-Base indices are used in field line representations for entries
with absolute indices greater than or equal to Base, starting at 0
for the entry with absolute index equal to Base and increasing in the
same direction as the absolute index.
Post-Base indices allow an encoder to process a field section in a
single pass and include references to entries added while processing
this (or other) field sections.
Base
|
V
+-----+-----+-----+-----+-----+
| n-1 | n-2 | n-3 | ... | d | Absolute Index
+-----+-----+-----+-----+-----+
| 1 | 0 | Post-Base Index
+-----+-----+
n = count of entries inserted
d = count of entries dropped
In this example, Base = n - 2
Figure 4: Example Dynamic Table Indexing - Post-Base Index in
Representation
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:
* An encoder stream is a unidirectional stream of type 0x02. It
carries an unframed sequence of encoder instructions from encoder
to decoder.
* A decoder stream is a unidirectional stream of type 0x03. It
carries an unframed sequence of decoder instructions from decoder
to encoder.
HTTP/3 endpoints contain a QPACK encoder and decoder. Each endpoint
MUST initiate, 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 NOT request 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
in Section 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;
see Section 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;
see Section 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.
4.4.1. Section Acknowledgment
After processing an encoded field section whose declared Required
Insert Count is not zero, the decoder emits a Section Acknowledgment
instruction. The instruction starts with the '1' 1-bit pattern,
followed by the field section's associated stream ID encoded as a
7-bit prefix integer; see Section 4.1.1.
This instruction is used as described in Sections 2.1.4 and 2.2.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
Figure 9: Section Acknowledgment
If an encoder receives a Section Acknowledgment instruction referring
to a stream on which every encoded field section with a non-zero
Required Insert Count has already been acknowledged, this MUST be
treated as a connection error of type QPACK_DECODER_STREAM_ERROR.
The Section Acknowledgment instruction might increase the Known
Received Count; see Section 2.1.4.
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.
4.5.1. Encoded Field Section Prefix
Each encoded field section is prefixed with two integers. The
Required Insert Count is encoded as an integer with an 8-bit prefix
using the encoding described in Section 4.5.1.1. The Base is encoded
as a Sign bit ('S') and a Delta Base value with a 7-bit prefix; see
Section 4.5.1.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Required Insert Count (8+) |
+---+---------------------------+
| S | Delta Base (7+) |
+---+---------------------------+
| Encoded Field Lines ...
+-------------------------------+
Figure 12: Encoded Field Section
4.5.1.1. Required Insert Count
Required Insert Count identifies the state of the dynamic table
needed to process the encoded field section. Blocking decoders use
the Required Insert Count to determine when it is safe to process the
rest of the field section.
The encoder transforms the Required Insert Count as follows before
encoding:
if ReqInsertCount == 0:
EncInsertCount = 0
else:
EncInsertCount = (ReqInsertCount mod (2 * MaxEntries)) + 1
Here 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. Hence, MaxEntries is calculated as:
MaxEntries = floor( MaxTableCapacity / 32 )
MaxTableCapacity is the maximum capacity of the dynamic table as
specified by the decoder; see Section 3.2.3.
This encoding limits the length of the prefix on long-lived
connections.
The decoder can reconstruct the Required Insert Count using an
algorithm such as the following. If the decoder encounters a value
of EncodedInsertCount that could not have been produced by a
conformant encoder, it MUST treat this as a connection error of type
QPACK_DECOMPRESSION_FAILED.
TotalNumberOfInserts is the total number of inserts into the
decoder's dynamic table.
FullRange = 2 * MaxEntries
if EncodedInsertCount == 0:
ReqInsertCount = 0
else:
if EncodedInsertCount > FullRange:
Error
MaxValue = TotalNumberOfInserts + MaxEntries
# MaxWrapped is the largest possible value of
# ReqInsertCount that is 0 mod 2 * MaxEntries
MaxWrapped = floor(MaxValue / FullRange) * FullRange
ReqInsertCount = MaxWrapped + EncodedInsertCount - 1
# If ReqInsertCount exceeds MaxValue, the Encoder's value
# must have wrapped one fewer time
if ReqInsertCount > MaxValue:
if ReqInsertCount <= FullRange:
Error
ReqInsertCount -= FullRange
# Value of 0 must be encoded as 0.
if ReqInsertCount == 0:
Error
For example, if the dynamic table is 100 bytes, then the Required
Insert Count will be encoded modulo 6. If a decoder has received 10
inserts, then an encoded value of 4 indicates that the Required
Insert Count is 9 for the field section.
4.5.1.2. Base
The Base is used to resolve references in the dynamic table as
described in Section 3.2.5.
To save space, the Base is encoded relative to the Required Insert
Count using a one-bit Sign ('S' in Figure 12) and the Delta Base
value. A Sign bit of 0 indicates that the Base is greater than or
equal to the value of the Required Insert Count; the decoder adds the
value of Delta Base to the Required Insert Count to determine the
value of the Base. A Sign bit of 1 indicates that the Base is less
than the Required Insert Count; the decoder subtracts the value of
Delta Base from the Required Insert Count and also subtracts one to
determine the value of the Base. That is:
if Sign == 0:
Base = ReqInsertCount + DeltaBase
else:
Base = ReqInsertCount - DeltaBase - 1
A single-pass encoder determines the Base before encoding a field
section. If the encoder inserted entries in the dynamic table while
encoding the field section and is referencing them, Required Insert
Count will be greater than the Base, so the encoded difference is
negative and the Sign bit is set to 1. If the field section was not
encoded using representations that reference the most recent entry in
the table and did not insert any new entries, the Base will be
greater than the Required Insert Count, so the encoded difference
will be positive and the Sign bit is set to 0.
The value of Base MUST NOT be negative. Though the protocol might
operate correctly with a negative Base using post-Base indexing, it
is unnecessary and inefficient. An endpoint MUST treat a field block
with a Sign bit of 1 as invalid if the value of Required Insert Count
is less than or equal to the value of Delta Base.
An encoder that produces table updates before encoding a field
section might set Base to the value of Required Insert Count. In
such a case, both the Sign bit and the Delta Base will be set to
zero.
A field section that was encoded without references to the dynamic
table can use any value for the Base; setting Delta Base to zero is
one of the most efficient encodings.
For example, with a Required Insert Count of 9, a decoder receives a
Sign bit of 1 and a Delta Base of 2. This sets the Base to 6 and
enables post-Base indexing for three entries. In this example, a
relative index of 1 refers to the fifth entry that was added to the
table; a post-Base index of 1 refers to the eighth entry.
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:
* Use of compression as a length-based oracle for verifying guesses
about secrets that are compressed into a shared compression
context.
* Denial of service resulting from exhausting processing or memory
capacity at a decoder.
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.
| Note: Padding schemes only provide limited protection against
| an attacker with these capabilities, potentially only forcing
| an increased number of guesses to learn the length associated
| with a given guess. Padding schemes also work directly against
| compression by increasing the number of bits that are
| transmitted.
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:
* sends requests from multiple clients on a single connection toward
an origin server, or
* takes responses from multiple origin servers and places them on a
shared connection toward a client.
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.
| Note: Simply removing entries corresponding to the field from
| the dynamic table can be ineffectual if the attacker has a
| reliable way of causing values to be reinstalled. For example,
| a request to load an image in a web browser typically includes
| the Cookie header field (a potentially highly valued target for
| this sort of attack), and websites can easily force an image to
| be loaded, thereby refreshing the entry in the dynamic table.
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 (see Section 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 (see Section 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; see Section 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, this MUST 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].
+==========================+======+===============+=========+
| Setting Name | Code | Specification | Default |
+==========================+======+===============+=========+
| QPACK_MAX_TABLE_CAPACITY | 0x01 | Section 5 | 0 |
+--------------------------+------+---------------+---------+
| QPACK_BLOCKED_STREAMS | 0x07 | Section 5 | 0 |
+--------------------------+------+---------------+---------+
Table 1: Additions to the HTTP/3 Settings Registry
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].
+======================+======+===============+========+
| Stream Type | Code | Specification | Sender |
+======================+======+===============+========+
| QPACK Encoder Stream | 0x02 | Section 4.2 | Both |
+----------------------+------+---------------+--------+
| QPACK Decoder Stream | 0x03 | Section 4.2 | Both |
+----------------------+------+---------------+--------+
Table 2: Additions to the HTTP/3 Stream Types Registry
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].
+============================+========+=============+===============+
| Name | Code |Description | Specification |
+============================+========+=============+===============+
| QPACK_DECOMPRESSION_FAILED | 0x0200 |Decoding of a| Section 6 |
| | |field section| |
| | |failed | |
+----------------------------+--------+-------------+---------------+
| QPACK_ENCODER_STREAM_ERROR | 0x0201 |Error on the | Section 6 |
| | |encoder | |
| | |stream | |
+----------------------------+--------+-------------+---------------+
| QPACK_DECODER_STREAM_ERROR | 0x0202 |Error on the | Section 6 |
| | |decoder | |
| | |stream | |
+----------------------------+--------+-------------+---------------+
Table 3: Additions to the HTTP/3 Error Codes Registry
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.
+=======+==================================+=======================+
| 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 |
+-------+----------------------------------+-----------------------+
Table 4: Static Table
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, and Dmitri Tikhonov are gratefully acknowledged.
The following people also provided substantial contributions to this
document:
* Bence Beky
* Alessandro Ghedini
* Ryan Hamilton
* Robin Marx
* Patrick McManus
* 奥 一穂 (Kazuho Oku)
* Lucas Pardue
* Biren Roy
* Ian Swett
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
Email: krasic@acm.org
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
Email: afrind@fb.com