One document matched: draft-ietf-httpbis-header-compression-08.xml
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<rfc category="std"
ipr="trust200902"
docName="draft-ietf-httpbis-header-compression-08"
x:maturity-level="proposed"
xmlns:x="http://purl.org/net/xml2rfc/ext">
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<front>
<title abbrev="HPACK">HPACK - Header Compression for HTTP/2</title>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google, Inc</organization>
<address>
<email>fenix@google.com</email>
</address>
</author>
<author initials="H." surname="Ruellan" fullname="Hervé Ruellan">
<organization>Canon CRF</organization>
<address>
<email>herve.ruellan@crf.canon.fr</email>
</address>
</author>
<date year="2014"/>
<area>Applications</area>
<workgroup>HTTPbis</workgroup>
<keyword>HTTP</keyword>
<keyword>Header</keyword>
<abstract>
<t>
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context of
HTTP/2.
</t>
</abstract>
<note title="Editorial Note (To be removed by RFC Editor)">
<t>
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at <eref
target="https://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
</t>
<t>
Working Group information can be found at <eref
target="http://tools.ietf.org/wg/httpbis/"/>; that specific to HTTP/2
are at <eref target="http://http2.github.io/"/>.
</t>
<t>
The changes in this draft are summarized in <xref
target="changes.since.draft-ietf-httpbis-header-compression-07"/>.
</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context of
HTTP/2 (see <xref target="HTTP2"/>).
</t>
</section>
<section title="HPACK Overview" anchor="overview">
<t>
In HTTP/1.1 (see <xref target="RFC7230"/>), header fields are
encoded without any form of compression. As web pages have
grown to include dozens to hundreds of requests, the redundant
header fields in these requests now measurably increase latency
and unnecessarily consume bandwidth (see <xref
target="SPDY-DESC-1"/>
and <xref target="SPDY-DESC-2"/>).
</t>
<t>
SPDY <xref target="SPDY"/> initially addressed this redundancy
by compressing header fields using the DEFLATE format <xref
target="DEFLATE"/>, which proved very effective at efficiently
representing the redundant header fields. However, that
approach exposed a security risk as demonstrated by the CRIME
attack (see <xref target="CRIME"/>).
</t>
<t>
This document describes HPACK, a new compressor for header
fields which eliminates redundant header fields, limits
vulnerability to known security attacks, and which has a bounded
memory requirement for use in constrained environments.
</t>
<section title="Outline">
<t>
The HTTP header field encoding defined in this document is
based on a header table that maps name-value pairs to index
values. The header table is incrementally updated as new
values are encoded or decoded.
</t>
<t>
A set of header fields is treated as an unordered collection
of name-value pairs that can include duplicates. Names and
values are considered to be opaque sequences of octets. The
order of header fields is not guaranteed to be preserved
after being compressed and decompressed.
</t>
<t>
In the encoded form, a header field is represented either
literally or as a reference to a name-value pair in a header
table. A set of header fields can therefore be encoded
using a mixture of references and literal values.
</t>
<t>
As two consecutive sets of header fields often have header
fields in common, each set is coded as a difference from the
previous set. The goal is to only encode the changes between
the two sets of header fields (that is, header fields that
are present in only one of the sets) and eliminate
redundancy (header fields present in both sets).
</t>
<t>
A subset of the header fields that are encoded as references to
the header table are maintained in a reference set that is
used as the initial set of header fields for the next
encoding.
</t>
<t>
The encoder is responsible for deciding which header fields
to insert as new entries in the header table. The decoder
executes the modifications to the header table and reference
set prescribed by the encoder, reconstructing the set of
header fields in the process. This enables decoders to
remain simple and understand a wide variety of encoders.
</t>
<t>
Examples illustrating the use of these different mechanisms
to represent header fields are available in <xref
target="examples"/>.
</t>
</section>
<section title="Conventions" anchor="conventions">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in <xref target="RFC2119">RFC 2119</xref>.
</t>
<t>
All numeric values are in network byte order. Values are
unsigned unless otherwise indicated. Literal values are
provided in decimal or hexadecimal as appropriate.
Hexadecimal literals are prefixed with <spanx
style="verb">0x</spanx> to distinguish them from decimal
literals.
</t>
</section>
<section title="Terminology" anchor="encoding.concepts">
<t>
This document uses the following terms:
<list style="hanging">
<t hangText="Header Field:">
A name-value pair. Both the name and value are
treated as opaque sequences of octets.
</t>
<t hangText="Header Table:">
The header table (see <xref target="header.table"/>)
is a component used to associate stored header
fields to index values.
</t>
<t hangText="Static Table:">
The static table (see <xref target="static.table"/>)
is a component used to associate static header
fields to index values. This data is ordered,
read-only, always accessible, and may be shared
amongst all encoding contexts.
</t>
<t hangText="Header Set:">
A header set is an unordered group of header fields
that are encoded jointly. It can contain duplicate
header fields. A complete set of key-value pairs
contained in a HTTP request or response is a header
set.
</t>
<t hangText="Reference Set:">
The reference set (see <xref
target="reference.set"/>) is a component containing
an unordered set of references to entries in the
header table. It doesn't contain duplicate
references. The reference set is used for the
differential encoding of a new header set.
</t>
<t hangText="Header Field Representation:">
A header field can be represented in encoded form
either as a literal or as an index (see <xref
target="header.representation"/>).
</t>
<t hangText="Header Block:">
The entire set of encoded header field
representations which, when decoded, yield a
complete header set.
</t>
<t hangText="Header Field Emission:">
When decoding a set of header field representations,
some operations emit a header field (see <xref
target="header.emission"/>). Emitted header fields
are added to the output header set and cannot be
removed.
</t>
</list>
</t>
</section>
</section>
<section title="Decoding Process Overview" anchor="header.encoding">
<t>
This specification does not describe a specific algorithm for an
encoder. Instead, it defines precisely how a decoder is expected
to operate, allowing encoders to produce any encoding that this
definition permits.
</t>
<section title="Encoding and Decoding Contexts"
anchor="encoding.context">
<t>
HPACK requires that a decoder maintains both a header table
and a reference set. No other state information is needed
to decode messages. An encoder that wishes to reference
entries in the header table, reference set, or static table
needs to maintain a copy of the information a decoder holds.
</t>
<t>
When used for bidirectional communication, such as in HTTP,
the encoding and decoding contexts maintained by an endpoint
are completely independent. Header fields are encoded
without any reference to the local decoding state; and
header fields are decoded without reference to the encoding
state.
</t>
<t>
Each endpoint maintains a header table and a reference set in
order to decode header blocks, and optionally a copy of the
information maintained by their peer.
</t>
</section>
<section title="Header Table" anchor="header.table">
<t>
A header table consists of a list of header fields
maintained in first-in, first-out order. The first and
newest entry in a header table is always at index 1, and the
oldest entry of a header table is at the index corresponding
to the number of entries in the header table.
</t>
<t>
The header table is initially empty.
</t>
<t>
The header table can contain duplicate entries. Therefore,
duplicate entries MUST NOT be treated as an error by a
decoder.
</t>
<t>
The encoder decides how to update the header table and as
such can control how much memory is used by the header
table. To limit the memory requirements of the decoder, the
header table size is strictly bounded (see <xref
target="maximum.table.size"/>).
</t>
<t>
The header table is updated during the processing of a set
of header field representations (see <xref
target="header.representation.processing"/>).
</t>
</section>
<section title="Reference Set" anchor="reference.set">
<t>
A reference set is an unordered set of references to entries
of the header table. It never contains duplicate references.
</t>
<t>
The reference set is initially empty.
</t>
<t>
The reference set is updated during the processing of a set
of header field representations (see <xref
target="header.representation.processing"/>).
</t>
<t>
The reference set enables differential encoding, where
only differences between the previous header set and the
current header set need to be encoded. The use of
differential encoding is optional for any header set.
</t>
<t>
When an entry is evicted from the header table, if it was
referenced from the reference set, its reference is removed
from the reference set.
</t>
<t>
To limit the memory requirements on the decoder side for
handling the reference set, only entries within the header
table can be contained in the reference set. To still allow
entries from the static table to take advantage of the
differential encoding, when a header field is represented as
a reference to an entry of the static table, this entry is
inserted into the header table (see <xref
target="header.representation.processing"/>).
</t>
</section>
<section title="Header Field Representation"
anchor="header.representation">
<t>
An encoded header field can be represented either as a
literal or as an index.
</t>
<t>
A literal representation defines a new header field. The
header field name can be represented literally or as a
reference to an entry of the header table. The header field
value is represented literally.
</t>
<t>
Three different literal representations are provided:
<list style="symbols">
<t>
A literal representation that does not add the
header field to the header table (see <xref
target="literal.header.without.indexing"/>).
</t>
<t>
A literal representation that does not add the
header field to the header table, with the
additional stipulation that this header field always
use a literal representation, in particular when
re-encoded by an intermediary (see <xref
target="literal.header.never.indexed"/>).
</t>
<t>
A literal representation that adds the header field
as a new entry at the beginning of the header table
(see <xref
target="literal.header.with.incremental.indexing"/>).
</t>
</list>
</t>
<t>
An indexed representation defines a header field as a
reference to an entry in either the header table or the
static table (see <xref
target="indexed.header.representation"/>).
</t>
<t>
Indices between 1 and the length of the header table
(inclusive) refer to elements in the header table, with
index 1 referring to the beginning of the table.
</t>
<t>
Indices between one higher than the length of the header
table represent indexes into the static table. The length
of the header table is subtracted to find the index into the
static table.
</t>
<t>
Indices that are greater than the sum of the lengths of both
tables MUST be treated as a decoding error.
</t>
<t>
An indexed representation using an entry of the static
table induces a copy of this entry into the header table
(see <xref target="header.representation.processing"/>)
for bounding memory requirements on the decoder side (see
<xref target="maximum.table.size"/>). For this reason, the
header table is accessed more frequently than the static
table and has the lower indices.
</t>
<figure title="Index Address Space">
<preamble>
For a header table size of k and a static table size of
s, the following diagram shows the entire valid index
address space.
</preamble>
<artwork type="drawing"><![CDATA[
<---------- Index Address Space ---------->
<-- Header Table --> <-- Static Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | k | |k+1| ... |k+s|
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Dropping Point
]]></artwork>
</figure>
</section>
<section title="Header Field Emission" anchor="header.emission">
<t>
A decoder processes an encoded header block sequentially.
As different instructions are processed, some might specify
that a header field is emitted.
</t>
<t>
The emission of a header field is the process of marking a
header field as belonging to the output header set. Once a
header has been emitted, it cannot be removed or retracted
from the decoder output.
</t>
<t>
An emitted header field can be safely
passed to the upper processing layer as part of the current
header set. The decoder can pass emitted header fields
to the upper processing layer in any order.
</t>
<t>
By emitting header fields instead of emitting header sets, a
decoder can be implemented with minimal memory commitment in
addition to the header table and the reference set. The
management of memory for handling very large sets of header
fields can therefore be deferred to the upper processing
layers.
</t>
</section>
</section>
<section title="Header Block Decoding"
anchor="header.block.decoding">
<t>
The processing of a header block to obtain a header set is
defined in this section. To ensure that the decoding will
successfully produce a header set, a decoder MUST obey the
following rules.
</t>
<section title="Header Field Representation Processing"
anchor="header.representation.processing">
<t>
All the header field representations contained in a header
block are processed in the order in which they appear, as
specified below. Details on the formatting of the various
header field representations, and some additional processing
instructions are found in <xref
target="detailed.format"/>.
</t>
<t>
An <spanx>indexed representation</spanx> corresponding to an
entry <spanx>present</spanx> in the reference set entails
the following actions:
<list style="symbols">
<t>The entry is removed from the reference set.</t>
</list>
</t>
<t>
An <spanx>indexed representation</spanx> corresponding to an
entry <spanx>not present</spanx> in the reference set
entails the following actions:
<list style="symbols">
<t>If referencing an element of the static table:
<list style="symbols">
<t>The header field corresponding to the referenced
entry is emitted.</t>
<t>The referenced static entry is inserted at the
beginning of the header table.</t>
<t>A reference to this new header table entry is
added to the reference set, unless this new entry
didn't fit in the header table.</t>
</list>
</t>
<t>If referencing an element of the header table:
<list style="symbols">
<t>The header field corresponding to the referenced
entry is emitted.</t>
<t>The referenced header table entry is added to the
reference set.</t>
</list>
</t>
</list>
</t>
<t>
A <spanx>literal representation</spanx> that is <spanx>not
added</spanx> to the header table entails the following
action:
<list style="symbols">
<t>The header field is emitted.</t>
</list>
</t>
<t>
A <spanx>literal representation</spanx> that is
<spanx>added</spanx> to the header table entails the
following actions:
<list style="symbols">
<t>The header field is emitted.</t>
<t>The header field is inserted at the beginning of the
header table.</t>
<t>A reference to the new entry is added to the
reference set, unless this new entry didn't fit in
the header table.</t>
</list>
</t>
</section>
<section title="Reference Set Emission"
anchor="reference.set.emission">
<t>
Once all the representations contained in a header block
have been processed, any header fields included in the
reference set that have not previously been emitted during
the processing of this header block are emitted.
</t>
<t>
After the emission of these remaining header fields, the
header set is complete.
</t>
</section>
</section>
<section title="Header Table Management"
anchor="header.table.management">
<section title="Maximum Table Size"
anchor="maximum.table.size">
<t>
To limit the memory requirements on the decoder side, the
mutable structures used in an encoding context are
constrained in size. These mutable structures are the header
table and the reference set.
</t>
<t>
The size of the header table is bounded by a maximum size
defined by the decoder. The size of the header table MUST
always be lower than or equal to this maximum size.
</t>
<t>
The reference set can only contain references to entries of
the header table, and can't contain references to entries of
the static table. In addition, it can't contain duplicate
references. Therefore, its maximum size is bounded by the
size of the header table.
</t>
<t>
By default, the maximum size of the header table is equal to
the value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE
defined by the decoder (see <xref target="HTTP2"
x:fmt="of" x:rel="#SettingValues"/>). The encoder can
change this maximum size (see <xref
target="encoding.context.update"/>), but it MUST stay
lower than or equal to the value of
SETTINGS_HEADER_TABLE_SIZE.
</t>
<t>
After applying an updated value of the HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE that changes the maximum size of
the header table used by the encoder, the encoder MUST
signal this change via an encoding context update (see <xref
target="encoding.context.update"/>). This encoding context
update MUST occur at the beginning of the first header
block following the SETTINGS frame sent to acknowledge the
application of the updated settings.
</t>
<t>
The size of the header table is the sum of the size of its
entries.
</t>
<t>
The size of an entry is the sum of its name's length in
octets (as defined in <xref
target="string.literal.representation" />), its value's
length in octets (<xref
target="string.literal.representation" />), plus 32.
</t>
<t>
The size of an entry is calculated using the length of the
name and value without any Huffman encoding applied.
</t>
<t>
The additional 32 octets account for overhead associated
with an entry. For example, an entry structure using two
64-bit pointers to reference the name and the value of the
entry, and two 64-bit integers for counting the number of
references to the name and value would have 32 octets of
overhead.
</t>
</section>
<section title="Entry Eviction When Header Table Size Changes"
anchor="entry.eviction">
<t>
Whenever the maximum size for the header table is reduced,
entries are evicted from the end of the header table until
the size of the header table is less than or equal to the
maximum size.
</t>
<t>
Whenever an entry is evicted from the header table, any
reference to that entry from the reference set is removed.
</t>
<t>
The eviction of an entry from the header table causes the
index of the entries in the static table to be reduced by
one.
</t>
</section>
<section title="Entry Eviction when Adding New Entries"
anchor="entry.addition">
<t>
Whenever a new entry is to be added to the header table
entries are evicted from the end of the header table until
the size of the header table is less than or equal to
(maximum size - new entry size), or until the table is
empty.
</t>
<t>
If the representation of the added entry references the name
of an entry in the header table, the referenced name is
cached prior to performing eviction to avoid having the name
inadvertently evicted.
</t>
<t>
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 not
an error to attempt to add an entry that is larger than the
maximum size; an attempt to add an entry larger than the
entire table causes the table to be emptied of all existing
entries.
</t>
</section>
</section>
<section title="Primitive Type Representations"
anchor="low-level.representation">
<t>
HPACK encoding uses two primitive types: unsigned variable
length integers, and strings of octets.
</t>
<section title="Integer representation"
anchor="integer.representation">
<t>
Integers are used to represent name indexes, pair indexes or
string lengths. To allow for optimized processing, an
integer representation always finishes at the end of an
octet.
</t>
<t>
An integer is represented in two parts: a prefix that fills
the current octet and an optional list of octets that are
used if the integer value does not fit within the prefix.
The number of bits of the prefix (called N) is a parameter
of the integer representation.
</t>
<t>
The N-bit prefix allows filling the current octet. If the
value is small enough (strictly less than
2<x:sup>N</x:sup>-1), it is encoded within the N-bit prefix.
Otherwise all the bits of the prefix are set to 1 and the
value is encoded using an unsigned variable length integer
representation (see <eref
target="http://en.wikipedia.org/wiki/Variable-length_quantity"/>).
N is always between 1 and 8 bits. An integer starting at an
octet-boundary will have an 8-bit prefix.
</t>
<figure>
<preamble>
The algorithm to represent an integer I is as follows:
</preamble>
<artwork type = "inline"><![CDATA[
if I < 2^N - 1, encode I on N bits
else
encode (2^N - 1) on N bits
I = I - (2^N - 1)
while I >= 128
encode (I % 128 + 128) on 8 bits
I = I / 128
encode I on 8 bits
]]></artwork>
</figure>
<figure>
<preamble>
For informational purpose, the algorithm to decode an
integer I is as follows:
</preamble>
<artwork type="inline"><![CDATA[
decode I from the next N bits
if I < 2^N - 1, return I
else
M = 0
repeat
B = next octet
I = I + (B & 127) * 2^M
M = M + 7
while B & 128 == 128
return I
]]></artwork>
</figure>
<t>
Examples illustrating the encoding of integers are available
in <xref target="integer.representation.examples"/>.
</t>
<t>
This integer representation allows for values of indefinite
size. It is also possible for an encoder to send a large
number of zero values, which can waste octets and could be
used to overflow integer values. Excessively large integer
encodings - in value or octet length - MUST be treated as a
decoding error. Different limits can be set for each of the
different uses of integers, based on implementation
constraints.
</t>
</section>
<section title="String Literal Representation"
anchor="string.literal.representation">
<t>
Header field names and header field values can be
represented as literal string. A literal string is encoded
as a sequence of octets, either by directly encoding the
literal string's octets, or by using a Huffman code
<xref target="HUFFMAN"/>.
</t>
<figure title="String Literal Representation">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| H | String Length (7+) |
+---+---------------------------+
| String Data (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal string representation contains the following
fields:
<list style="hanging">
<t hangText="H:">
A one bit flag, H, indicating whether or not the
octets of the string are Huffman encoded.
</t>
<t hangText="String Length:">
The number of octets used to encode the string
literal, encoded as an integer with 7-bit prefix
(see <xref target="integer.representation"/>).
</t>
<t hangText="String Data:">
The encoded data of the string literal. If H is
'0', then the encoded data is the raw octets of
the string literal. If H is '1', then the
encoded data is the Huffman encoding of the
string literal.
</t>
</list>
</t>
<t>
String literals which use Huffman encoding are encoded with
the Huffman code defined in <xref target="huffman.code"/>
(see examples in <xref
target="request.examples.with.huffman.coding">Request
Examples with Huffman Coding</xref> and in <xref
target="response.examples.with.huffman.coding">Response
Examples with Huffman Coding</xref>). The encoded data
is the bitwise concatenation of the codes corresponding to
each octet of the string literal.
</t>
<t>
As the Huffman encoded data doesn't always end at an octet
boundary, some padding is inserted after it up to the next
octet boundary. To prevent this padding to be misinterpreted
as part of the string literal, the most significant bits of
code corresponding to the EOS (end-of-string) symbol are
used.
</t>
<t>
Upon decoding, an incomplete code at the end of the
encoded data is to be considered as padding and discarded. A
padding strictly longer than 7 bits MUST be treated as a
decoding error. A padding not corresponding to the most
significant bits of the code for the EOS symbol MUST be
treated as a decoding error. A Huffman encoded string
literal containing the EOS symbol MUST be treated as a
decoding error.
</t>
</section>
</section>
<section title="Binary Format" anchor="detailed.format">
<t>
This section describes the detailed format of each of the
different header field representations, plus the encoding
context update instruction.
</t>
<section title="Indexed Header Field Representation"
anchor="indexed.header.representation">
<t>
An indexed header field representation identifies an entry
in either the header table or the static table.
</t>
<t>
An indexed header field representation can either causes a
header field to be emitted or to be removed from the
reference set, as described in <xref
target="header.representation.processing"/>.
</t>
<figure title="Indexed Header Field">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
]]></artwork>
</figure>
<t>
An indexed header field starts with the '1' 1-bit pattern,
followed by the index of the matching pair, represented as
an integer with a 7-bit prefix.
</t>
<t>
The index value of 0 is not used. It MUST be treated as a
decoding error if found in an indexed header field
representation.
</t>
</section>
<section title="Literal Header Field Representation"
anchor="literal.header.representation">
<t>
A literal header field representation contains a literal
header field value. Header field names are either provided
as a literal or by reference to an existing table entry,
either from the header table or the static table.
</t>
<t>
A literal representation always result in the emission of a
header field when decoded.
</t>
<section title="Literal Header Field with Incremental Indexing"
anchor="literal.header.with.incremental.indexing">
<t>
A literal header field with incremental indexing
representation causes the emission of a header field,
adding it as a new entry to the header table.
</t>
<figure title="Literal Header Field with Incremental Indexing -
Indexed Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<figure title="Literal Header Field with Incremental Indexing -
New Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal header field with incremental indexing
representation starts with the '01' 2-bit pattern.
</t>
<t>
If the header field name matches the header field name
of an entry stored in the header table or the static
table, the header field name can be represented using
the index of that entry. In this case, the index of the
entry is represented as an integer with a 6-bit prefix
(see <xref target="integer.representation"/>). This
value is always non-zero.
</t>
<t>
Otherwise, the header field name is represented as a
literal. A value 0 is used in place of the 6-bit index,
followed by the header field name (see <xref
target="string.literal.representation" />).
</t>
<t>
Either form of header field name representation is
followed by the header field value represented as a
literal string as described in <xref
target="string.literal.representation" />.
</t>
</section>
<section title="Literal Header Field without Indexing"
anchor="literal.header.without.indexing">
<t>
A literal header field without indexing representation
causes the emission of a header field without altering
the header table.
</t>
<figure title="Literal Header Field without Indexing - Indexed Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<figure title="Literal Header Field without Indexing - New Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal header field without indexing representation
starts with the '0000' 4-bit pattern.
</t>
<t>
If the header field name matches the header field name
of an entry stored in the header table or the static
table, the header field name can be represented using
the index of that entry. In this case, the index of the
entry is represented as an integer with a 4-bit prefix
(see <xref target="integer.representation"/>). This
value is always non-zero.
</t>
<t>
Otherwise, the header field name is represented as a
literal. A value 0 is used in place of the 4-bit index,
followed by the header field name (see <xref
target="string.literal.representation" />).
</t>
<t>
Either form of header field name representation is
followed by the header field value represented as a
literal string as described in <xref
target="string.literal.representation" />.
</t>
</section>
<section title="Literal Header Field Never Indexed"
anchor="literal.header.never.indexed">
<t>
A literal header field never indexed representation
causes the emission of a header field without altering
the header table. Intermediaries MUST use the same
representation for encoding this header field.
</t>
<figure title="Literal Header Field Never Indexed - Indexed Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<figure title="Literal Header Field Never Indexed - New Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal header field never indexed representation
starts with the '0001' 4-bit pattern.
</t>
<t>
When a header field is represented as a literal header
field never indexed, it MUST always be encoded with
this specific literal representation. In particular,
when a peer sends a header field that it received
represented as a literal header field never indexed, it
MUST use the same representation to forward this header
field.
</t>
<t>
This representation is intended for protecting header
field values that are not to be put at risk by
compressing them (see <xref
target="compression.based.attacks"/> for more details).
</t>
<t>
The encoding of the representation is identical to the
literal header field without indexing
(see <xref target="literal.header.without.indexing"/>).
</t>
</section>
</section>
<section title="Encoding Context Update"
anchor="encoding.context.update">
<t>
An encoding context update causes the immediate application
of a change to the encoding context.
</t>
<figure title="Context Update">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | F | ... |
+---+---------------------------+
]]></artwork>
</figure>
<t>
An encoding context update starts with the '001' 3-bit
pattern.
</t>
<t>
It is followed by a flag specifying the type of the change,
and by any data necessary to describe the change itself.
</t>
<figure title="Reference Set Emptying">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 1 | 0 |
+---+---------------------------+
]]></artwork>
</figure>
<t>
The flag bit being set to '1' signals that the reference set
is emptied. The remaining bits MUST be set to '0', non-zero
values MUST be treated as a decoding error.
</t>
<figure title="Maximum Header Table Size Change">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 0 | Max size (4+) |
+---+---------------------------+
]]></artwork>
</figure>
<t>
The flag bit being set to '0' signals that a change to the
maximum size of the header table. This new maximum size MUST
be lower than or equal to the maximum set by the decoder.
That is, the value of the HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE, defined in <xref target="HTTP2"
x:fmt="of" x:rel="#SettingValues"/>.
</t>
<t>
The new maximum size is encoded as an integer with a 4-bit
prefix (see <xref target="integer.representation"/>).
</t>
<t>
Reducing the maximum size of the header table causes entries
to be evicted (see <xref target="entry.eviction"/>).
</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>
This section describes potential areas of security concern
with HPACK:
<list style="symbols">
<t>
Use of compression as a length-based oracle for
verifying guesses about secrets that are compressed
into a shared compression context.
</t>
<t>
Denial of service resulting from exhausting processing
or memory capacity at a decoder.
</t>
</list>
</t>
<section title="Probing Header Table State"
anchor="compression.based.attacks">
<t>
HPACK reduces the length of header field encodings 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.
</t>
<t>
The compression context used to encode header fields can be
probed by an attacker that has the following capabilities:
to define header fields to be encoded and transmitted; and
to observe the length of those fields once they are encoded.
This allows an attacker to adaptively modify requests in
order to confirm guesses about the header 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.
</t>
<t>
This is possible because while TLS provides confidentiality
protection for content, it only provides a limited amount of
protection for the length of that content.
<list style="hanging">
<t hangText="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.
</t>
</list>
</t>
<t>
Attacks like <xref target="CRIME"/> demonstrated the
existence of these general attacker capabilities. The
specific attack exploited the fact that <xref
target="DEFLATE"/> 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 constant time attack.
</t>
<section title="Applicability to HPACK and HTTP">
<t>
HPACK mitigates but does not completely prevent attacks
modelled on <xref target="CRIME"/> by forcing a guess to
match an entire header field value, rather than
individual characters. An attacker can only learn
whether a guess is correct or not, so is reduced to a
brute force guess for the header field values.
</t>
<t>
The viability of recovering specific header field values
therefore 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.
</t>
<t>
Attacks of this nature are possible any time that two
mutually distrustful entities control requests or
responses that are placed onto a single HTTP/2
connection. If the shared HPACK compressor permits one
entity to add entries to the header table, and the other
to access those entries, then the state of the table can
be learned.
</t>
<t>
Having requests or responses from mutually distrustful
entities occurs when an intermediary either:
<list style="symbols">
<t>
sends requests from multiple clients on a single
connection toward an origin server, or
</t>
<t>
takes responses from multiple origin servers and
places them on a shared connection toward a
client.
</t>
</list>
Web browsers also need to assume that requests made on
the same connection by different <xref
target="ORIGIN">web origins</xref> are made by mutually
distrustful entities.
</t>
</section>
<section title="Mitigation">
<t>
Users of HTTP that require confidentiality for header
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.
</t>
<t>
Rather than impose constraints on users of HTTP, an
implementation of HPACK can instead constrain how
compression is applied in order to limit the potential
for header table probing.
</t>
<t>
An ideal solution segregates access to the header table
based on the entity that is constructing header fields.
Header field values that are added to the table are
attributed to an entity, and only the entity that
created an particular value can extract that value.
</t>
<t>
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.
</t>
<t>
An encoder without good knowledge of the provenance of
header fields might instead introduce a penalty for bad
guesses, such that attempts to guess a header field
value results in all values being removed from
consideration in all future requests, effectively
preventing further guesses.
<list style="hanging">
<t hangText="Note:">
Simply removing values from the header 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 web sites can easily
force an image to be loaded, thereby refreshing
the entry in the header table.
</t>
</list>
</t>
<t>
This response might be made inversely proportional to
the length of the header field. Marking as inaccessible
might occur for shorter values more quickly or with
higher probability than for longer values.
</t>
<t>
Implementations might also choose to protect certain
header fields that are known to be highly valued, such
as the Authorization or Cookie header fields, by
disabling or further limiting compression.
</t>
</section>
<section title="Never Indexed Literals">
<t>
Refusing to generate an indexed representation for a
header field is only effective if compression is avoided
on all hops. The <xref
target="literal.header.never.indexed">never indexed
literal</xref> 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 the never indexed literal as an indexed
representation.
</t>
</section>
</section>
<section title="Static Huffman Encoding">
<t>
There is currently no known threat taking advantage of the
use of a fixed Huffman encoding. A study has shown that
using a fixed 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 <xref
target="PETAL"/>).
</t>
</section>
<section title="Memory Consumption">
<t>
An attacker can try to cause an endpoint to exhaust its
memory. HPACK is designed to limit both the peak and state
amounts of memory allocated by an endpoint.
</t>
<t>
The amount of memory used by the compressor state is limited
by the decoder using the value of the HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE (see <xref target="HTTP2"
x:fmt="of" x:rel="#SettingValues"/>). This limit takes into
account both the size of the data stored in the header
table, plus a small allowance for overhead.
</t>
<t>
A decoder can limit the amount of state memory used by
setting an appropriate value for the setting
SETTINGS_HEADER_TABLE_SIZE. An encoder can limit the amount
of state memory it uses by signaling lower header table size
than the decoder allows (see <xref
target="encoding.context.update"/>).
</t>
<t>
The amount of temporary memory consumed by an encoder or
decoder can be limited by processing header fields
sequentially. An implementation does not need to retain a
complete set of header fields. Note however that it might
be necessary for an application to retain a complete header
set for other reasons; even though HPACK does not force this
to occur, application constraints might make this necessary.
</t>
</section>
<section title="Implementation Limits">
<t>
An implementation of HPACK needs to ensure that large values
for integers, long encoding for integers, or long string
literals do not create security weaknesses.
</t>
<t>
An implementation has to set a limit for the values it
accepts for integers, as well as for the encoded length (see
<xref target="integer.representation"/>). In the same way,
it has to set a limit to the length it accepts for string
literals (see <xref
target="string.literal.representation"/>).
</t>
</section>
</section>
<section title="Acknowledgements">
<t>
This document includes substantial input from the following
individuals:
<list style="symbols">
<t>
Mike Bishop, Jeff Pinner, Julian Reschke, Martin Thomson
(substantial editorial contributions).
</t>
<t>
Johnny Graettinger (Huffman code statistics).
</t>
</list>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="HTTP2">
<front>
<title>Hypertext Transfer Protocol version 2</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe">
<organization>Twist</organization>
</author>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google</organization>
</author>
<author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor">
<organization>Mozilla</organization>
</author>
<date month="June" year="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-http2-13"/>
<x:source href="refs/draft-ietf-httpbis-http2-13.xml"/>
</reference>
<reference anchor="RFC7230">
<front>
<title>
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and
Routing
</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="June" year="2014" />
</front>
<seriesInfo name="RFC" value="7230" />
<x:source href="refs/rfc7230.xml"
basename="https://svn.tools.ietf.org/svn/wg/httpbis/specs/rfc7230"/>
</reference>
<reference anchor="RFC2119">
<front>
<title>
Key words for use in RFCs to Indicate Requirement Levels
</title>
<author initials="S." surname="Bradner" fullname="Scott Bradner">
<organization>Harvard University</organization>
<address><email>sob@harvard.edu</email></address>
</author>
<date month="March" year="1997"/>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="SPDY">
<front>
<title>SPDY Protocol</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe">
<organization>Twist</organization>
</author>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google</organization>
</author>
<date month="February" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-mbelshe-httpbis-spdy-00"/>
</reference>
<reference anchor="ORIGIN">
<front>
<title>The Web Origin Concept</title>
<author initials="A." surname="Barth" fullname="Adam Barth"/>
<date month="December" year="2011"/>
</front>
<seriesInfo name="RFC" value="6454"/>
</reference>
<reference anchor="DEFLATE">
<front>
<title>DEFLATE Compressed Data Format Specification version 1.3</title>
<author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
<organization>Aladdin Enterprises</organization>
</author>
<date month="May" year="1996"/>
</front>
<seriesInfo name="RFC" value="1951"/>
</reference>
<reference anchor="CRIME" target="https://docs.google.com/a/twist.com/presentation/d/11eBmGiHbYcHR9gL5nDyZChu_-lCa2GizeuOfaLU2HOU/edit#slide=id.g1eb6c1b5_3_6">
<front>
<title>The CRIME Attack</title>
<author initials="J." surname="Rizzo" fullname="Juliano Rizzo"></author>
<author initials="T." surname="Duong" fullname="Thai Duong"></author>
<date month="September" year="2012"/>
</front>
</reference>
<reference anchor="SPDY-DESC-1" target="https://www.ietf.org/proceedings/83/slides/slides-83-httpbis-3">
<front>
<title>
IETF83: SPDY and What to Consider for HTTP/2.0
</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe">
</author>
<date month="March" year="2012"/>
</front>
</reference>
<reference anchor="SPDY-DESC-2" target="https://bitsup.blogspot.com/2011/09/spdy-what-i-like-about-you.html">
<front>
<title>SPDY: What I Like About You</title>
<author initials="P." surname="McManus" fullname="Patrick McManus">
</author>
<date month="September" year="2011"/>
</front>
</reference>
<reference anchor="HUFFMAN" target="https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4051119">
<front>
<title>A Method for the Construction of Minimum Redundancy
Codes</title>
<author surname="Huffman" initials="D. A." fullname="David A. Huffman"/>
<date month="September" year="1952"/>
</front>
<seriesInfo name="Proceedings of the Institute of Radio Engineers" value="Volume 40, Number 9, pp. 1098-1101"/>
</reference>
<reference anchor="CANONICAL" target="https://dl.acm.org/citation.cfm?id=363991">
<front>
<title>Generating a canonical prefix encoding</title>
<author surname="Schwartz" initials="E. S." fullname="Eugene. S. Schwartz"/>
<author surname="Kallick" initials="B." fullname="Bruce Kallick"/>
<date month="March" year="1964"/>
</front>
<seriesInfo name="Communications of the ACM" value="Volume 7 Issue 3, pp. 166-169"/>
</reference>
<reference anchor="PETAL" target="http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-13-106.pdf">
<front>
<title>PETAL: Preset Encoding Table Information
Leakage</title>
<author surname="Tan" initials="J." fullname="Jiaqi Tan"/>
<author surname="Nahata" initials="J."
fullname="Jayvardhan Nahata"/>
<date month="April" year="2013"/>
</front>
</reference>
</references>
<section title="Change Log (to be removed by RFC Editor before publication">
<section title="Since draft-ietf-httpbis-header-compression-07"
anchor="changes.since.draft-ietf-httpbis-header-compression-07">
<t><list style="symbols">
<t>
Removed old text on index value of 0.
</t>
<t>
Added clarification for signalling of maximum table size
after a SETTINGS_HEADER_TABLE_SIZE update.
</t>
<t>
Rewrote security considerations.
</t>
<t>
Many editorial clarifications or improvements.
</t>
<t>
Added convention section.
</t>
<t>
Reworked document's outline.
</t>
<t>
Updated static table. Entry 16 has now "gzip, deflate"
for value.
</t>
<t>
Updated Huffman table, using data set provided by
Google.
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-06"
anchor="changes.since.draft-ietf-httpbis-header-compression-06">
<t><list style="symbols">
<t>
Updated format to include literal headers that must
never be compressed.
</t>
<t>
Updated security considerations.
</t>
<t>
Moved integer encoding examples to the appendix.
</t>
<t>
Updated Huffman table.
</t>
<t>
Updated static header table (adding and removing status
values).
</t>
<t>
Updated examples.
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-05"
anchor="changes.since.draft-ietf-httpbis-header-compression-05">
<t><list style="symbols">
<t>
Regenerated examples.
</t>
<t>
Only one Huffman table for requests and responses.
</t>
<t>
Added maximum size for header table, independent of
SETTINGS_HEADER_TABLE_SIZE.
</t>
<t>
Added pseudo-code for integer decoding.
</t>
<t>
Improved examples (removing unnecessary removals).
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-04"
anchor="changes.since.draft-ietf-httpbis-header-compression-04">
<t><list style="symbols">
<t>
Updated examples: take into account changes in the spec,
and show more features.
</t>
<t>
Use 'octet' everywhere instead of having both 'byte' and
'octet'.
</t>
<t>
Added reference set emptying.
</t>
<t>
Editorial changes and clarifications.
</t>
<t>
Added "host" header to the static table.
</t>
<t>
Ordering for list of values (either NULL- or
comma-separated).
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-03"
anchor="changes.since.draft-ietf-httpbis-header-compression-03">
<t><list style="symbols">
<t>
A large number of editorial changes; changed the
description of evicting/adding new entries.
</t>
<t>
Removed substitution indexing
</t>
<t>
Changed 'initial headers' to 'static headers', as per
issue #258
</t>
<t>
Merged 'request' and 'response' static headers, as per
issue #259
</t>
<t>
Changed text to indicate that new headers are added at
index 0 and expire from the largest index, as per issue
#233
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-02">
<t><list style="symbols">
<t>
Corrected error in integer encoding pseudocode.
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-01">
<t>
<list style="symbols">
<t>
Refactored of Header Encoding Section: split
definitions and processing rule.
</t>
<t>
Backward incompatible change: Updated reference set
management as per issue #214. This changes how the
interaction between the reference set and eviction
works. This also changes the working of the
reference set in some specific cases.
</t>
<t>
Backward incompatible change: modified initial
header list, as per issue #188.
</t>
<t>
Added example of 32 octets entry structure (issue
#191).
</t>
<t>
Added Header Set Completion section. Reflowed some
text. Clarified some writing which was akward.
Added text about duplicate header entry encoding.
Clarified some language w.r.t Header Set. Changed
x-my-header to mynewheader. Added text in the
HeaderEmission section indicating that the
application may also be able to free up memory more
quickly. Added information in Security
Considerations section.
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-00">
<t>
<list>
<t>Fixed bug/omission in integer representation
algorithm.</t>
<t>Changed the document title.</t>
<t>Header matching text rewritten.</t>
<t>Changed the definition of header emission.</t>
<t>Changed the name of the setting which dictates how
much memory the compression context should use.</t>
<t>Removed "specific use cases" section</t>
<t>Corrected erroneous statement about what index can be
contained in one octet</t>
<t>Added descriptions of opcodes</t>
<t>Removed security claims from introduction.</t>
</list>
</t>
</section>
</section>
<section title="Static Table" anchor="static.table">
<t>
The static table consists of an unchangeable ordered list of
(name, value) pairs. The first entry in the table is always
represented by the index len(header table) + 1, and the last
entry in the table is represented by the index len(header table)
+ len(static table).
</t>
<t>
The static table was created by listing the most common
header fields that are valid for messages exchanged inside a
HTTP/2 connection. For header fields with a few frequent
values, an entry was added for each of these frequent values.
For other header fields, an entry was added with an empty
value.
</t>
<t>
The following table lists the pre-defined header fields that
make-up the static table.
</t>
<texttable title="Static Table Entries"
anchor="static.table.entries">
<!-- An easy way to renumber these in vim, with mark a and b
delimiting the table entries:
:let @a=1 | 'a,'bs/>[0-9 ][0-9 ]*</\='>'.(@a+setreg('a',@a+1)).'<'/
-->
<ttcol>Index</ttcol>
<ttcol>Header Name</ttcol>
<ttcol>Header Value</ttcol>
<c>1</c><c>:authority</c><c></c>
<c>2</c><c>:method</c><c>GET</c>
<c>3</c><c>:method</c><c>POST</c>
<c>4</c><c>:path</c><c>/</c>
<c>5</c><c>:path</c><c>/index.html</c>
<c>6</c><c>:scheme</c><c>http</c>
<c>7</c><c>:scheme</c><c>https</c>
<c>8</c><c>:status</c><c>200</c>
<c>9</c><c>:status</c><c>204</c>
<c>10</c><c>:status</c><c>206</c>
<c>11</c><c>:status</c><c>304</c>
<c>12</c><c>:status</c><c>400</c>
<c>13</c><c>:status</c><c>404</c>
<c>14</c><c>:status</c><c>500</c>
<c>15</c><c>accept-charset</c><c></c>
<c>16</c><c>accept-encoding</c><c>gzip, deflate</c>
<c>17</c><c>accept-language</c><c></c>
<c>18</c><c>accept-ranges</c><c></c>
<c>19</c><c>accept</c><c></c>
<c>20</c><c>access-control-allow-origin</c><c></c>
<c>21</c><c>age</c><c></c>
<c>22</c><c>allow</c><c></c>
<c>23</c><c>authorization</c><c></c>
<c>24</c><c>cache-control</c><c></c>
<c>25</c><c>content-disposition</c><c></c>
<c>26</c><c>content-encoding</c><c></c>
<c>27</c><c>content-language</c><c></c>
<c>28</c><c>content-length</c><c></c>
<c>29</c><c>content-location</c><c></c>
<c>30</c><c>content-range</c><c></c>
<c>31</c><c>content-type</c><c></c>
<c>32</c><c>cookie</c><c></c>
<c>33</c><c>date</c><c></c>
<c>34</c><c>etag</c><c></c>
<c>35</c><c>expect</c><c></c>
<c>36</c><c>expires</c><c></c>
<c>37</c><c>from</c><c></c>
<c>38</c><c>host</c><c></c>
<c>39</c><c>if-match</c><c></c>
<c>40</c><c>if-modified-since</c><c></c>
<c>41</c><c>if-none-match</c><c></c>
<c>42</c><c>if-range</c><c></c>
<c>43</c><c>if-unmodified-since</c><c></c>
<c>44</c><c>last-modified</c><c></c>
<c>45</c><c>link</c><c></c>
<c>46</c><c>location</c><c></c>
<c>47</c><c>max-forwards</c><c></c>
<c>48</c><c>proxy-authenticate</c><c></c>
<c>49</c><c>proxy-authorization</c><c></c>
<c>50</c><c>range</c><c></c>
<c>51</c><c>referer</c><c></c>
<c>52</c><c>refresh</c><c></c>
<c>53</c><c>retry-after</c><c></c>
<c>54</c><c>server</c><c></c>
<c>55</c><c>set-cookie</c><c></c>
<c>56</c><c>strict-transport-security</c><c></c>
<c>57</c><c>transfer-encoding</c><c></c>
<c>58</c><c>user-agent</c><c></c>
<c>59</c><c>vary</c><c></c>
<c>60</c><c>via</c><c></c>
<c>61</c><c>www-authenticate</c><c></c>
</texttable>
<t>
<xref target="static.table.entries"/> gives the index of each
entry in the static table. The full index of each entry, to be
used for encoding a reference to this entry, is computed by
adding the number of entries in the header table to this index.
</t>
</section>
<section title="Huffman Code" anchor="huffman.code">
<t>
The following Huffman code is used when encoding string literals
with a Huffman coding (see <xref
target="string.literal.representation"/>).
</t>
<t>
This Huffman code was generated from statistics obtained on a
large sample of HTTP headers. It is a canonical Huffman code
<xref target="CANONICAL"/> with some tweaking to ensure that no
symbol has a unique code length.
</t>
<t>
Each row in the table defines the code used to represent a
symbol:
<list style="hanging">
<t hangText="sym:">
The symbol to be represented. It is the decimal value of
an octet, possibly prepended with its ASCII
representation. A specific symbol, "EOS", is used to
indicate the end of a string literal.
</t>
<t hangText="code as bits:">
The Huffman code for the symbol represented as a base-2
integer, aligned on the most significant bit (MSB).
</t>
<t hangText="code as hex:">
The Huffman code for the symbol, represented as a
hexadecimal integer, aligned on the least significant
bit (LSB).
</t>
<t hangText="len:">
The number of bits for the code representing the symbol.
</t>
</list>
</t>
<t>
As an example, the code for the symbol 47 (corresponding to the
ASCII character "/") consists in the 6 bits "0", "1", "1", "0",
"0", "0". This corresponds to the value 0x18 (in hexadecimal)
encoded on 6 bits.
</t>
<figure>
<artwork><![CDATA[
code
code as bits as hex len
sym aligned to MSB aligned in
to LSB bits
( 0) |11111111|11000 1ff8 [13]
( 1) |11111111|11111111|1011000 7fffd8 [23]
( 2) |11111111|11111111|11111110|0010 fffffe2 [28]
( 3) |11111111|11111111|11111110|0011 fffffe3 [28]
( 4) |11111111|11111111|11111110|0100 fffffe4 [28]
( 5) |11111111|11111111|11111110|0101 fffffe5 [28]
( 6) |11111111|11111111|11111110|0110 fffffe6 [28]
( 7) |11111111|11111111|11111110|0111 fffffe7 [28]
( 8) |11111111|11111111|11111110|1000 fffffe8 [28]
( 9) |11111111|11111111|11101010 ffffea [24]
( 10) |11111111|11111111|11111111|111100 3ffffffc [30]
( 11) |11111111|11111111|11111110|1001 fffffe9 [28]
( 12) |11111111|11111111|11111110|1010 fffffea [28]
( 13) |11111111|11111111|11111111|111101 3ffffffd [30]
( 14) |11111111|11111111|11111110|1011 fffffeb [28]
( 15) |11111111|11111111|11111110|1100 fffffec [28]
( 16) |11111111|11111111|11111110|1101 fffffed [28]
( 17) |11111111|11111111|11111110|1110 fffffee [28]
( 18) |11111111|11111111|11111110|1111 fffffef [28]
( 19) |11111111|11111111|11111111|0000 ffffff0 [28]
( 20) |11111111|11111111|11111111|0001 ffffff1 [28]
( 21) |11111111|11111111|11111111|0010 ffffff2 [28]
( 22) |11111111|11111111|11111111|111110 3ffffffe [30]
( 23) |11111111|11111111|11111111|0011 ffffff3 [28]
( 24) |11111111|11111111|11111111|0100 ffffff4 [28]
( 25) |11111111|11111111|11111111|0101 ffffff5 [28]
( 26) |11111111|11111111|11111111|0110 ffffff6 [28]
( 27) |11111111|11111111|11111111|0111 ffffff7 [28]
( 28) |11111111|11111111|11111111|1000 ffffff8 [28]
( 29) |11111111|11111111|11111111|1001 ffffff9 [28]
( 30) |11111111|11111111|11111111|1010 ffffffa [28]
( 31) |11111111|11111111|11111111|1011 ffffffb [28]
' ' ( 32) |010100 14 [ 6]
'!' ( 33) |11111110|00 3f8 [10]
'"' ( 34) |11111110|01 3f9 [10]
'#' ( 35) |11111111|1010 ffa [12]
'$' ( 36) |11111111|11001 1ff9 [13]
'%' ( 37) |010101 15 [ 6]
'&' ( 38) |11111000 f8 [ 8]
''' ( 39) |11111111|010 7fa [11]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111110|11 3fb [10]
'*' ( 42) |11111001 f9 [ 8]
'+' ( 43) |11111111|011 7fb [11]
',' ( 44) |11111010 fa [ 8]
'-' ( 45) |010110 16 [ 6]
'.' ( 46) |010111 17 [ 6]
'/' ( 47) |011000 18 [ 6]
'0' ( 48) |00000 0 [ 5]
'1' ( 49) |00001 1 [ 5]
'2' ( 50) |00010 2 [ 5]
'3' ( 51) |011001 19 [ 6]
'4' ( 52) |011010 1a [ 6]
'5' ( 53) |011011 1b [ 6]
'6' ( 54) |011100 1c [ 6]
'7' ( 55) |011101 1d [ 6]
'8' ( 56) |011110 1e [ 6]
'9' ( 57) |011111 1f [ 6]
':' ( 58) |1011100 5c [ 7]
';' ( 59) |11111011 fb [ 8]
'<' ( 60) |11111111|1111100 7ffc [15]
'=' ( 61) |100000 20 [ 6]
'>' ( 62) |11111111|1011 ffb [12]
'?' ( 63) |11111111|00 3fc [10]
'@' ( 64) |11111111|11010 1ffa [13]
'A' ( 65) |100001 21 [ 6]
'B' ( 66) |1011101 5d [ 7]
'C' ( 67) |1011110 5e [ 7]
'D' ( 68) |1011111 5f [ 7]
'E' ( 69) |1100000 60 [ 7]
'F' ( 70) |1100001 61 [ 7]
'G' ( 71) |1100010 62 [ 7]
'H' ( 72) |1100011 63 [ 7]
'I' ( 73) |1100100 64 [ 7]
'J' ( 74) |1100101 65 [ 7]
'K' ( 75) |1100110 66 [ 7]
'L' ( 76) |1100111 67 [ 7]
'M' ( 77) |1101000 68 [ 7]
'N' ( 78) |1101001 69 [ 7]
'O' ( 79) |1101010 6a [ 7]
'P' ( 80) |1101011 6b [ 7]
'Q' ( 81) |1101100 6c [ 7]
'R' ( 82) |1101101 6d [ 7]
'S' ( 83) |1101110 6e [ 7]
'T' ( 84) |1101111 6f [ 7]
'U' ( 85) |1110000 70 [ 7]
'V' ( 86) |1110001 71 [ 7]
'W' ( 87) |1110010 72 [ 7]
'X' ( 88) |11111100 fc [ 8]
'Y' ( 89) |1110011 73 [ 7]
'Z' ( 90) |11111101 fd [ 8]
'[' ( 91) |11111111|11011 1ffb [13]
'\' ( 92) |11111111|11111110|000 7fff0 [19]
']' ( 93) |11111111|11100 1ffc [13]
'^' ( 94) |11111111|111100 3ffc [14]
'_' ( 95) |100010 22 [ 6]
'`' ( 96) |11111111|1111101 7ffd [15]
'a' ( 97) |00011 3 [ 5]
'b' ( 98) |100011 23 [ 6]
'c' ( 99) |00100 4 [ 5]
'd' (100) |100100 24 [ 6]
'e' (101) |00101 5 [ 5]
'f' (102) |100101 25 [ 6]
'g' (103) |100110 26 [ 6]
'h' (104) |100111 27 [ 6]
'i' (105) |00110 6 [ 5]
'j' (106) |1110100 74 [ 7]
'k' (107) |1110101 75 [ 7]
'l' (108) |101000 28 [ 6]
'm' (109) |101001 29 [ 6]
'n' (110) |101010 2a [ 6]
'o' (111) |00111 7 [ 5]
'p' (112) |101011 2b [ 6]
'q' (113) |1110110 76 [ 7]
'r' (114) |101100 2c [ 6]
's' (115) |01000 8 [ 5]
't' (116) |01001 9 [ 5]
'u' (117) |101101 2d [ 6]
'v' (118) |1110111 77 [ 7]
'w' (119) |1111000 78 [ 7]
'x' (120) |1111001 79 [ 7]
'y' (121) |1111010 7a [ 7]
'z' (122) |1111011 7b [ 7]
'{' (123) |11111111|1111110 7ffe [15]
'|' (124) |11111111|100 7fc [11]
'}' (125) |11111111|111101 3ffd [14]
'~' (126) |11111111|11101 1ffd [13]
(127) |11111111|11111111|11111111|1100 ffffffc [28]
(128) |11111111|11111110|0110 fffe6 [20]
(129) |11111111|11111111|010010 3fffd2 [22]
(130) |11111111|11111110|0111 fffe7 [20]
(131) |11111111|11111110|1000 fffe8 [20]
(132) |11111111|11111111|010011 3fffd3 [22]
(133) |11111111|11111111|010100 3fffd4 [22]
(134) |11111111|11111111|010101 3fffd5 [22]
(135) |11111111|11111111|1011001 7fffd9 [23]
(136) |11111111|11111111|010110 3fffd6 [22]
(137) |11111111|11111111|1011010 7fffda [23]
(138) |11111111|11111111|1011011 7fffdb [23]
(139) |11111111|11111111|1011100 7fffdc [23]
(140) |11111111|11111111|1011101 7fffdd [23]
(141) |11111111|11111111|1011110 7fffde [23]
(142) |11111111|11111111|11101011 ffffeb [24]
(143) |11111111|11111111|1011111 7fffdf [23]
(144) |11111111|11111111|11101100 ffffec [24]
(145) |11111111|11111111|11101101 ffffed [24]
(146) |11111111|11111111|010111 3fffd7 [22]
(147) |11111111|11111111|1100000 7fffe0 [23]
(148) |11111111|11111111|11101110 ffffee [24]
(149) |11111111|11111111|1100001 7fffe1 [23]
(150) |11111111|11111111|1100010 7fffe2 [23]
(151) |11111111|11111111|1100011 7fffe3 [23]
(152) |11111111|11111111|1100100 7fffe4 [23]
(153) |11111111|11111110|11100 1fffdc [21]
(154) |11111111|11111111|011000 3fffd8 [22]
(155) |11111111|11111111|1100101 7fffe5 [23]
(156) |11111111|11111111|011001 3fffd9 [22]
(157) |11111111|11111111|1100110 7fffe6 [23]
(158) |11111111|11111111|1100111 7fffe7 [23]
(159) |11111111|11111111|11101111 ffffef [24]
(160) |11111111|11111111|011010 3fffda [22]
(161) |11111111|11111110|11101 1fffdd [21]
(162) |11111111|11111110|1001 fffe9 [20]
(163) |11111111|11111111|011011 3fffdb [22]
(164) |11111111|11111111|011100 3fffdc [22]
(165) |11111111|11111111|1101000 7fffe8 [23]
(166) |11111111|11111111|1101001 7fffe9 [23]
(167) |11111111|11111110|11110 1fffde [21]
(168) |11111111|11111111|1101010 7fffea [23]
(169) |11111111|11111111|011101 3fffdd [22]
(170) |11111111|11111111|011110 3fffde [22]
(171) |11111111|11111111|11110000 fffff0 [24]
(172) |11111111|11111110|11111 1fffdf [21]
(173) |11111111|11111111|011111 3fffdf [22]
(174) |11111111|11111111|1101011 7fffeb [23]
(175) |11111111|11111111|1101100 7fffec [23]
(176) |11111111|11111111|00000 1fffe0 [21]
(177) |11111111|11111111|00001 1fffe1 [21]
(178) |11111111|11111111|100000 3fffe0 [22]
(179) |11111111|11111111|00010 1fffe2 [21]
(180) |11111111|11111111|1101101 7fffed [23]
(181) |11111111|11111111|100001 3fffe1 [22]
(182) |11111111|11111111|1101110 7fffee [23]
(183) |11111111|11111111|1101111 7fffef [23]
(184) |11111111|11111110|1010 fffea [20]
(185) |11111111|11111111|100010 3fffe2 [22]
(186) |11111111|11111111|100011 3fffe3 [22]
(187) |11111111|11111111|100100 3fffe4 [22]
(188) |11111111|11111111|1110000 7ffff0 [23]
(189) |11111111|11111111|100101 3fffe5 [22]
(190) |11111111|11111111|100110 3fffe6 [22]
(191) |11111111|11111111|1110001 7ffff1 [23]
(192) |11111111|11111111|11111000|00 3ffffe0 [26]
(193) |11111111|11111111|11111000|01 3ffffe1 [26]
(194) |11111111|11111110|1011 fffeb [20]
(195) |11111111|11111110|001 7fff1 [19]
(196) |11111111|11111111|100111 3fffe7 [22]
(197) |11111111|11111111|1110010 7ffff2 [23]
(198) |11111111|11111111|101000 3fffe8 [22]
(199) |11111111|11111111|11110110|0 1ffffec [25]
(200) |11111111|11111111|11111000|10 3ffffe2 [26]
(201) |11111111|11111111|11111000|11 3ffffe3 [26]
(202) |11111111|11111111|11111001|00 3ffffe4 [26]
(203) |11111111|11111111|11111011|110 7ffffde [27]
(204) |11111111|11111111|11111011|111 7ffffdf [27]
(205) |11111111|11111111|11111001|01 3ffffe5 [26]
(206) |11111111|11111111|11110001 fffff1 [24]
(207) |11111111|11111111|11110110|1 1ffffed [25]
(208) |11111111|11111110|010 7fff2 [19]
(209) |11111111|11111111|00011 1fffe3 [21]
(210) |11111111|11111111|11111001|10 3ffffe6 [26]
(211) |11111111|11111111|11111100|000 7ffffe0 [27]
(212) |11111111|11111111|11111100|001 7ffffe1 [27]
(213) |11111111|11111111|11111001|11 3ffffe7 [26]
(214) |11111111|11111111|11111100|010 7ffffe2 [27]
(215) |11111111|11111111|11110010 fffff2 [24]
(216) |11111111|11111111|00100 1fffe4 [21]
(217) |11111111|11111111|00101 1fffe5 [21]
(218) |11111111|11111111|11111010|00 3ffffe8 [26]
(219) |11111111|11111111|11111010|01 3ffffe9 [26]
(220) |11111111|11111111|11111111|1101 ffffffd [28]
(221) |11111111|11111111|11111100|011 7ffffe3 [27]
(222) |11111111|11111111|11111100|100 7ffffe4 [27]
(223) |11111111|11111111|11111100|101 7ffffe5 [27]
(224) |11111111|11111110|1100 fffec [20]
(225) |11111111|11111111|11110011 fffff3 [24]
(226) |11111111|11111110|1101 fffed [20]
(227) |11111111|11111111|00110 1fffe6 [21]
(228) |11111111|11111111|101001 3fffe9 [22]
(229) |11111111|11111111|00111 1fffe7 [21]
(230) |11111111|11111111|01000 1fffe8 [21]
(231) |11111111|11111111|1110011 7ffff3 [23]
(232) |11111111|11111111|101010 3fffea [22]
(233) |11111111|11111111|101011 3fffeb [22]
(234) |11111111|11111111|11110111|0 1ffffee [25]
(235) |11111111|11111111|11110111|1 1ffffef [25]
(236) |11111111|11111111|11110100 fffff4 [24]
(237) |11111111|11111111|11110101 fffff5 [24]
(238) |11111111|11111111|11111010|10 3ffffea [26]
(239) |11111111|11111111|1110100 7ffff4 [23]
(240) |11111111|11111111|11111010|11 3ffffeb [26]
(241) |11111111|11111111|11111100|110 7ffffe6 [27]
(242) |11111111|11111111|11111011|00 3ffffec [26]
(243) |11111111|11111111|11111011|01 3ffffed [26]
(244) |11111111|11111111|11111100|111 7ffffe7 [27]
(245) |11111111|11111111|11111101|000 7ffffe8 [27]
(246) |11111111|11111111|11111101|001 7ffffe9 [27]
(247) |11111111|11111111|11111101|010 7ffffea [27]
(248) |11111111|11111111|11111101|011 7ffffeb [27]
(249) |11111111|11111111|11111111|1110 ffffffe [28]
(250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27]
(252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27]
(254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
]]></artwork>
</figure>
</section>
<section title="Examples" anchor="examples">
<t>
A number of examples are worked through here, covering integer
encoding, header field representation, and the encoding of whole
sets of header fields, for both requests and responses, and with
and without Huffman coding.
</t>
<section title="Integer Representation Examples"
anchor="integer.representation.examples">
<t>
This section shows the representation of integer values in
details (see <xref target="integer.representation"/>).
</t>
<section title="Example 1: Encoding 10 Using a 5-bit Prefix"
anchor="integer.representation.example1">
<t>
The value 10 is to be encoded with a 5-bit prefix.
<list style="symbols">
<t>
10 is less than 31 (2<x:sup>5</x:sup> - 1) and
is represented using the 5-bit prefix.
</t>
</list>
</t>
<figure>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits
+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
</section>
<section title="Example 2: Encoding 1337 Using a 5-bit Prefix"
anchor="integer.representation.example2">
<t>
The value I=1337 is to be encoded with a 5-bit prefix.
<list>
<t>
1337 is greater than 31 (2<x:sup>5</x:sup> - 1).
</t>
<t>
<list>
<t>The 5-bit prefix is filled with its max
value (31).</t>
</list>
</t>
<t>I = 1337 - (2<x:sup>5</x:sup> - 1) = 1306.</t>
<t>
<list>
<t>I (1306) is greater than or equal to 128,
the while loop body executes:</t>
<t>
<list>
<t>I % 128 == 26</t>
<t>26 + 128 == 154</t>
<t>154 is encoded in 8 bits as:
10011010</t>
<t>I is set to 10 (1306 / 128 ==
10)</t>
<t>I is no longer greater than or
equal to 128, the while loop
terminates.</t>
</list>
</t>
<t>
I, now 10, is encoded on 8 bits as:
00001010.
</t>
</list>
</t>
<t>The process ends.</t>
</list>
</t>
<figure>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done
+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
</section>
<section title="Example 3: Encoding 42 Starting at an Octet Boundary"
anchor="integer.representation.example3">
<t>
The value 42 is to be encoded starting at an
octet-boundary. This implies that a 8-bit prefix is
used.
<list style="symbols">
<t>
42 is less than 255 (2<x:sup>8</x:sup> - 1) and
is represented using the 8-bit prefix.
</t>
</list>
</t>
<figure>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits
+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
</section>
</section>
<!-- example-start -->
<section title="Header Field Representation Examples"
anchor="header.field.representation.examples">
<t>
This section shows several independent representation examples.
</t>
<section title="Literal Header Field with Indexing">
<t>
The header field representation uses a literal name and a literal
value. The header field is added to the header table.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[custom-key: custom-header]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus
746f 6d2d 6865 6164 6572 | tom-header]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0d | Literal value (len = 13)
6375 7374 6f6d 2d68 6561 6465 72 | custom-header
| -> custom-key: custom-head\
| er]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 55) custom-key: custom-header
Table size: 55]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[custom-key: custom-header]]></artwork>
</figure>
</t>
</section>
<section title="Literal Header Field without Indexing">
<t>
The header field representation uses an indexed name and a literal
value. The header field is not added to the header table.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:path: /sample/path]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[040c 2f73 616d 706c 652f 7061 7468 | ../sample/path]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[04 | == Literal not indexed ==
| Indexed name (idx = 4)
| :path
0c | Literal value (len = 12)
2f73 616d 706c 652f 7061 7468 | /sample/path
| -> :path: /sample/path]]></artwork>
</figure>
</t>
<t>
Header table (after decoding): empty.
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:path: /sample/path]]></artwork>
</figure>
</t>
</section>
<section title="Literal Header Field never Indexed">
<t>
The header field representation uses a literal name and a literal
value. The header field is not added to the header table, and must
use the same representation if re-encoded by an intermediary.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[password: secret]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre
74 | t]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[10 | == Literal never indexed ==
08 | Literal name (len = 8)
7061 7373 776f 7264 | password
06 | Literal value (len = 6)
7365 6372 6574 | secret
| -> password: secret]]></artwork>
</figure>
</t>
<t>
Header table (after decoding): empty.
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[password: secret]]></artwork>
</figure>
</t>
</section>
<section title="Indexed Header Field">
<t>
The header field representation uses an indexed header field, from
the static table. Upon using it, the static table entry is copied
into the header table.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[82 | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[82 | == Indexed - Add ==
| idx = 2
| -> :method: GET]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 42) :method: GET
Table size: 42]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:method: GET]]></artwork>
</figure>
</t>
</section>
<section title="Indexed Header Field from Static Table">
<t>
The header field representation uses an indexed header field, from
the static table. In this example, the HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE is set to 0, therefore, the entry is not
copied into the header table.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[82 | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[82 | == Indexed - Add ==
| idx = 2
| -> :method: GET]]></artwork>
</figure>
</t>
<t>
Header table (after decoding): empty.
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:method: GET]]></artwork>
</figure>
</t>
</section>
</section>
<section title="Request Examples without Huffman Coding"
anchor="request.examples.without.huffman.coding">
<t>
This section shows several consecutive header sets, corresponding to
HTTP requests, on the same connection.
</t>
<section title="First Request">
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[8287 8644 0f77 7777 2e65 7861 6d70 6c65 | ...D.www.example
2e63 6f6d | .com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
44 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
0f | Literal value (len = 15)
7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
| -> :authority: www.example\
| .com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 57) :authority: www.example.com
[ 2] (s = 38) :path: /
[ 3] (s = 43) :scheme: http
[ 4] (s = 42) :method: GET
Table size: 180]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Request">
<t>
This request takes advantage of the differential encoding of header
sets.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] :authority: www.example.com
[ 2] :path: /
[ 3] :scheme: http
[ 4] :method: GET]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[5c08 6e6f 2d63 6163 6865 | \.no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[5c | == Literal indexed ==
| Indexed name (idx = 28)
| cache-control
08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | no-cache
| -> cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
[ 3] (s = 38) :path: /
[ 4] (s = 43) :scheme: http
[ 5] (s = 42) :method: GET
Table size: 233]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[cache-control: no-cache
:authority: www.example.com
:path: /
:scheme: http
:method: GET]]></artwork>
</figure>
</t>
</section>
<section title="Third Request">
<t>
This request has not enough headers in common with the previous
request to take advantage of the differential encoding. Therefore,
the reference set is emptied before encoding the header fields.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] cache-control: no-cache
[ 2] :authority: www.example.com
[ 3] :path: /
[ 4] :scheme: http
[ 5] :method: GET]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[3085 8c8b 8440 0a63 7573 746f 6d2d 6b65 | 0....@.custom-ke
790c 6375 7374 6f6d 2d76 616c 7565 | y.custom-value]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[30 | == Empty reference set ==
| idx = 0
| flag = 1
85 | == Indexed - Add ==
| idx = 5
| -> :method: GET
8c | == Indexed - Add ==
| idx = 12
| -> :scheme: https
8b | == Indexed - Add ==
| idx = 11
| -> :path: /index.html
84 | == Indexed - Add ==
| idx = 4
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0c | Literal value (len = 12)
6375 7374 6f6d 2d76 616c 7565 | custom-value
| -> custom-key: custom-valu\
| e]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 48) :path: /index.html
[ 3] (s = 44) :scheme: https
[ 4] (s = 53) cache-control: no-cache
[ 5] (s = 57) :authority: www.example.com
[ 6] (s = 38) :path: /
[ 7] (s = 43) :scheme: http
[ 8] (s = 42) :method: GET
Table size: 379]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
</section>
</section>
<section title="Request Examples with Huffman Coding"
anchor="request.examples.with.huffman.coding">
<t>
This section shows the same examples as the previous section, but using
Huffman encoding for the literal values.
</t>
<section title="First Request">
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[8287 8644 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...D......:k....
ff | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
44 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
8c | Literal value (len = 15)
| Huffman encoded:
f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k.....
| Decoded:
| www.example.com
| -> :authority: www.example\
| .com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 57) :authority: www.example.com
[ 2] (s = 38) :path: /
[ 3] (s = 43) :scheme: http
[ 4] (s = 42) :method: GET
Table size: 180]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Request">
<t>
This request takes advantage of the differential encoding of header
sets.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] :authority: www.example.com
[ 2] :path: /
[ 3] :scheme: http
[ 4] :method: GET]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[5c86 a8eb 1064 9cbf | \....d..]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[5c | == Literal indexed ==
| Indexed name (idx = 28)
| cache-control
86 | Literal value (len = 8)
| Huffman encoded:
a8eb 1064 9cbf | ...d..
| Decoded:
| no-cache
| -> cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
[ 3] (s = 38) :path: /
[ 4] (s = 43) :scheme: http
[ 5] (s = 42) :method: GET
Table size: 233]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[cache-control: no-cache
:authority: www.example.com
:path: /
:scheme: http
:method: GET]]></artwork>
</figure>
</t>
</section>
<section title="Third Request">
<t>
This request has not enough headers in common with the previous
request to take advantage of the differential encoding. Therefore,
the reference set is emptied before encoding the header fields.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] cache-control: no-cache
[ 2] :authority: www.example.com
[ 3] :path: /
[ 4] :scheme: http
[ 5] :method: GET]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[3085 8c8b 8440 8825 a849 e95b a97d 7f89 | 0....@.%.I.[.}..
25a8 49e9 5bb8 e8b4 bf | %.I.[....]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[30 | == Empty reference set ==
| idx = 0
| flag = 1
85 | == Indexed - Add ==
| idx = 5
| -> :method: GET
8c | == Indexed - Add ==
| idx = 12
| -> :scheme: https
8b | == Indexed - Add ==
| idx = 11
| -> :path: /index.html
84 | == Indexed - Add ==
| idx = 4
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
88 | Literal name (len = 10)
| Huffman encoded:
25a8 49e9 5ba9 7d7f | %.I.[.}.
| Decoded:
| custom-key
89 | Literal value (len = 12)
| Huffman encoded:
25a8 49e9 5bb8 e8b4 bf | %.I.[....
| Decoded:
| custom-value
| -> custom-key: custom-valu\
| e]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 48) :path: /index.html
[ 3] (s = 44) :scheme: https
[ 4] (s = 53) cache-control: no-cache
[ 5] (s = 57) :authority: www.example.com
[ 6] (s = 38) :path: /
[ 7] (s = 43) :scheme: http
[ 8] (s = 42) :method: GET
Table size: 379]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
</section>
</section>
<section title="Response Examples without Huffman Coding"
anchor="response.examples.without.huffman.coding">
<t>
This section shows several consecutive header sets, corresponding to
HTTP responses, on the same connection. The HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing
some evictions to occur.
</t>
<section title="First Response">
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[4803 3330 3259 0770 7269 7661 7465 631d | H.302Y.privatec.
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 5471 1768 | 20:13:21 GMTq.h
7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
6c65 2e63 6f6d | le.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 32 | 302
| -> :status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
71 | == Literal indexed ==
| Indexed name (idx = 49)
| location
17 | Literal value (len = 23)
6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
706c 652e 636f 6d | ple.com
| -> location: https://www.e\
| xample.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Response">
<t>
The (":status", "302") header field is evicted from the header table
to free space to allow adding the (":status", "200") header field,
copied from the static table into the header table. The (":status",
"302") header field doesn't need to be removed from the reference
set as it is evicted from the header table.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] location: https://www.example.com
[ 2] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] cache-control: private
[ 4] :status: 302]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[8c | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[8c | == Indexed - Add ==
| idx = 12
| - evict: :status: 302
| -> :status: 200]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 42) :status: 200
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:status: 200
location: https://www.example.com
date: Mon, 21 Oct 2013 20:13:21 GMT
cache-control: private]]></artwork>
</figure>
</t>
</section>
<section title="Third Response">
<t>
Several header fields are evicted from the header table during the
processing of this header set. Before evicting a header belonging to
the reference set, it is emitted, by coding it twice as an Indexed
Representation. The first representation removes the header field
from the reference set, the second one adds it again to the
reference set, also emitting it.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com
content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] :status: 200
[ 2] location: https://www.example.com
[ 3] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] cache-control: private]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[8484 431d 4d6f 6e2c 2032 3120 4f63 7420 | ..C.Mon, 21 Oct
3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
545e 0467 7a69 7084 8483 837b 3866 6f6f | T^.gzip....{8foo
3d41 5344 4a4b 4851 4b42 5a58 4f51 5745 | =ASDJKHQKBZXOQWE
4f50 4955 4158 5157 454f 4955 3b20 6d61 | OPIUAXQWEOIU; ma
782d 6167 653d 3336 3030 3b20 7665 7273 | x-age=3600; vers
696f 6e3d 31 | ion=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[84 | == Indexed - Remove ==
| idx = 4
| -> cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
5e | == Literal indexed ==
| Indexed name (idx = 30)
| content-encoding
04 | Literal value (len = 4)
677a 6970 | gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
84 | == Indexed - Remove ==
| idx = 4
| -> location: https://www.e\
| xample.com
84 | == Indexed - Add ==
| idx = 4
| -> location: https://www.e\
| xample.com
83 | == Indexed - Remove ==
| idx = 3
| -> :status: 200
83 | == Indexed - Add ==
| idx = 3
| -> :status: 200
7b | == Literal indexed ==
| Indexed name (idx = 59)
| set-cookie
38 | Literal value (len = 56)
666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
6572 7369 6f6e 3d31 | ersion=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 200
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
content-encoding: gzip
location: https://www.example.com
:status: 200
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
</section>
</section>
<section title="Response Examples with Huffman Coding"
anchor="response.examples.with.huffman.coding">
<t>
This section shows the same examples as the previous section, but using
Huffman encoding for the literal values. The HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing
some evictions to occur. The eviction mechanism uses the length of the
decoded literal values, so the same evictions occurs as in the previous
section.
</t>
<section title="First Response">
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
Reference set: empty.
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[4882 6402 5985 aec3 771a 4b63 96d0 7abe | H.d.Y...w.Kc..z.
9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f...
2d1b ff71 919d 29ad 1718 63c7 8f0b 97c8 | -..q..)...c.....
e9ae 82ae 43d3 | ....C.]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 3)
| Huffman encoded:
6402 | d.
| Decoded:
| 302
| -> :status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
85 | Literal value (len = 7)
| Huffman encoded:
aec3 771a 4b | ..w.K
| Decoded:
| private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
96 | Literal value (len = 29)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e082 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:21 \
| GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
71 | == Literal indexed ==
| Indexed name (idx = 49)
| location
91 | Literal value (len = 23)
| Huffman encoded:
9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
d3 | .
| Decoded:
| https://www.example.com
| -> location: https://www.e\
| xample.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Response">
<t>
The (":status", "302") header field is evicted from the header table
to free space to allow adding the (":status", "200") header field,
copied from the static table into the header table. The (":status",
"302") header field doesn't need to be removed from the reference
set as it is evicted from the header table.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] location: https://www.example.com
[ 2] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] cache-control: private
[ 4] :status: 302]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[8c | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[8c | == Indexed - Add ==
| idx = 12
| - evict: :status: 302
| -> :status: 200]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 42) :status: 200
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[:status: 200
location: https://www.example.com
date: Mon, 21 Oct 2013 20:13:21 GMT
cache-control: private]]></artwork>
</figure>
</t>
</section>
<section title="Third Response">
<t>
Several header fields are evicted from the header table during the
processing of this header set. Before evicting a header belonging to
the reference set, it is emitted, by coding it twice as an Indexed
Representation. The first representation removes the header field
from the reference set, the second one adds it again to the
reference set, also emitting it.
</t>
<t>
<figure>
<preamble>Header set to encode:</preamble>
<artwork>
<![CDATA[:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com
content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Reference set:</preamble>
<artwork>
<![CDATA[[ 1] :status: 200
[ 2] location: https://www.example.com
[ 3] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] cache-control: private]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork>
<![CDATA[8484 4396 d07a be94 1054 d444 a820 0595 | ..C..z...T.D. ..
040b 8166 e084 a62d 1bff 5e83 9bd9 ab84 | ...f...-..^.....
8483 837b ad94 e782 1dd7 f2e6 c7b3 35df | ...{..........5.
dfcd 5b39 60d5 af27 087f 3672 c1ab 270f | ..[9`..'..6r..'.
b529 1f95 8731 6065 c003 ed4e e5b1 063d | .)...1`e...N...=
5007 | P.]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork>
<![CDATA[84 | == Indexed - Remove ==
| idx = 4
| -> cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
96 | Literal value (len = 29)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e084 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:22 \
| GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
5e | == Literal indexed ==
| Indexed name (idx = 30)
| content-encoding
83 | Literal value (len = 4)
| Huffman encoded:
9bd9 ab | ...
| Decoded:
| gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
84 | == Indexed - Remove ==
| idx = 4
| -> location: https://www.e\
| xample.com
84 | == Indexed - Add ==
| idx = 4
| -> location: https://www.e\
| xample.com
83 | == Indexed - Remove ==
| idx = 3
| -> :status: 200
83 | == Indexed - Add ==
| idx = 3
| -> :status: 200
7b | == Literal indexed ==
| Indexed name (idx = 59)
| set-cookie
ad | Literal value (len = 56)
| Huffman encoded:
94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P.
| Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ\
| WEOIU; max-age=3600; versi\
| on=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 200
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Header Table (after decoding):</preamble>
<artwork>
<![CDATA[[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header set:</preamble>
<artwork>
<![CDATA[cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
content-encoding: gzip
location: https://www.example.com
:status: 200
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
</section>
</section>
<!-- example-end -->
</section>
</back>
</rfc>
<!--
vim:et:tw=78:sw=4:
-->
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