HTTPbis Working Group R. Peon
Internet-Draft Google, Inc
Intended status: Standards Track H. Ruellan
Expires: October 05, 2014 Canon CRF
April 03, 2014
HPACK - Header Compression for HTTP/2
draft-ietf-httpbis-header-compression-07
Abstract
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context of HTTP/2.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at .
Working Group information can be found at ; that specific to HTTP/2 are at .
The changes in this draft are summarized in Appendix A.1.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 05, 2014.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Header Field Encoding . . . . . . . . . . . . . . . . . . . . 5
3.1. Encoding Concepts . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Encoding Context . . . . . . . . . . . . . . . . . . 6
3.1.2. Header Table . . . . . . . . . . . . . . . . . . . . 6
3.1.3. Reference Set . . . . . . . . . . . . . . . . . . . . 6
3.1.4. Header Field Representation . . . . . . . . . . . . . 7
3.1.5. Header Field Emission . . . . . . . . . . . . . . . . 8
3.2. Header Block Decoding . . . . . . . . . . . . . . . . . . 8
3.2.1. Header Field Representation Processing . . . . . . . 8
3.2.2. Reference Set Emission . . . . . . . . . . . . . . . 10
3.2.3. Header Set Completion . . . . . . . . . . . . . . . . 10
3.3. Header Table Management . . . . . . . . . . . . . . . . . 10
3.3.1. Maximum Table Size . . . . . . . . . . . . . . . . . 10
3.3.2. Entry Eviction When Header Table Size Changes . . . . 10
3.3.3. Entry Eviction when Adding New Entries . . . . . . . 11
4. Detailed Format . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Low-level representations . . . . . . . . . . . . . . . . 11
4.1.1. Integer representation . . . . . . . . . . . . . . . 11
4.1.2. String Literal Representation . . . . . . . . . . . . 12
4.2. Indexed Header Field Representation . . . . . . . . . . . 13
4.3. Literal Header Field Representation . . . . . . . . . . . 14
4.3.1. Literal Header Field with Incremental Indexing . . . 14
4.3.2. Literal Header Field without Indexing . . . . . . . . 15
4.3.3. Literal Header Field never Indexed . . . . . . . . . 16
4.4. Encoding Context Update . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5.1. Compression-based Attacks . . . . . . . . . . . . . . . . 18
5.2. Memory Consumption . . . . . . . . . . . . . . . . . . . 19
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5.3. Implementation Limits . . . . . . . . . . . . . . . . . . 19
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Normative References . . . . . . . . . . . . . . . . . . 20
7.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Change Log (to be removed by RFC Editor before
publication . . . . . . . . . . . . . . . . . . . . 21
A.1. Since draft-ietf-httpbis-header-compression-06 . . . . . 21
A.2. Since draft-ietf-httpbis-header-compression-05 . . . . . 21
A.3. Since draft-ietf-httpbis-header-compression-04 . . . . . 21
A.4. Since draft-ietf-httpbis-header-compression-03 . . . . . 21
A.5. Since draft-ietf-httpbis-header-compression-02 . . . . . 22
A.6. Since draft-ietf-httpbis-header-compression-01 . . . . . 22
A.7. Since draft-ietf-httpbis-header-compression-00 . . . . . 22
Appendix B. Static Table . . . . . . . . . . . . . . . . . . . . 23
Appendix C. Huffman Codes . . . . . . . . . . . . . . . . . . . 25
Appendix D. Examples . . . . . . . . . . . . . . . . . . . . . . 31
D.1. Integer Representation Examples . . . . . . . . . . . . . 31
D.1.1. Example 1: Encoding 10 using a 5-bit prefix . . . . . 31
D.1.2. Example 2: Encoding 1337 using a 5-bit prefix . . . . 31
D.1.3. Example 3: Encoding 42 starting at an
octet-boundary . . . . . . . . . . . . . . . . . . . 32
D.2. Header Field Representation Examples . . . . . . . . . . 32
D.2.1. Literal Header Field with Indexing . . . . . . . . . 32
D.2.2. Literal Header Field without Indexing . . . . . . . . 33
D.2.3. Indexed Header Field . . . . . . . . . . . . . . . . 34
D.2.4. Indexed Header Field from Static Table . . . . . . . 35
D.3. Request Examples without Huffman . . . . . . . . . . . . 35
D.3.1. First request . . . . . . . . . . . . . . . . . . . . 35
D.3.2. Second request . . . . . . . . . . . . . . . . . . . 37
D.3.3. Third request . . . . . . . . . . . . . . . . . . . . 38
D.4. Request Examples with Huffman . . . . . . . . . . . . . . 40
D.4.1. First request . . . . . . . . . . . . . . . . . . . . 40
D.4.2. Second request . . . . . . . . . . . . . . . . . . . 41
D.4.3. Third request . . . . . . . . . . . . . . . . . . . . 42
D.5. Response Examples without Huffman . . . . . . . . . . . . 44
D.5.1. First response . . . . . . . . . . . . . . . . . . . 44
D.5.2. Second response . . . . . . . . . . . . . . . . . . . 46
D.5.3. Third response . . . . . . . . . . . . . . . . . . . 47
D.6. Response Examples with Huffman . . . . . . . . . . . . . 49
D.6.1. First response . . . . . . . . . . . . . . . . . . . 49
D.6.2. Second response . . . . . . . . . . . . . . . . . . . 52
D.6.3. Third response . . . . . . . . . . . . . . . . . . . 53
1. Introduction
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This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context of HTTP/2
(see [HTTP2]).
2. Overview
In HTTP/1.1 (see [HTTP-p1]), 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 [PERF1] and [PERF2]).
SPDY [SPDY] initially addressed this redundancy by compressing header
fields using the DEFLATE format [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 [CRIME]).
This document describes HPACK, a new compressor for header fields
which eliminates redundant header fields, is not vulnerable to known
security attacks, and which also has a bounded memory requirement for
use in constrained environments.
2.1. Outline
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 during the HTTP/2 connection.
A set of header fields is treated as an unordered collection of name-
value pairs. 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.
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 (header fields present in one of
the sets that are absent from the other) between the two sets of
header fields.
A header field is represented either literally or as a reference to a
name-value pair in the header table. A set of header fields is
stored as a set of references to entries in the header table
(possibly keeping only a subset of it, as some header fields may be
missing a corresponding entry in the header table). Differences
between consecutive sets of header fields are encoded as changes to
the set of references.
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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.
Examples illustrating the use of these different mechanisms to
represent header fields are available in Appendix D.
3. Header Field Encoding
3.1. Encoding Concepts
The encoding and decoding of header fields relies on some components
and concepts:
Header Field: A name-value pair. Both the name and value are
treated as opaque sequences of octets.
Header Table: The header table (see Section 3.1.2) is a component
used to associate stored header fields to index values.
Static Table: The static table (see Appendix B) 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.
Reference Set: The reference set (see Section 3.1.3) is a component
containing an unordered set of references to entries in the header
table. This is used for the differential encoding of a new header
set.
Header Set: A header set is an unordered group of header fields that
are encoded jointly. A complete set of key-value pairs contained
in a HTTP request or response is a header set.
Header Field Representation: A header field can be represented in
encoded form either as a literal or as an index (see
Section 3.1.4).
Header Block: The entire set of encoded header field representations
which, when decoded, yield a complete header set.
Header Field Emission: When decoding a set of header field
representations, some operations emit a header field (see
Section 3.1.5). Emitted header fields are added to the current
header set and cannot be removed.
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3.1.1. Encoding Context
The set of mutable structures used within an encoding context include
a header table and a reference set. Everything else is either
immutable or conceptual.
HTTP messages are exchanged between a client and a server in both
directions. The encoding of header fields in each direction is
independent from the other direction. There is a single encoding
context for each direction used to encode all header fields sent in
that direction.
3.1.2. Header Table
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 len(header table).
The header table is initially empty.
There is typically no need for the header table to contain duplicate
entries. However, duplicate entries MUST NOT be treated as an error
by a decoder.
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 Section 3.3.1).
The header table is updated during the processing of a set of header
field representations (see Section 3.2.1).
3.1.3. Reference Set
A reference set is an unordered set of references to entries of the
header table.
The reference set is initially empty.
The reference set is updated during the processing of a set of header
field representations (see Section 3.2.1).
The reference set enables differential encoding, whereby 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.
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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.
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 Section 3.2.1).
3.1.4. Header Field Representation
An encoded header field can be represented either as a literal or as
an index.
Literal Representation: A literal representation defines a new
header field. The header field name is represented either
literally or as a reference to an entry of the header table. The
header field value is represented literally.
Three different literal representations are provided:
* A literal representation that does not add the header field to
the header table (see Section 4.3.2).
* A literal representation that does not add the header field to
the header table and require that this header field always use
a literal representation, in particular when re-encoded by an
intermediary (see Section 4.3.3).
* A literal representation that adds the header field as a new
entry at the beginning of the header table (see Section 4.3.1).
Indexed Representation: The indexed representation defines a header
field as a reference to an entry in either the header table or the
static table (see Section 4.2).
Indices between 1 and len(header table), inclusive, refer to
elements in the header table, with index 1 referring to the
beginning of the table.
Indices between len(header table) + 1 and len(header table) +
len(static table), inclusive, refer to elements in the static
table, where the index len(header table) + 1 refers to the first
entry in the static table.
Any other indices MUST be treated as a decoding error.
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<---------- Index Address Space ---------->
<-- Header Table --> <-- Static Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | k | |k+1| ... | n |
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Drop Point
Index Address Space
3.1.5. Header Field Emission
The emission of a header field is the process of marking a header
field as belonging to the current header set. Once a header has been
emitted, it cannot be removed from the current header set.
On the decoding side, an emitted header field can be safely passed to
the upper processing layer as part of the current header set. The
decoder MAY pass the emitted header fields to the upper processing
layer in any order.
By emitting header fields instead of emitting header sets, the
decoder can be implemented in a streaming way, and as such has only
to keep in memory the header table and the reference set. This
bounds the amount of memory used by the decoder, even in presence of
a very large set of header fields. The management of memory for
handling very large sets of header fields can therefore be deferred
to the upper processing layers.
3.2. Header Block Decoding
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.
3.2.1. Header Field Representation Processing
All the header field representations contained in a header block are
processed in the order in which they are presented, as specified
below.
An _indexed representation_ with an index value of 0 entails one of
the following actions, depending on what is encoded next:
o The reference set is emptied.
o The maximum size of the header table is updated.
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An _indexed representation_ corresponding to an entry _present_ in
the reference set entails the following actions:
o The entry is removed from the reference set.
An _indexed representation_ corresponding to an entry _not present_
in the reference set entails the following actions:
o If referencing an element of the static table:
* The header field corresponding to the referenced entry is
emitted.
* The referenced static entry is inserted at the beginning of the
header table.
* A reference to this new header table entry is added to the
reference set, except if this new entry didn't fit in the
header table.
o If referencing an element of the header table:
* The header field corresponding to the referenced entry is
emitted.
* The referenced header table entry is added to the reference
set.
A _literal representation_ that is _not added_ to the header table
entails the following action:
o The header field is emitted.
A _literal representation_ that is _added_ to the header table
entails the following actions:
o The header field is emitted.
o The header field is inserted at the beginning of the header table.
o A reference to the new entry is added to the reference set (except
if this new entry didn't fit in the header table).
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3.2.2. Reference Set Emission
Once all the representations contained in a header block have been
processed, the header fields referenced in the reference set which
have not previously been emitted during this processing are emitted.
3.2.3. Header Set Completion
Once all of the header field representations have been processed, and
the remaining items in the reference set have been emitted, the
header set is complete.
3.3. Header Table Management
3.3.1. Maximum Table Size
To limit the memory requirements on the decoder side, the size of the
header table is bounded. The size of the header table MUST stay
lower than or equal to its maximum size.
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 [HTTP2]). The encoder can change this maximum size (see
Section 4.4), but it must stay lower than or equal to the value of
SETTINGS_HEADER_TABLE_SIZE.
The size of the header table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in octets (as
defined in Section 4.1.2), of its value's length in octets
(Section 4.1.2) and of 32 octets.
The lengths are measured on the non-encoded entry name and entry
value (for the case when a Huffman encoding is used to transmit
string values).
The 32 octets are an accounting for the entry structure overhead.
For example, an entry structure using two 64-bits pointers to
reference the name and the value and the entry, and two 64-bits
integer for counting the number of references to these name and value
would use 32 octets.
3.3.2. Entry Eviction When Header Table Size Changes
Whenever an entry is evicted from the header table, any reference to
that entry contained by the reference set is removed.
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Whenever the maximum size for the header table is made smaller,
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.
The eviction of an entry from the header table causes the index of
the entries in the static table to be reduced by one.
3.3.3. Entry Eviction when Adding New Entries
Whenever a new entry is to be added to the table, any name referenced
by the representation of this new entry is cached, and then 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.
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.
4. Detailed Format
4.1. Low-level representations
4.1.1. Integer representation
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.
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.
The N-bit prefix allows filling the current octet. If the value is
small enough (strictly less than 2^N-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 ). N is always between 1 and 8 bits. An integer
starting at an octet-boundary will have an 8-bit prefix.
The algorithm to represent an integer I is as follows:
if I < 2^N - 1, encode I on N bits
else
encode (2^N - 1) on N bits
I = I - (2^N - 1)
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while I >= 128
encode (I % 128 + 128) on 8 bits
I = I / 128
encode I on 8 bits
For informational purpose, the algorithm to decode an integer I is as
follows:
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
Examples illustrating the encoding of integers are available in
Appendix D.1.
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.
4.1.2. String Literal Representation
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 canonical [CANON] Huffman encoding [HUFF].
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| H | String Length (7+) |
+---+---------------------------+
| String Data (Length octets) |
+-------------------------------+
String Literal Representation
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A literal string representation contains the following fields:
H: A one bit flag, H, indicating whether or not the octets of the
string are Huffman encoded.
String Length: The number of octets used to encode the string
literal, encoded as an integer with 7-bit prefix (see
Section 4.1.1).
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.
String literals which use Huffman encoding are encoded with the
Huffman codes defined in Appendix C (see examples inRequest Examples
with Huffman Appendix D.4 and in Response Examples with Huffman
Appendix D.6). The encoded data is the bitwise concatenation of the
Huffman codes corresponding to each octet of the string literal.
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 the EOS (end-of-string) entry
in the Huffman table are used.
Upon decoding, an incomplete Huffman 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 EOS
entry MUST be treated as a decoding error. A Huffman encoded string
literal containing the EOS entry MUST be treated as a decoding error.
4.2. Indexed Header Field Representation
An indexed header field representation either identifies an entry in
the header table or static table. The processing of an indexed
header field representation is described in Section 3.2.1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
Indexed Header Field
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This representation starts with the '1' 1-bit pattern, followed by
the index of the matching pair, represented as an integer with a
7-bit prefix.
The index value of 0 is not used. It MUST be treated as a decoding
error if found in an indexed header field representation.
4.3. Literal Header Field Representation
Literal header field representations contain a literal header field
value. Header field names are either provided as a literal or by
reference to an existing header table or static table entry.
Literal representations all result in the emission of a header field
when decoded.
4.3.1. Literal Header Field with Incremental Indexing
A literal header field with incremental indexing adds a new entry to
the header table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field with Incremental Indexing - Indexed Name
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) |
+-------------------------------+
Literal Header Field with Incremental Indexing - New Name
This representation starts with the '01' 2-bit pattern.
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If the header field name matches the header field name of a (name,
value) pair stored in the Header Table or Static Table, the header
field name can be represented using the index of that entry. In this
case, the index of the entry, index (which is strictly greater than
0), is represented as an integer with a 6-bit prefix (see
Section 4.1.1).
Otherwise, the header field name is represented as a literal. The
value 0 is represented on 6 bits followed by the header field name
(see Section 4.1.2).
The header field name representation is followed by the header field
value represented as a literal string as described in Section 4.1.2.
4.3.2. Literal Header Field without Indexing
A literal header field without indexing causes the emission of a
header field without altering the header table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field without Indexing - Indexed Name
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) |
+-------------------------------+
Literal Header Field without Indexing - New Name
The literal header field without indexing representation starts with
the '0000' 4-bit pattern.
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If the header field name matches the header field name of a (name,
value) pair stored in the Header Table or Static Table, the header
field name can be represented using the index of that entry. In this
case, the index of the entry, index (which is strictly greater than
0), is represented as an integer with a 6-bit prefix (see
Section 4.1.1).
Otherwise, the header field name is represented as a literal. The
value 0 is represented on 4 bits followed by the header field name
(see Section 4.1.2).
The header field name representation is followed by the header field
value represented as a literal string as described in Section 4.1.2.
4.3.3. Literal Header Field never Indexed
A literal header field never indexed causes the emission of a header
field without altering the header table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field never Indexed - Indexed Name
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) |
+-------------------------------+
Literal Header Field never Indexed - New Name
The literal header field never indexed representation starts with the
'0001' 4-bit pattern.
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When a header field is represented as a literal header field never
indexed, it MUST always be encoded with this same 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.
This representation is intended for protecting header field values
that are not to be put at risk by compressing them (see Section 5.1
for more details).
The encoding of the representation is the same as for the literal
header field without indexing representation (see Section 4.3.2).
4.4. Encoding Context Update
An encoding context update causes the immediate application of a
change to the encoding context.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | F | ... |
+---+---------------------------+
Context Update
An encoding context update starts with the '001' 3-bit pattern.
It is followed by a flag specifying the type of the change, and by
any data necessary to describe the change itself.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 1 | 0 |
+---+---------------------------+
Reference Set Emptying
The flag bit being set to '1' signals that the reference set is
emptied. The remaining bits are set to '0'.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 0 | Max size (4+) |
+---+---------------------------+
Maximum Header Table Size Change
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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 value of the setting SETTINGS_HEADER_TABLE_SIZE (see
[HTTP2]).
The new maximum size is encoded as an integer with a 4-bit prefix.
Change in the maximum size of the header table can trigger entry
evictions (see Section 3.3.2).
5. Security Considerations
5.1. Compression-based Attacks
Compression can create a weak point allowing an attacker to recover
secret data. For example, the CRIME attack (see [CRIME]) took
advantage of the DEFLATE mechanism (see [DEFLATE]) of SPDY (see
[SPDY]) to efficiently probe the compression context. The full-text
compression mechanism of DEFLATE allowed the attacker to learn some
information from each failed attempt at guessing the secret.
For this reason, HPACK provides only limited compression mechanisms
in the form of an indexing table and of a static Huffman encoding.
The indexing table can still provide information to an attacker that
would be able to probe the compression context. However, this
information is limited to the knowledge of whether the attacker's
guess is correct or not.
Still, an attacker could take advantage of this limited information
for breaking low-entropy secrets using a brute-force attack. A
server usually has some protections against such brute-force attack.
Here, the attack would target the client, where it would be harder to
detect. The attack would be even more dangerous if the attacker is
able to prevent the traffic generated by its brute-force attack from
reaching the server.
To offer some protection against such type of attacks, HPACK enables
an endpoint to indicate that a header field must never be compressed,
across any hop up to the other endpoint (see Section 4.3.3). An
endpoint MUST use this feature to prevent the compression of any
header field whose value contains a secret which could be put at risk
by a brute-force attack.
For optimal processing, a sensitive value (for example a cookie)
needs to have an entropy high enough to not be endangered by a brute-
force attack, in order to take advantage of HPACK indexing.
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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 [PETAL]).
5.2. Memory Consumption
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.
The amount of memory used by the compressor state is limited by the
value of the setting SETTINGS_HEADER_TABLE_SIZE. This limitation
takes into account both the size of the data stored in the header
table, and the overhead required by the table structure itself.
For the decoding side, an endpoint can limit the amount of state
memory used by setting an appropriate value for
SETTINGS_HEADER_TABLE_SIZE. For the encoding side, the endpoint can
limit the amount of state memory it uses by defining a header table
maximum size lower than the value of SETTINGS_HEADER_TABLE_SIZE
defined by its peer (see Section 4.4).
The amount of temporary memory consumed is linked to the set of
header fields emitted or received. However, this amount of temporary
memory can be limited by processing these header fields in a
streaming manner.
5.3. Implementation Limits
An implementation of HPACK needs to ensure that large values for
integers, long encoding for integers, or long string literal do not
create security weaknesses.
An implementation has to set a limit for the values it accepts for
integers, as well as for the encoded length (see Section 4.1.1). In
the same way, it has to set a limit to the length it accepts for
string literals (see Section 4.1.2).
6. Acknowledgements
This document includes substantial editorial contributions from the
following individuals: Mike Bishop, Jeff Pinner, Julian Reschke,
Martin Thomson.
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7. References
7.1. Normative References
[HTTP-p1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", draft-
ietf-httpbis-p1-messaging-26 (work in progress), February
2014.
[HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol version 2", draft-ietf-httpbis-http2-10
(work in progress), February 2014.
7.2. Informative References
[CANON] Schwartz, E. and B. Kallick, "Generating a canonical
prefix encoding", Communications of the ACM Volume 7 Issue
3, pp. 166-169, March 1964,
.
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", September
2012, .
[DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[HUFF] Huffman, D., "A Method for the Construction of Minimum
Redundancy Codes", Proceedings of the Institute of Radio
Engineers Volume 40, Number 9, pp. 1098-1101, September
1952, .
[PERF1] Belshe, M., "IETF83: SPDY and What to Consider for HTTP/
2.0", March 2012, .
[PERF2] McManus, P., "SPDY: What I Like About You", September
2011, .
[PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding Table
Information Leakage", April 2013, .
[SPDY] Belshe, M. and R. Peon, "SPDY Protocol", draft-mbelshe-
httpbis-spdy-00 (work in progress), February 2012.
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Appendix A. Change Log (to be removed by RFC Editor before publication
A.1. Since draft-ietf-httpbis-header-compression-06
o Updated format to include literal headers that must never be
compressed.
o Updated security considerations.
o Moved integer encoding examples to the appendix.
o Updated Huffman table.
o Updated static header table (adding and removing status values).
o Updated examples.
A.2. Since draft-ietf-httpbis-header-compression-05
o Regenerated examples.
o Only one Huffman table for requests and responses.
o Added maximum size for header table, independent of
SETTINGS_HEADER_TABLE_SIZE.
o Added pseudo-code for integer decoding.
o Improved examples (removing unnecessary removals).
A.3. Since draft-ietf-httpbis-header-compression-04
o Updated examples: take into account changes in the spec, and show
more features.
o Use 'octet' everywhere instead of having both 'byte' and 'octet'.
o Added reference set emptying.
o Editorial changes and clarifications.
o Added "host" header to the static table.
o Ordering for list of values (either NULL- or comma-separated).
A.4. Since draft-ietf-httpbis-header-compression-03
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o A large number of editorial changes; changed the description of
evicting/adding new entries.
o Removed substitution indexing
o Changed 'initial headers' to 'static headers', as per issue #258
o Merged 'request' and 'response' static headers, as per issue #259
o Changed text to indicate that new headers are added at index 0 and
expire from the largest index, as per issue #233
A.5. Since draft-ietf-httpbis-header-compression-02
o Corrected error in integer encoding pseudocode.
A.6. Since draft-ietf-httpbis-header-compression-01
o Refactored of Header Encoding Section: split definitions and
processing rule.
o 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.
o Backward incompatible change: modified initial header list, as per
issue #188.
o Added example of 32 octets entry structure (issue #191).
o 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.
A.7. Since draft-ietf-httpbis-header-compression-00
Fixed bug/omission in integer representation algorithm.
Changed the document title.
Header matching text rewritten.
Changed the definition of header emission.
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Changed the name of the setting which dictates how much memory the
compression context should use.
Removed "specific use cases" section
Corrected erroneous statement about what index can be contained in
one octet
Added descriptions of opcodes
Removed security claims from introduction.
Appendix B. Static Table
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).
The following table lists the pre-defined header fields that make-up
the static table.
+-------+-----------------------------+--------------+
| Index | Header Name | Header Value |
+-------+-----------------------------+--------------+
| 1 | :authority | |
| 2 | :method | GET |
| 3 | :method | POST |
| 4 | :path | / |
| 5 | :path | /index.html |
| 6 | :scheme | http |
| 7 | :scheme | https |
| 8 | :status | 200 |
| 9 | :status | 204 |
| 10 | :status | 206 |
| 11 | :status | 304 |
| 12 | :status | 400 |
| 13 | :status | 404 |
| 14 | :status | 500 |
| 15 | accept-charset | |
| 16 | accept-encoding | |
| 17 | accept-language | |
| 18 | accept-ranges | |
| 19 | accept | |
| 20 | access-control-allow-origin | |
| 21 | age | |
| 22 | allow | |
| 23 | authorization | |
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| 24 | cache-control | |
| 25 | content-disposition | |
| 26 | content-encoding | |
| 27 | content-language | |
| 28 | content-length | |
| 29 | content-location | |
| 30 | content-range | |
| 31 | content-type | |
| 32 | cookie | |
| 33 | date | |
| 34 | etag | |
| 35 | expect | |
| 36 | expires | |
| 37 | from | |
| 38 | host | |
| 39 | if-match | |
| 40 | if-modified-since | |
| 41 | if-none-match | |
| 42 | if-range | |
| 43 | if-unmodified-since | |
| 44 | last-modified | |
| 45 | link | |
| 46 | location | |
| 47 | max-forwards | |
| 48 | proxy-authenticate | |
| 49 | proxy-authorization | |
| 50 | range | |
| 51 | referer | |
| 52 | refresh | |
| 53 | retry-after | |
| 54 | server | |
| 55 | set-cookie | |
| 56 | strict-transport-security | |
| 57 | transfer-encoding | |
| 58 | user-agent | |
| 59 | vary | |
| 60 | via | |
| 61 | www-authenticate | |
+-------+-----------------------------+--------------+
Table 1: Static Table Entries
The table give 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.
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Appendix C. Huffman Codes
The following codes are used when encoding string literals with an
Huffman coding (see Section 4.1.2).
Each row in the table specifies one Huffman code:
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.
code as bits: The Huffman code for the symbol represented as a
base-2 integer.
code as hex: The Huffman code for the symbol, represented as a
hexadecimal integer, aligned on the least significant bit.
len: The number of bits for the Huffman code of the symbol.
As an example, the Huffman code for the symbol 48 (corresponding to
the ASCII character "0") consists in the 5 bits "0", "0", "1", "0",
"1". This corresponds to the value 5 encoded on 5 bits.
code
code as bits as hex len
sym aligned to MSB aligned in
to LSB bits
( 0) |11111111|11111111|11101110|10 3ffffba [26]
( 1) |11111111|11111111|11101110|11 3ffffbb [26]
( 2) |11111111|11111111|11101111|00 3ffffbc [26]
( 3) |11111111|11111111|11101111|01 3ffffbd [26]
( 4) |11111111|11111111|11101111|10 3ffffbe [26]
( 5) |11111111|11111111|11101111|11 3ffffbf [26]
( 6) |11111111|11111111|11110000|00 3ffffc0 [26]
( 7) |11111111|11111111|11110000|01 3ffffc1 [26]
( 8) |11111111|11111111|11110000|10 3ffffc2 [26]
( 9) |11111111|11111111|11110000|11 3ffffc3 [26]
( 10) |11111111|11111111|11110001|00 3ffffc4 [26]
( 11) |11111111|11111111|11110001|01 3ffffc5 [26]
( 12) |11111111|11111111|11110001|10 3ffffc6 [26]
( 13) |11111111|11111111|11110001|11 3ffffc7 [26]
( 14) |11111111|11111111|11110010|00 3ffffc8 [26]
( 15) |11111111|11111111|11110010|01 3ffffc9 [26]
( 16) |11111111|11111111|11110010|10 3ffffca [26]
( 17) |11111111|11111111|11110010|11 3ffffcb [26]
( 18) |11111111|11111111|11110011|00 3ffffcc [26]
( 19) |11111111|11111111|11110011|01 3ffffcd [26]
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( 20) |11111111|11111111|11110011|10 3ffffce [26]
( 21) |11111111|11111111|11110011|11 3ffffcf [26]
( 22) |11111111|11111111|11110100|00 3ffffd0 [26]
( 23) |11111111|11111111|11110100|01 3ffffd1 [26]
( 24) |11111111|11111111|11110100|10 3ffffd2 [26]
( 25) |11111111|11111111|11110100|11 3ffffd3 [26]
( 26) |11111111|11111111|11110101|00 3ffffd4 [26]
( 27) |11111111|11111111|11110101|01 3ffffd5 [26]
( 28) |11111111|11111111|11110101|10 3ffffd6 [26]
( 29) |11111111|11111111|11110101|11 3ffffd7 [26]
( 30) |11111111|11111111|11110110|00 3ffffd8 [26]
( 31) |11111111|11111111|11110110|01 3ffffd9 [26]
' ' ( 32) |00110 6 [ 5]
'!' ( 33) |11111111|11100 1ffc [13]
'"' ( 34) |11111000|0 1f0 [ 9]
'#' ( 35) |11111111|111100 3ffc [14]
'$' ( 36) |11111111|1111100 7ffc [15]
'%' ( 37) |011110 1e [ 6]
'&' ( 38) |1100100 64 [ 7]
''' ( 39) |11111111|11101 1ffd [13]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111000|1 1f1 [ 9]
'*' ( 42) |11111110|11 3fb [10]
'+' ( 43) |11111111|00 3fc [10]
',' ( 44) |1100101 65 [ 7]
'-' ( 45) |1100110 66 [ 7]
'.' ( 46) |011111 1f [ 6]
'/' ( 47) |00111 7 [ 5]
'0' ( 48) |0000 0 [ 4]
'1' ( 49) |0001 1 [ 4]
'2' ( 50) |0010 2 [ 4]
'3' ( 51) |01000 8 [ 5]
'4' ( 52) |100000 20 [ 6]
'5' ( 53) |100001 21 [ 6]
'6' ( 54) |100010 22 [ 6]
'7' ( 55) |100011 23 [ 6]
'8' ( 56) |100100 24 [ 6]
'9' ( 57) |100101 25 [ 6]
':' ( 58) |100110 26 [ 6]
';' ( 59) |11101100| ec [ 8]
'<' ( 60) |11111111|11111110|0 1fffc [17]
'=' ( 61) |100111 27 [ 6]
'>' ( 62) |11111111|1111101 7ffd [15]
'?' ( 63) |11111111|01 3fd [10]
'@' ( 64) |11111111|1111110 7ffe [15]
'A' ( 65) |1100111 67 [ 7]
'B' ( 66) |11101101| ed [ 8]
'C' ( 67) |11101110| ee [ 8]
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'D' ( 68) |1101000 68 [ 7]
'E' ( 69) |11101111| ef [ 8]
'F' ( 70) |1101001 69 [ 7]
'G' ( 71) |1101010 6a [ 7]
'H' ( 72) |11111001|0 1f2 [ 9]
'I' ( 73) |11110000| f0 [ 8]
'J' ( 74) |11111001|1 1f3 [ 9]
'K' ( 75) |11111010|0 1f4 [ 9]
'L' ( 76) |11111010|1 1f5 [ 9]
'M' ( 77) |1101011 6b [ 7]
'N' ( 78) |1101100 6c [ 7]
'O' ( 79) |11110001| f1 [ 8]
'P' ( 80) |11110010| f2 [ 8]
'Q' ( 81) |11111011|0 1f6 [ 9]
'R' ( 82) |11111011|1 1f7 [ 9]
'S' ( 83) |1101101 6d [ 7]
'T' ( 84) |101000 28 [ 6]
'U' ( 85) |11110011| f3 [ 8]
'V' ( 86) |11111100|0 1f8 [ 9]
'W' ( 87) |11111100|1 1f9 [ 9]
'X' ( 88) |11110100| f4 [ 8]
'Y' ( 89) |11111101|0 1fa [ 9]
'Z' ( 90) |11111101|1 1fb [ 9]
'[' ( 91) |11111111|100 7fc [11]
'\' ( 92) |11111111|11111111|11110110|10 3ffffda [26]
']' ( 93) |11111111|101 7fd [11]
'^' ( 94) |11111111|111101 3ffd [14]
'_' ( 95) |1101110 6e [ 7]
'`' ( 96) |11111111|11111111|10 3fffe [18]
'a' ( 97) |01001 9 [ 5]
'b' ( 98) |1101111 6f [ 7]
'c' ( 99) |01010 a [ 5]
'd' (100) |101001 29 [ 6]
'e' (101) |01011 b [ 5]
'f' (102) |1110000 70 [ 7]
'g' (103) |101010 2a [ 6]
'h' (104) |101011 2b [ 6]
'i' (105) |01100 c [ 5]
'j' (106) |11110101| f5 [ 8]
'k' (107) |11110110| f6 [ 8]
'l' (108) |101100 2c [ 6]
'm' (109) |101101 2d [ 6]
'n' (110) |101110 2e [ 6]
'o' (111) |01101 d [ 5]
'p' (112) |101111 2f [ 6]
'q' (113) |11111110|0 1fc [ 9]
'r' (114) |110000 30 [ 6]
's' (115) |110001 31 [ 6]
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't' (116) |01110 e [ 5]
'u' (117) |1110001 71 [ 7]
'v' (118) |1110010 72 [ 7]
'w' (119) |1110011 73 [ 7]
'x' (120) |1110100 74 [ 7]
'y' (121) |1110101 75 [ 7]
'z' (122) |11110111| f7 [ 8]
'{' (123) |11111111|11111110|1 1fffd [17]
'|' (124) |11111111|1100 ffc [12]
'}' (125) |11111111|11111111|0 1fffe [17]
'~' (126) |11111111|1101 ffd [12]
(127) |11111111|11111111|11110110|11 3ffffdb [26]
(128) |11111111|11111111|11110111|00 3ffffdc [26]
(129) |11111111|11111111|11110111|01 3ffffdd [26]
(130) |11111111|11111111|11110111|10 3ffffde [26]
(131) |11111111|11111111|11110111|11 3ffffdf [26]
(132) |11111111|11111111|11111000|00 3ffffe0 [26]
(133) |11111111|11111111|11111000|01 3ffffe1 [26]
(134) |11111111|11111111|11111000|10 3ffffe2 [26]
(135) |11111111|11111111|11111000|11 3ffffe3 [26]
(136) |11111111|11111111|11111001|00 3ffffe4 [26]
(137) |11111111|11111111|11111001|01 3ffffe5 [26]
(138) |11111111|11111111|11111001|10 3ffffe6 [26]
(139) |11111111|11111111|11111001|11 3ffffe7 [26]
(140) |11111111|11111111|11111010|00 3ffffe8 [26]
(141) |11111111|11111111|11111010|01 3ffffe9 [26]
(142) |11111111|11111111|11111010|10 3ffffea [26]
(143) |11111111|11111111|11111010|11 3ffffeb [26]
(144) |11111111|11111111|11111011|00 3ffffec [26]
(145) |11111111|11111111|11111011|01 3ffffed [26]
(146) |11111111|11111111|11111011|10 3ffffee [26]
(147) |11111111|11111111|11111011|11 3ffffef [26]
(148) |11111111|11111111|11111100|00 3fffff0 [26]
(149) |11111111|11111111|11111100|01 3fffff1 [26]
(150) |11111111|11111111|11111100|10 3fffff2 [26]
(151) |11111111|11111111|11111100|11 3fffff3 [26]
(152) |11111111|11111111|11111101|00 3fffff4 [26]
(153) |11111111|11111111|11111101|01 3fffff5 [26]
(154) |11111111|11111111|11111101|10 3fffff6 [26]
(155) |11111111|11111111|11111101|11 3fffff7 [26]
(156) |11111111|11111111|11111110|00 3fffff8 [26]
(157) |11111111|11111111|11111110|01 3fffff9 [26]
(158) |11111111|11111111|11111110|10 3fffffa [26]
(159) |11111111|11111111|11111110|11 3fffffb [26]
(160) |11111111|11111111|11111111|00 3fffffc [26]
(161) |11111111|11111111|11111111|01 3fffffd [26]
(162) |11111111|11111111|11111111|10 3fffffe [26]
(163) |11111111|11111111|11111111|11 3ffffff [26]
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(164) |11111111|11111111|11000000|0 1ffff80 [25]
(165) |11111111|11111111|11000000|1 1ffff81 [25]
(166) |11111111|11111111|11000001|0 1ffff82 [25]
(167) |11111111|11111111|11000001|1 1ffff83 [25]
(168) |11111111|11111111|11000010|0 1ffff84 [25]
(169) |11111111|11111111|11000010|1 1ffff85 [25]
(170) |11111111|11111111|11000011|0 1ffff86 [25]
(171) |11111111|11111111|11000011|1 1ffff87 [25]
(172) |11111111|11111111|11000100|0 1ffff88 [25]
(173) |11111111|11111111|11000100|1 1ffff89 [25]
(174) |11111111|11111111|11000101|0 1ffff8a [25]
(175) |11111111|11111111|11000101|1 1ffff8b [25]
(176) |11111111|11111111|11000110|0 1ffff8c [25]
(177) |11111111|11111111|11000110|1 1ffff8d [25]
(178) |11111111|11111111|11000111|0 1ffff8e [25]
(179) |11111111|11111111|11000111|1 1ffff8f [25]
(180) |11111111|11111111|11001000|0 1ffff90 [25]
(181) |11111111|11111111|11001000|1 1ffff91 [25]
(182) |11111111|11111111|11001001|0 1ffff92 [25]
(183) |11111111|11111111|11001001|1 1ffff93 [25]
(184) |11111111|11111111|11001010|0 1ffff94 [25]
(185) |11111111|11111111|11001010|1 1ffff95 [25]
(186) |11111111|11111111|11001011|0 1ffff96 [25]
(187) |11111111|11111111|11001011|1 1ffff97 [25]
(188) |11111111|11111111|11001100|0 1ffff98 [25]
(189) |11111111|11111111|11001100|1 1ffff99 [25]
(190) |11111111|11111111|11001101|0 1ffff9a [25]
(191) |11111111|11111111|11001101|1 1ffff9b [25]
(192) |11111111|11111111|11001110|0 1ffff9c [25]
(193) |11111111|11111111|11001110|1 1ffff9d [25]
(194) |11111111|11111111|11001111|0 1ffff9e [25]
(195) |11111111|11111111|11001111|1 1ffff9f [25]
(196) |11111111|11111111|11010000|0 1ffffa0 [25]
(197) |11111111|11111111|11010000|1 1ffffa1 [25]
(198) |11111111|11111111|11010001|0 1ffffa2 [25]
(199) |11111111|11111111|11010001|1 1ffffa3 [25]
(200) |11111111|11111111|11010010|0 1ffffa4 [25]
(201) |11111111|11111111|11010010|1 1ffffa5 [25]
(202) |11111111|11111111|11010011|0 1ffffa6 [25]
(203) |11111111|11111111|11010011|1 1ffffa7 [25]
(204) |11111111|11111111|11010100|0 1ffffa8 [25]
(205) |11111111|11111111|11010100|1 1ffffa9 [25]
(206) |11111111|11111111|11010101|0 1ffffaa [25]
(207) |11111111|11111111|11010101|1 1ffffab [25]
(208) |11111111|11111111|11010110|0 1ffffac [25]
(209) |11111111|11111111|11010110|1 1ffffad [25]
(210) |11111111|11111111|11010111|0 1ffffae [25]
(211) |11111111|11111111|11010111|1 1ffffaf [25]
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(212) |11111111|11111111|11011000|0 1ffffb0 [25]
(213) |11111111|11111111|11011000|1 1ffffb1 [25]
(214) |11111111|11111111|11011001|0 1ffffb2 [25]
(215) |11111111|11111111|11011001|1 1ffffb3 [25]
(216) |11111111|11111111|11011010|0 1ffffb4 [25]
(217) |11111111|11111111|11011010|1 1ffffb5 [25]
(218) |11111111|11111111|11011011|0 1ffffb6 [25]
(219) |11111111|11111111|11011011|1 1ffffb7 [25]
(220) |11111111|11111111|11011100|0 1ffffb8 [25]
(221) |11111111|11111111|11011100|1 1ffffb9 [25]
(222) |11111111|11111111|11011101|0 1ffffba [25]
(223) |11111111|11111111|11011101|1 1ffffbb [25]
(224) |11111111|11111111|11011110|0 1ffffbc [25]
(225) |11111111|11111111|11011110|1 1ffffbd [25]
(226) |11111111|11111111|11011111|0 1ffffbe [25]
(227) |11111111|11111111|11011111|1 1ffffbf [25]
(228) |11111111|11111111|11100000|0 1ffffc0 [25]
(229) |11111111|11111111|11100000|1 1ffffc1 [25]
(230) |11111111|11111111|11100001|0 1ffffc2 [25]
(231) |11111111|11111111|11100001|1 1ffffc3 [25]
(232) |11111111|11111111|11100010|0 1ffffc4 [25]
(233) |11111111|11111111|11100010|1 1ffffc5 [25]
(234) |11111111|11111111|11100011|0 1ffffc6 [25]
(235) |11111111|11111111|11100011|1 1ffffc7 [25]
(236) |11111111|11111111|11100100|0 1ffffc8 [25]
(237) |11111111|11111111|11100100|1 1ffffc9 [25]
(238) |11111111|11111111|11100101|0 1ffffca [25]
(239) |11111111|11111111|11100101|1 1ffffcb [25]
(240) |11111111|11111111|11100110|0 1ffffcc [25]
(241) |11111111|11111111|11100110|1 1ffffcd [25]
(242) |11111111|11111111|11100111|0 1ffffce [25]
(243) |11111111|11111111|11100111|1 1ffffcf [25]
(244) |11111111|11111111|11101000|0 1ffffd0 [25]
(245) |11111111|11111111|11101000|1 1ffffd1 [25]
(246) |11111111|11111111|11101001|0 1ffffd2 [25]
(247) |11111111|11111111|11101001|1 1ffffd3 [25]
(248) |11111111|11111111|11101010|0 1ffffd4 [25]
(249) |11111111|11111111|11101010|1 1ffffd5 [25]
(250) |11111111|11111111|11101011|0 1ffffd6 [25]
(251) |11111111|11111111|11101011|1 1ffffd7 [25]
(252) |11111111|11111111|11101100|0 1ffffd8 [25]
(253) |11111111|11111111|11101100|1 1ffffd9 [25]
(254) |11111111|11111111|11101101|0 1ffffda [25]
(255) |11111111|11111111|11101101|1 1ffffdb [25]
EOS (256) |11111111|11111111|11101110|0 1ffffdc [25]
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Appendix D. Examples
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.
D.1. Integer Representation Examples
This section shows the representation of integer values in details
(see Section 4.1.1).
D.1.1. Example 1: Encoding 10 using a 5-bit prefix
The value 10 is to be encoded with a 5-bit prefix.
o 10 is less than 31 (2^5 - 1) and is represented using the 5-bit
prefix.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits
+---+---+---+---+---+---+---+---+
D.1.2. Example 2: Encoding 1337 using a 5-bit prefix
The value I=1337 is to be encoded with a 5-bit prefix.
1337 is greater than 31 (2^5 - 1).
The 5-bit prefix is filled with its max value (31).
I = 1337 - (2^5 - 1) = 1306.
I (1306) is greater than or equal to 128, the while loop body
executes:
I % 128 == 26
26 + 128 == 154
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154 is encoded in 8 bits as: 10011010
I is set to 10 (1306 / 128 == 10)
I is no longer greater than or equal to 128, the while loop
terminates.
I, now 10, is encoded on 8 bits as: 00001010.
The process ends.
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
+---+---+---+---+---+---+---+---+
D.1.3. Example 3: Encoding 42 starting at an octet-boundary
The value 42 is to be encoded starting at an octet-boundary. This
implies that a 8-bit prefix is used.
o 42 is less than 255 (2^8 - 1) and is represented using the 8-bit
prefix.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits
+---+---+---+---+---+---+---+---+
D.2. Header Field Representation Examples
This section shows several independent representation examples.
D.2.1. Literal Header Field with Indexing
The header field representation uses a literal name and a literal
value.
Header set to encode:
custom-key: custom-header
Reference set: empty.
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Hex dump of encoded data:
400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus
746f 6d2d 6865 6164 6572 | tom-header
Decoding process:
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
Header Table (after decoding):
[ 1] (s = 55) custom-key: custom-header
Table size: 55
Decoded header set:
custom-key: custom-header
D.2.2. Literal Header Field without Indexing
The header field representation uses an indexed name and a literal
value.
Header set to encode:
:path: /sample/path
Reference set: empty.
Hex dump of encoded data:
040c 2f73 616d 706c 652f 7061 7468 | ../sample/path
Decoding process:
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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
Header table (after decoding): empty.
Decoded header set:
:path: /sample/path
D.2.3. Indexed Header Field
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.
Header set to encode:
:method: GET
Reference set: empty.
Hex dump of encoded data:
82 | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Header Table (after decoding):
[ 1] (s = 42) :method: GET
Table size: 42
Decoded header set:
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:method: GET
D.2.4. Indexed Header Field from Static Table
The header field representation uses an indexed header field, from
the static table. In this example, the SETTINGS_HEADER_TABLE_SIZE is
set to 0, therefore, the entry is not copied into the header table.
Header set to encode:
:method: GET
Reference set: empty.
Hex dump of encoded data:
82 | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Header table (after decoding): empty.
Decoded header set:
:method: GET
D.3. Request Examples without Huffman
This section shows several consecutive header sets, corresponding to
HTTP requests, on the same connection.
D.3.1. First request
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Header set to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
Reference set: empty.
Hex dump of encoded data:
8287 8644 0f77 7777 2e65 7861 6d70 6c65 | ...D.www.example
2e63 6f6d | .com
Decoding process:
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
Header Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
[ 2] (s = 38) :path: /
[ 3] (s = 43) :scheme: http
[ 4] (s = 42) :method: GET
Table size: 180
Decoded header set:
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:method: GET
:scheme: http
:path: /
:authority: www.example.com
D.3.2. Second request
This request takes advantage of the differential encoding of header
sets.
Header set to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
Reference set:
[ 1] :authority: www.example.com
[ 2] :path: /
[ 3] :scheme: http
[ 4] :method: GET
Hex dump of encoded data:
5c08 6e6f 2d63 6163 6865 | \.no-cache
Decoding process:
5c | == Literal indexed ==
| Indexed name (idx = 28)
| cache-control
08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | no-cache
| -> cache-control: no-cache
Header Table (after decoding):
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[ 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
Decoded header set:
cache-control: no-cache
:authority: www.example.com
:path: /
:scheme: http
:method: GET
D.3.3. Third request
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.
Header set to encode:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
Reference set:
[ 1] cache-control: no-cache
[ 2] :authority: www.example.com
[ 3] :path: /
[ 4] :scheme: http
[ 5] :method: GET
Hex dump of encoded data:
3085 8c8b 8440 0a63 7573 746f 6d2d 6b65 | 0....@.custom-ke
790c 6375 7374 6f6d 2d76 616c 7565 | y.custom-value
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Decoding process:
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
Header Table (after decoding):
[ 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
Decoded header set:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
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D.4. Request Examples with Huffman
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values.
D.4.1. First request
Header set to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
Reference set: empty.
Hex dump of encoded data:
8287 8644 8ce7 cf9b ebe8 9b6f b16f a9b6 | ...D.......o.o..
ff | .
Decoding process:
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:
e7cf 9beb e89b 6fb1 6fa9 b6ff | ......o.o...
| Decoded:
| www.example.com
| -> :authority: www.example\
| .com
Header Table (after decoding):
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[ 1] (s = 57) :authority: www.example.com
[ 2] (s = 38) :path: /
[ 3] (s = 43) :scheme: http
[ 4] (s = 42) :method: GET
Table size: 180
Decoded header set:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
D.4.2. Second request
This request takes advantage of the differential encoding of header
sets.
Header set to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
Reference set:
[ 1] :authority: www.example.com
[ 2] :path: /
[ 3] :scheme: http
[ 4] :method: GET
Hex dump of encoded data:
5c86 b9b9 9495 56bf | \.....V.
Decoding process:
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5c | == Literal indexed ==
| Indexed name (idx = 28)
| cache-control
86 | Literal value (len = 8)
| Huffman encoded:
b9b9 9495 56bf | ....V.
| Decoded:
| no-cache
| -> cache-control: no-cache
Header Table (after decoding):
[ 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
Decoded header set:
cache-control: no-cache
:authority: www.example.com
:path: /
:scheme: http
:method: GET
D.4.3. Third request
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.
Header set to encode:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
Reference set:
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[ 1] cache-control: no-cache
[ 2] :authority: www.example.com
[ 3] :path: /
[ 4] :scheme: http
[ 5] :method: GET
Hex dump of encoded data:
3085 8c8b 8440 8857 1c5c db73 7b2f af89 | 0....@.W.\.s{/..
571c 5cdb 7372 4d9c 57 | W.\.srM.W
Decoding process:
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:
571c 5cdb 737b 2faf | W.\.s{/.
| Decoded:
| custom-key
89 | Literal value (len = 12)
| Huffman encoded:
571c 5cdb 7372 4d9c 57 | W.\.srM.W
| Decoded:
| custom-value
| -> custom-key: custom-valu\
| e
Header Table (after decoding):
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[ 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
Decoded header set:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
D.5. Response Examples without Huffman
This section shows several consecutive header sets, corresponding to
HTTP responses, on the same connection. SETTINGS_HEADER_TABLE_SIZE
is set to the value of 256 octets, causing some evictions to occur.
D.5.1. First response
Header set to encode:
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Reference set: empty.
Hex dump of encoded data:
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
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Decoding process:
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
Header Table (after decoding):
[ 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
Decoded header set:
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
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D.5.2. Second response
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.
Header set to encode:
:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Reference set:
[ 1] location: https://www.example.com
[ 2] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] cache-control: private
[ 4] :status: 302
Hex dump of encoded data:
8c | .
Decoding process:
8c | == Indexed - Add ==
| idx = 12
| - evict: :status: 302
| -> :status: 200
Header Table (after decoding):
[ 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
Decoded header set:
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:status: 200
location: https://www.example.com
date: Mon, 21 Oct 2013 20:13:21 GMT
cache-control: private
D.5.3. Third response
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.
Header set to encode:
: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
Reference set:
[ 1] :status: 200
[ 2] location: https://www.example.com
[ 3] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] cache-control: private
Hex dump of encoded data:
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
Decoding process:
84 | == Indexed - Remove ==
| idx = 4
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| -> 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\
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| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1
Header Table (after decoding):
[ 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
Decoded header set:
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
D.6. Response Examples with Huffman
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values. The eviction
mechanism uses the length of the decoded literal values, so the same
evictions occurs as in the previous section.
D.6.1. First response
Header set to encode:
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Reference set: empty.
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Hex dump of encoded data:
4882 4017 5985 bf06 724b 9763 93d6 dbb2 | H.@.Y...rK.c....
9884 de2a 7188 0506 2098 5131 09b5 6ba3 | ...*q... .Q1..k.
7191 adce bf19 8e7e 7cf9 bebe 89b6 fb16 | q.......|.......
fa9b 6f | ..o
Decoding process:
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48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 3)
| Huffman encoded:
4017 | @.
| Decoded:
| 302
| -> :status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
85 | Literal value (len = 7)
| Huffman encoded:
bf06 724b 97 | ..rK.
| Decoded:
| private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
93 | Literal value (len = 29)
| Huffman encoded:
d6db b298 84de 2a71 8805 0620 9851 3109 | ......*q... .Q1.
b56b a3 | .k.
| 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:
adce bf19 8e7e 7cf9 bebe 89b6 fb16 fa9b | ......|.........
6f | o
| Decoded:
| https://www.example.com
| -> location: https://www.e\
| xample.com
Header Table (after decoding):
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[ 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
Decoded header set:
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
D.6.2. Second response
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.
Header set to encode:
:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Reference set:
[ 1] location: https://www.example.com
[ 2] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] cache-control: private
[ 4] :status: 302
Hex dump of encoded data:
8c | .
Decoding process:
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8c | == Indexed - Add ==
| idx = 12
| - evict: :status: 302
| -> :status: 200
Header Table (after decoding):
[ 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
Decoded header set:
:status: 200
location: https://www.example.com
date: Mon, 21 Oct 2013 20:13:21 GMT
cache-control: private
D.6.3. Third response
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.
Header set to encode:
: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
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Reference set:
[ 1] :status: 200
[ 2] location: https://www.example.com
[ 3] date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] cache-control: private
Hex dump of encoded data:
8484 4393 d6db b298 84de 2a71 8805 0620 | ..C.......*q...
9851 3111 b56b a35e 84ab dd97 ff84 8483 | .Q1..k.^........
837b b1e0 d6cf 9f6e 8f9f d3e5 f6fa 76fe | .{.....n......v.
fd3c 7edf 9eff 1f2f 0f3c fe9f 6fcf 7f8f | ......./....o...
879f 61ad 4f4c c9a9 73a2 200e c372 5e18 | ..a.OL..s. ..r^.
b1b7 4e3f | ..N?
Decoding process:
84 | == Indexed - Remove ==
| idx = 4
| -> cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
93 | Literal value (len = 29)
| Huffman encoded:
d6db b298 84de 2a71 8805 0620 9851 3111 | ......*q... .Q1.
b56b a3 | .k.
| 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
84 | Literal value (len = 4)
| Huffman encoded:
abdd 97ff | ....
| Decoded:
| gzip
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| - 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
b1 | Literal value (len = 56)
| Huffman encoded:
e0d6 cf9f 6e8f 9fd3 e5f6 fa76 fefd 3c7e | ....n......v....
df9e ff1f 2f0f 3cfe 9f6f cf7f 8f87 9f61 | ..../....o.....a
ad4f 4cc9 a973 a220 0ec3 725e 18b1 b74e | .OL..s. ..r^...N
3f | ?
| 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
Header Table (after decoding):
[ 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
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Decoded header set:
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
Authors' Addresses
Roberto Peon
Google, Inc
EMail: fenix@google.com
Herve Ruellan
Canon CRF
EMail: herve.ruellan@crf.canon.fr
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