HTTPbis Working Group R. Peon
Internet-Draft Google, Inc
Intended status: Standards Track H. Ruellan
Expires: August 17, 2014 Canon CRF
February 13, 2014
HPACK - Header Compression for HTTP/2
draft-ietf-httpbis-header-compression-06
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 [1].
Working Group information and related documents can be found at [2]
(Wiki) and [3] (source code and issues tracker).
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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This Internet-Draft will expire on August 17, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Header Field Encoding . . . . . . . . . . . . . . . . . . . . 4
3.1. Encoding Concepts . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Encoding Context . . . . . . . . . . . . . . . . . . 5
3.1.2. Header Table . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . 9
3.2.3. Header Set Completion . . . . . . . . . . . . . . . . 9
3.3. Header Table Management . . . . . . . . . . . . . . . . . 9
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 . . . . . . . 10
4. Detailed Format . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Low-level representations . . . . . . . . . . . . . . . . 11
4.1.1. Integer representation . . . . . . . . . . . . . . . 11
4.1.2. String Literal Representation . . . . . . . . . . . . 13
4.2. Indexed Header Field Representation . . . . . . . . . . . 15
4.3. Literal Header Field Representation . . . . . . . . . . . 15
4.3.1. Literal Header Field without Indexing . . . . . . . . 15
4.3.2. Literal Header Field with Incremental Indexing . . . 16
4.4. Encoding Context Update . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1. Normative References . . . . . . . . . . . . . . . . . . 19
7.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Change Log (to be removed by RFC Editor before
publication . . . . . . . . . . . . . . . . . . . . 20
A.1. Since draft-ietf-httpbis-header-compression-05 . . . . . 20
A.2. Since draft-ietf-httpbis-header-compression-04 . . . . . 20
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A.3. Since draft-ietf-httpbis-header-compression-03 . . . . . 21
A.4. Since draft-ietf-httpbis-header-compression-02 . . . . . 21
A.5. Since draft-ietf-httpbis-header-compression-01 . . . . . 21
A.6. Since draft-ietf-httpbis-header-compression-00 . . . . . 21
Appendix B. Static Table . . . . . . . . . . . . . . . . . . . . 22
Appendix C. Huffman Codes . . . . . . . . . . . . . . . . . . . 24
Appendix D. Examples . . . . . . . . . . . . . . . . . . . . . . 29
D.1. Header Field Representation Examples . . . . . . . . . . 30
D.1.1. Literal Header Field with Indexing . . . . . . . . . 30
D.1.2. Literal Header Field without Indexing . . . . . . . . 30
D.1.3. Indexed Header Field . . . . . . . . . . . . . . . . 31
D.1.4. Indexed Header Field from Static Table . . . . . . . 32
D.2. Request Examples without Huffman . . . . . . . . . . . . 33
D.2.1. First request . . . . . . . . . . . . . . . . . . . . 33
D.2.2. Second request . . . . . . . . . . . . . . . . . . . 34
D.2.3. Third request . . . . . . . . . . . . . . . . . . . . 35
D.3. Request Examples with Huffman . . . . . . . . . . . . . . 37
D.3.1. First request . . . . . . . . . . . . . . . . . . . . 37
D.3.2. Second request . . . . . . . . . . . . . . . . . . . 38
D.3.3. Third request . . . . . . . . . . . . . . . . . . . . 40
D.4. Response Examples without Huffman . . . . . . . . . . . . 42
D.4.1. First response . . . . . . . . . . . . . . . . . . . 42
D.4.2. Second response . . . . . . . . . . . . . . . . . . . 44
D.4.3. Third response . . . . . . . . . . . . . . . . . . . 45
D.5. Response Examples with Huffman . . . . . . . . . . . . . 47
D.5.1. First response . . . . . . . . . . . . . . . . . . . 47
D.5.2. Second response . . . . . . . . . . . . . . . . . . . 50
D.5.3. Third response . . . . . . . . . . . . . . . . . . . 51
1. Introduction
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]).
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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.
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:
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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.
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
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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.
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).
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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.
Two different literal representations are provided:
* A literal representation that does not add the header field to
the header table (see Section 4.3.1).
* A literal representation that adds the header field as a new
entry at the beginning of the header table (see Section 4.3.2).
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).
<---------- Index Address Space ---------->
<-- Header Table --> <-- Static Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | k | |k+1| ... | n |
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Drop Point
Index Address Space
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.
Index 0 signals a modification of the encoding context: either
the reference set is emptied, or the maximum size of the header
table is updated (see Section 4.4).
Any other indices MUST be treated as erroneous, and the
compression context considered corrupt and unusable.
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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.
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:
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* 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).
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
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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.
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.
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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 [4]
representation. 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)
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
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M = M + 7
while B & 128 == 128
return I
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.
4.1.1.1. 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
+---+---+---+---+---+---+---+---+
4.1.1.2. 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
+---+---+---+---+---+---+---+---+
4.1.1.3. 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
+---+---+---+---+---+---+---+---+
4.1.2. String Literal Representation
Header field names and header field values are encoded as sequences
of octets. A header field name or a header field value is encoded in
three parts:
1. One bit, H, indicating whether or not the octets are Huffman
encoded.
2. The number of octets required to hold the result of the next
step, represented as an integer with a 7-bit prefix (see
Section 4.1.1), immediately following the first bit.
3. The encoded data of the string:
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* If H is '1', then the encoded string data is the bitwise
concatenation of the canonical [CANON] Huffman code [HUFF]
corresponding to each octet of the data, followed by between
0-7 bits of padding.
* If H is '0', then the encoded string is the octets of the
field value without modification.
Padding is necessary when doing Huffman encoding to ensure that the
remaining bits between the actual end of the data and the next octet
boundary are not misinterpreted as part of the input data.
When padding for Huffman encoding, the bits from the EOS (end-of-
string) entry in the Huffman table are used, starting with the MSB
(most significant bit). This entry is guaranteed to be at least 8
bits long.
String literals which use Huffman encoding are encoded with the
Huffman Codes Appendix C (see examples in Request Examples with
Huffman Appendix D.3 and in Response Examples with Huffman
Appendix D.5).
The EOS symbol is represented with value 256, and is used solely to
signal the end of the Huffman-encoded key data or the end of the
Huffman-encoded value data. Given that only between 0-7 bits of the
EOS symbol is included in any Huffman-encoded string, and given that
the EOS symbol is at least 8 bits long, it is expected that it should
never be successfully decoded.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Value Length Prefix (7) |
+---+---+---+---+---+---+---+---+
| Value Length (0-N octets) |
+---+---+---+---+---+---+---+---+
...
+---+---+---+---+---+---+---+---+
| Huffman Encoded Data |Padding|
+---+---+---+---+---+---+---+---+
String Literal with Huffman Encoding
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | Value Length Prefix (7) |
+---+---+---+---+---+---+---+---+
| Value Length (0-N octets) |
+---+---+---+---+---+---+---+---+
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...
+---+---+---+---+---+---+---+---+
| Field Bytes without Encoding |
+---+---+---+---+---+---+---+---+
String Literal without Huffman Encoding
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
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 reserved for signalling changes in the
encoding context (see Section 4.4).
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 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 | 1 | Index (6+) |
+---+---+---+-------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
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+-------------------------------+
Literal Header Field without 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 without Indexing - New Name
This representation starts with the '01' 2-bit pattern.
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 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 | 0 | Index (6+) |
+---+---+---+-------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
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Literal Header Field with Incremental Indexing - Indexed Name
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 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 '00' 2-bit pattern.
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.
4.4. Encoding Context Update
An indexed value of 0 is reserved for signalling changes in the
encoding context. The type of the change is encoded on the following
octet(s). Any change in the encoding context is applied immediately.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 0 |
+---+---------------------------+
Reference Set Emptying
An octet with its high bit set to '1' signals that the reference set
is emptied. The remaining bits are set to '0'.
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | New maximum size (7+) |
+---+---------------------------+
Maximum Header Table Size Change
An octet with its high bit set to '0' signals the new 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 7-bit prefix.
Change in the maximum size of the header table can trigger entry
evictions (see Section 3.3.2).
5. Security Considerations
This compressor exists to solve security issues present in stream
compressors such as DEFLATE whereby the compression context can be
efficiently probed to reveal secrets. A conformant implementation of
this specification should be fairly safe against that kind of attack,
as the reaping of any information from the compression context
requires more work than guessing and verifying the plain text data
directly with the server. As with any secret, however, the longer
the length of the secret, the more difficult the secret is to guess.
It is inadvisable to have short cookies that are relied upon to
remain secret for any duration of time.
A proper security-conscious implementation will also need to prevent
timing attacks by ensuring that the amount of time it takes to do
string comparisons is always a function of the total length of the
strings, and not a function of the number of matched characters.
A decoder needs to ensure that larger values or encodings of integers
do not permit exploitation. Decoders MUST limit the size of
integers, both in value and encoded length, that it accepts (see
Section 4.1.1).
Another common security problem is when the remote endpoint
successfully causes the local endpoint to exhaust its memory. This
compressor attempts to deal with the most obvious ways that this
could occur by limiting both the peak and the steady-state amount of
memory consumed in the compressor state, by providing ways for the
application to consume/flush the emitted header fields in small
chunks, and by considering overhead in the state size calculation.
Implementors must still be careful in the creation of APIs to an
implementation of this compressor by ensuring that header field keys
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and values are either emitted as a stream, or that the compression
implementation have a limit on the maximum size of a key or value.
Failure to implement these kinds of safeguards may still result in a
scenario where the local endpoint exhausts its memory.
A particular care should be used for the maximum size of the header
table. While an endpoint can fully control the maximum size of its
header table for the decoding size, by using
SETTINGS_HEADER_TABLE_SIZE, the maximum size of the encoding size is
controlled by the remote peer. The endpoint should check the
SETTINGS_HEADER_TABLE_SIZE defined by the remote peer, and decrease
the maximum size for the encoding size if needed.
6. Acknowledgements
This document includes substantial editorial contributions from the
following individuals: Mike Bishop, Jeff Pinner, Julian Reschke,
Martin Thomson.
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,
<http://dl.acm.org/citation.cfm?id=363991>.
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", September
2012, <https://docs.google.com/a/twist.com/presentation/d/
11eBmGiHbYcHR9gL5nDyZChu_-lCa2GizeuOfaLU2HOU/
edit#slide=id.g1eb6c1b5_3_6>.
[DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
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[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, <http://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=4051119>.
[PERF1] Belshe, M., "IETF83: SPDY and What to Consider for HTTP/
2.0", March 2012, <http://www.ietf.org/proceedings/83/
slides/slides-83-httpbis-3>.
[PERF2] McManus, P., "SPDY: What I Like About You", September
2011, <http://bitsup.blogspot.com/2011/09/spdy-what-i
-like-about-you.html>.
[SPDY] Belshe, M. and R. Peon, "SPDY Protocol", draft-mbelshe-
httpbis-spdy-00 (work in progress), February 2012.
Appendix A. Change Log (to be removed by RFC Editor before publication
A.1. 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.2. 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).
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A.3. Since draft-ietf-httpbis-header-compression-03
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.4. Since draft-ietf-httpbis-header-compression-02
o Corrected error in integer encoding pseudocode.
A.5. 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.6. Since draft-ietf-httpbis-header-compression-00
Fixed bug/omission in integer representation algorithm.
Changed the document title.
Header matching text rewritten.
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Changed the definition of header emission.
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 ordering of these tables is currently arbitrary. The tables in
this section should be updated and ordered such that the table
entries with the smallest indices are those which, based on a
statistical analysis of the frequency of use weighted by size,
achieve the largest decrease in octets transmitted subject to HTTP 2
header field rules (like removal of some header fields). This set of
header fields is currently very likely incomplete, and should be made
complete. ]]
The following table lists the pre-defined header fields that make-up
the static header 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 | 500 |
| 10 | :status | 404 |
| 11 | :status | 403 |
| 12 | :status | 400 |
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| 13 | :status | 401 |
| 14 | accept-charset | |
| 15 | accept-encoding | |
| 16 | accept-language | |
| 17 | accept-ranges | |
| 18 | accept | |
| 19 | access-control-allow-origin | |
| 20 | age | |
| 21 | allow | |
| 22 | authorization | |
| 23 | cache-control | |
| 24 | content-disposition | |
| 25 | content-encoding | |
| 26 | content-language | |
| 27 | content-length | |
| 28 | content-location | |
| 29 | content-range | |
| 30 | content-type | |
| 31 | cookie | |
| 32 | date | |
| 33 | etag | |
| 34 | expect | |
| 35 | expires | |
| 36 | from | |
| 37 | host | |
| 38 | if-match | |
| 39 | if-modified-since | |
| 40 | if-none-match | |
| 41 | if-range | |
| 42 | if-unmodified-since | |
| 43 | last-modified | |
| 44 | link | |
| 45 | location | |
| 46 | max-forwards | |
| 47 | proxy-authenticate | |
| 48 | proxy-authorization | |
| 49 | range | |
| 50 | referer | |
| 51 | refresh | |
| 52 | retry-after | |
| 53 | server | |
| 54 | set-cookie | |
| 55 | strict-transport-security | |
| 56 | transfer-encoding | |
| 57 | user-agent | |
| 58 | vary | |
| 59 | via | |
| 60 | www-authenticate | |
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+-------+-----------------------------+--------------+
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.
Appendix C. Huffman Codes
The following Huffman codes are used when encoding string literals.
[[This table will be regenerated. ]]
aligned aligned
to len to len
MSB in LSB in
sym as bits bits as hex bits
( 0) |11111111|11111111|11110111|010 [27] 7ffffba [27]
( 1) |11111111|11111111|11110111|011 [27] 7ffffbb [27]
( 2) |11111111|11111111|11110111|100 [27] 7ffffbc [27]
( 3) |11111111|11111111|11110111|101 [27] 7ffffbd [27]
( 4) |11111111|11111111|11110111|110 [27] 7ffffbe [27]
( 5) |11111111|11111111|11110111|111 [27] 7ffffbf [27]
( 6) |11111111|11111111|11111000|000 [27] 7ffffc0 [27]
( 7) |11111111|11111111|11111000|001 [27] 7ffffc1 [27]
( 8) |11111111|11111111|11111000|010 [27] 7ffffc2 [27]
( 9) |11111111|11111111|11111000|011 [27] 7ffffc3 [27]
( 10) |11111111|11111111|11111000|100 [27] 7ffffc4 [27]
( 11) |11111111|11111111|11111000|101 [27] 7ffffc5 [27]
( 12) |11111111|11111111|11111000|110 [27] 7ffffc6 [27]
( 13) |11111111|11111111|11111000|111 [27] 7ffffc7 [27]
( 14) |11111111|11111111|11111001|000 [27] 7ffffc8 [27]
( 15) |11111111|11111111|11111001|001 [27] 7ffffc9 [27]
( 16) |11111111|11111111|11111001|010 [27] 7ffffca [27]
( 17) |11111111|11111111|11111001|011 [27] 7ffffcb [27]
( 18) |11111111|11111111|11111001|100 [27] 7ffffcc [27]
( 19) |11111111|11111111|11111001|101 [27] 7ffffcd [27]
( 20) |11111111|11111111|11111001|110 [27] 7ffffce [27]
( 21) |11111111|11111111|11111001|111 [27] 7ffffcf [27]
( 22) |11111111|11111111|11111010|000 [27] 7ffffd0 [27]
( 23) |11111111|11111111|11111010|001 [27] 7ffffd1 [27]
( 24) |11111111|11111111|11111010|010 [27] 7ffffd2 [27]
( 25) |11111111|11111111|11111010|011 [27] 7ffffd3 [27]
( 26) |11111111|11111111|11111010|100 [27] 7ffffd4 [27]
( 27) |11111111|11111111|11111010|101 [27] 7ffffd5 [27]
( 28) |11111111|11111111|11111010|110 [27] 7ffffd6 [27]
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( 29) |11111111|11111111|11111010|111 [27] 7ffffd7 [27]
( 30) |11111111|11111111|11111011|000 [27] 7ffffd8 [27]
( 31) |11111111|11111111|11111011|001 [27] 7ffffd9 [27]
' ' ( 32) |11101000| [8] e8 [8]
'!' ( 33) |11111111|1100 [12] ffc [12]
'"' ( 34) |11111111|111010 [14] 3ffa [14]
'#' ( 35) |11111111|1111100 [15] 7ffc [15]
'$' ( 36) |11111111|1111101 [15] 7ffd [15]
'%' ( 37) |100100 [6] 24 [6]
'&' ( 38) |1101110 [7] 6e [7]
''' ( 39) |11111111|1111110 [15] 7ffe [15]
'(' ( 40) |11111111|010 [11] 7fa [11]
')' ( 41) |11111111|011 [11] 7fb [11]
'*' ( 42) |11111110|10 [10] 3fa [10]
'+' ( 43) |11111111|100 [11] 7fc [11]
',' ( 44) |11101001| [8] e9 [8]
'-' ( 45) |100101 [6] 25 [6]
'.' ( 46) |00100 [5] 4 [5]
'/' ( 47) |0000 [4] 0 [4]
'0' ( 48) |00101 [5] 5 [5]
'1' ( 49) |00110 [5] 6 [5]
'2' ( 50) |00111 [5] 7 [5]
'3' ( 51) |100110 [6] 26 [6]
'4' ( 52) |100111 [6] 27 [6]
'5' ( 53) |101000 [6] 28 [6]
'6' ( 54) |101001 [6] 29 [6]
'7' ( 55) |101010 [6] 2a [6]
'8' ( 56) |101011 [6] 2b [6]
'9' ( 57) |101100 [6] 2c [6]
':' ( 58) |11110110|0 [9] 1ec [9]
';' ( 59) |11101010| [8] ea [8]
'<' ( 60) |11111111|11111111|10 [18] 3fffe [18]
'=' ( 61) |101101 [6] 2d [6]
'>' ( 62) |11111111|11111110|0 [17] 1fffc [17]
'?' ( 63) |11110110|1 [9] 1ed [9]
'@' ( 64) |11111111|111011 [14] 3ffb [14]
'A' ( 65) |1101111 [7] 6f [7]
'B' ( 66) |11101011| [8] eb [8]
'C' ( 67) |11101100| [8] ec [8]
'D' ( 68) |11101101| [8] ed [8]
'E' ( 69) |11101110| [8] ee [8]
'F' ( 70) |1110000 [7] 70 [7]
'G' ( 71) |11110111|0 [9] 1ee [9]
'H' ( 72) |11110111|1 [9] 1ef [9]
'I' ( 73) |11111000|0 [9] 1f0 [9]
'J' ( 74) |11111000|1 [9] 1f1 [9]
'K' ( 75) |11111110|11 [10] 3fb [10]
'L' ( 76) |11111001|0 [9] 1f2 [9]
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'M' ( 77) |11101111| [8] ef [8]
'N' ( 78) |11111001|1 [9] 1f3 [9]
'O' ( 79) |11111010|0 [9] 1f4 [9]
'P' ( 80) |11111010|1 [9] 1f5 [9]
'Q' ( 81) |11111011|0 [9] 1f6 [9]
'R' ( 82) |11111011|1 [9] 1f7 [9]
'S' ( 83) |11110000| [8] f0 [8]
'T' ( 84) |11110001| [8] f1 [8]
'U' ( 85) |11111100|0 [9] 1f8 [9]
'V' ( 86) |11111100|1 [9] 1f9 [9]
'W' ( 87) |11111101|0 [9] 1fa [9]
'X' ( 88) |11111101|1 [9] 1fb [9]
'Y' ( 89) |11111110|0 [9] 1fc [9]
'Z' ( 90) |11111111|00 [10] 3fc [10]
'[' ( 91) |11111111|111100 [14] 3ffc [14]
'\' ( 92) |11111111|11111111|11111011|010 [27] 7ffffda [27]
']' ( 93) |11111111|11100 [13] 1ffc [13]
'^' ( 94) |11111111|111101 [14] 3ffd [14]
'_' ( 95) |101110 [6] 2e [6]
'`' ( 96) |11111111|11111111|110 [19] 7fffe [19]
'a' ( 97) |01000 [5] 8 [5]
'b' ( 98) |101111 [6] 2f [6]
'c' ( 99) |01001 [5] 9 [5]
'd' (100) |110000 [6] 30 [6]
'e' (101) |0001 [4] 1 [4]
'f' (102) |110001 [6] 31 [6]
'g' (103) |110010 [6] 32 [6]
'h' (104) |110011 [6] 33 [6]
'i' (105) |01010 [5] a [5]
'j' (106) |1110001 [7] 71 [7]
'k' (107) |1110010 [7] 72 [7]
'l' (108) |01011 [5] b [5]
'm' (109) |110100 [6] 34 [6]
'n' (110) |01100 [5] c [5]
'o' (111) |01101 [5] d [5]
'p' (112) |01110 [5] e [5]
'q' (113) |11110010| [8] f2 [8]
'r' (114) |01111 [5] f [5]
's' (115) |10000 [5] 10 [5]
't' (116) |10001 [5] 11 [5]
'u' (117) |110101 [6] 35 [6]
'v' (118) |1110011 [7] 73 [7]
'w' (119) |110110 [6] 36 [6]
'x' (120) |11110011| [8] f3 [8]
'y' (121) |11110100| [8] f4 [8]
'z' (122) |11110101| [8] f5 [8]
'{' (123) |11111111|11111110|1 [17] 1fffd [17]
'|' (124) |11111111|101 [11] 7fd [11]
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'}' (125) |11111111|11111111|0 [17] 1fffe [17]
'~' (126) |11111111|1101 [12] ffd [12]
(127) |11111111|11111111|11111011|011 [27] 7ffffdb [27]
(128) |11111111|11111111|11111011|100 [27] 7ffffdc [27]
(129) |11111111|11111111|11111011|101 [27] 7ffffdd [27]
(130) |11111111|11111111|11111011|110 [27] 7ffffde [27]
(131) |11111111|11111111|11111011|111 [27] 7ffffdf [27]
(132) |11111111|11111111|11111100|000 [27] 7ffffe0 [27]
(133) |11111111|11111111|11111100|001 [27] 7ffffe1 [27]
(134) |11111111|11111111|11111100|010 [27] 7ffffe2 [27]
(135) |11111111|11111111|11111100|011 [27] 7ffffe3 [27]
(136) |11111111|11111111|11111100|100 [27] 7ffffe4 [27]
(137) |11111111|11111111|11111100|101 [27] 7ffffe5 [27]
(138) |11111111|11111111|11111100|110 [27] 7ffffe6 [27]
(139) |11111111|11111111|11111100|111 [27] 7ffffe7 [27]
(140) |11111111|11111111|11111101|000 [27] 7ffffe8 [27]
(141) |11111111|11111111|11111101|001 [27] 7ffffe9 [27]
(142) |11111111|11111111|11111101|010 [27] 7ffffea [27]
(143) |11111111|11111111|11111101|011 [27] 7ffffeb [27]
(144) |11111111|11111111|11111101|100 [27] 7ffffec [27]
(145) |11111111|11111111|11111101|101 [27] 7ffffed [27]
(146) |11111111|11111111|11111101|110 [27] 7ffffee [27]
(147) |11111111|11111111|11111101|111 [27] 7ffffef [27]
(148) |11111111|11111111|11111110|000 [27] 7fffff0 [27]
(149) |11111111|11111111|11111110|001 [27] 7fffff1 [27]
(150) |11111111|11111111|11111110|010 [27] 7fffff2 [27]
(151) |11111111|11111111|11111110|011 [27] 7fffff3 [27]
(152) |11111111|11111111|11111110|100 [27] 7fffff4 [27]
(153) |11111111|11111111|11111110|101 [27] 7fffff5 [27]
(154) |11111111|11111111|11111110|110 [27] 7fffff6 [27]
(155) |11111111|11111111|11111110|111 [27] 7fffff7 [27]
(156) |11111111|11111111|11111111|000 [27] 7fffff8 [27]
(157) |11111111|11111111|11111111|001 [27] 7fffff9 [27]
(158) |11111111|11111111|11111111|010 [27] 7fffffa [27]
(159) |11111111|11111111|11111111|011 [27] 7fffffb [27]
(160) |11111111|11111111|11111111|100 [27] 7fffffc [27]
(161) |11111111|11111111|11111111|101 [27] 7fffffd [27]
(162) |11111111|11111111|11111111|110 [27] 7fffffe [27]
(163) |11111111|11111111|11111111|111 [27] 7ffffff [27]
(164) |11111111|11111111|11100000|00 [26] 3ffff80 [26]
(165) |11111111|11111111|11100000|01 [26] 3ffff81 [26]
(166) |11111111|11111111|11100000|10 [26] 3ffff82 [26]
(167) |11111111|11111111|11100000|11 [26] 3ffff83 [26]
(168) |11111111|11111111|11100001|00 [26] 3ffff84 [26]
(169) |11111111|11111111|11100001|01 [26] 3ffff85 [26]
(170) |11111111|11111111|11100001|10 [26] 3ffff86 [26]
(171) |11111111|11111111|11100001|11 [26] 3ffff87 [26]
(172) |11111111|11111111|11100010|00 [26] 3ffff88 [26]
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(173) |11111111|11111111|11100010|01 [26] 3ffff89 [26]
(174) |11111111|11111111|11100010|10 [26] 3ffff8a [26]
(175) |11111111|11111111|11100010|11 [26] 3ffff8b [26]
(176) |11111111|11111111|11100011|00 [26] 3ffff8c [26]
(177) |11111111|11111111|11100011|01 [26] 3ffff8d [26]
(178) |11111111|11111111|11100011|10 [26] 3ffff8e [26]
(179) |11111111|11111111|11100011|11 [26] 3ffff8f [26]
(180) |11111111|11111111|11100100|00 [26] 3ffff90 [26]
(181) |11111111|11111111|11100100|01 [26] 3ffff91 [26]
(182) |11111111|11111111|11100100|10 [26] 3ffff92 [26]
(183) |11111111|11111111|11100100|11 [26] 3ffff93 [26]
(184) |11111111|11111111|11100101|00 [26] 3ffff94 [26]
(185) |11111111|11111111|11100101|01 [26] 3ffff95 [26]
(186) |11111111|11111111|11100101|10 [26] 3ffff96 [26]
(187) |11111111|11111111|11100101|11 [26] 3ffff97 [26]
(188) |11111111|11111111|11100110|00 [26] 3ffff98 [26]
(189) |11111111|11111111|11100110|01 [26] 3ffff99 [26]
(190) |11111111|11111111|11100110|10 [26] 3ffff9a [26]
(191) |11111111|11111111|11100110|11 [26] 3ffff9b [26]
(192) |11111111|11111111|11100111|00 [26] 3ffff9c [26]
(193) |11111111|11111111|11100111|01 [26] 3ffff9d [26]
(194) |11111111|11111111|11100111|10 [26] 3ffff9e [26]
(195) |11111111|11111111|11100111|11 [26] 3ffff9f [26]
(196) |11111111|11111111|11101000|00 [26] 3ffffa0 [26]
(197) |11111111|11111111|11101000|01 [26] 3ffffa1 [26]
(198) |11111111|11111111|11101000|10 [26] 3ffffa2 [26]
(199) |11111111|11111111|11101000|11 [26] 3ffffa3 [26]
(200) |11111111|11111111|11101001|00 [26] 3ffffa4 [26]
(201) |11111111|11111111|11101001|01 [26] 3ffffa5 [26]
(202) |11111111|11111111|11101001|10 [26] 3ffffa6 [26]
(203) |11111111|11111111|11101001|11 [26] 3ffffa7 [26]
(204) |11111111|11111111|11101010|00 [26] 3ffffa8 [26]
(205) |11111111|11111111|11101010|01 [26] 3ffffa9 [26]
(206) |11111111|11111111|11101010|10 [26] 3ffffaa [26]
(207) |11111111|11111111|11101010|11 [26] 3ffffab [26]
(208) |11111111|11111111|11101011|00 [26] 3ffffac [26]
(209) |11111111|11111111|11101011|01 [26] 3ffffad [26]
(210) |11111111|11111111|11101011|10 [26] 3ffffae [26]
(211) |11111111|11111111|11101011|11 [26] 3ffffaf [26]
(212) |11111111|11111111|11101100|00 [26] 3ffffb0 [26]
(213) |11111111|11111111|11101100|01 [26] 3ffffb1 [26]
(214) |11111111|11111111|11101100|10 [26] 3ffffb2 [26]
(215) |11111111|11111111|11101100|11 [26] 3ffffb3 [26]
(216) |11111111|11111111|11101101|00 [26] 3ffffb4 [26]
(217) |11111111|11111111|11101101|01 [26] 3ffffb5 [26]
(218) |11111111|11111111|11101101|10 [26] 3ffffb6 [26]
(219) |11111111|11111111|11101101|11 [26] 3ffffb7 [26]
(220) |11111111|11111111|11101110|00 [26] 3ffffb8 [26]
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(221) |11111111|11111111|11101110|01 [26] 3ffffb9 [26]
(222) |11111111|11111111|11101110|10 [26] 3ffffba [26]
(223) |11111111|11111111|11101110|11 [26] 3ffffbb [26]
(224) |11111111|11111111|11101111|00 [26] 3ffffbc [26]
(225) |11111111|11111111|11101111|01 [26] 3ffffbd [26]
(226) |11111111|11111111|11101111|10 [26] 3ffffbe [26]
(227) |11111111|11111111|11101111|11 [26] 3ffffbf [26]
(228) |11111111|11111111|11110000|00 [26] 3ffffc0 [26]
(229) |11111111|11111111|11110000|01 [26] 3ffffc1 [26]
(230) |11111111|11111111|11110000|10 [26] 3ffffc2 [26]
(231) |11111111|11111111|11110000|11 [26] 3ffffc3 [26]
(232) |11111111|11111111|11110001|00 [26] 3ffffc4 [26]
(233) |11111111|11111111|11110001|01 [26] 3ffffc5 [26]
(234) |11111111|11111111|11110001|10 [26] 3ffffc6 [26]
(235) |11111111|11111111|11110001|11 [26] 3ffffc7 [26]
(236) |11111111|11111111|11110010|00 [26] 3ffffc8 [26]
(237) |11111111|11111111|11110010|01 [26] 3ffffc9 [26]
(238) |11111111|11111111|11110010|10 [26] 3ffffca [26]
(239) |11111111|11111111|11110010|11 [26] 3ffffcb [26]
(240) |11111111|11111111|11110011|00 [26] 3ffffcc [26]
(241) |11111111|11111111|11110011|01 [26] 3ffffcd [26]
(242) |11111111|11111111|11110011|10 [26] 3ffffce [26]
(243) |11111111|11111111|11110011|11 [26] 3ffffcf [26]
(244) |11111111|11111111|11110100|00 [26] 3ffffd0 [26]
(245) |11111111|11111111|11110100|01 [26] 3ffffd1 [26]
(246) |11111111|11111111|11110100|10 [26] 3ffffd2 [26]
(247) |11111111|11111111|11110100|11 [26] 3ffffd3 [26]
(248) |11111111|11111111|11110101|00 [26] 3ffffd4 [26]
(249) |11111111|11111111|11110101|01 [26] 3ffffd5 [26]
(250) |11111111|11111111|11110101|10 [26] 3ffffd6 [26]
(251) |11111111|11111111|11110101|11 [26] 3ffffd7 [26]
(252) |11111111|11111111|11110110|00 [26] 3ffffd8 [26]
(253) |11111111|11111111|11110110|01 [26] 3ffffd9 [26]
(254) |11111111|11111111|11110110|10 [26] 3ffffda [26]
(255) |11111111|11111111|11110110|11 [26] 3ffffdb [26]
EOS (256) |11111111|11111111|11110111|00 [26] 3ffffdc [26]
Appendix D. Examples
A number of examples are worked through here, for both requests and
responses, and with and without Huffman coding.
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D.1. Header Field Representation Examples
This section show several independent representation examples.
D.1.1. 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.
Hex dump of encoded data:
000a 6375 7374 6f6d 2d6b 6579 0d63 7573 | ..custom-key.cus
746f 6d2d 6865 6164 6572 | tom-header
Decoding process:
00 | == 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.1.2. Literal Header Field without Indexing
The header field representation uses an indexed name and a literal
value.
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Header set to encode:
:path: /sample/path
Reference set: empty.
Hex dump of encoded data:
440c 2f73 616d 706c 652f 7061 7468 | D./sample/path
Decoding process:
44 | == 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.1.3. 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 | .
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Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Header Table (after decoding):
[ 1] (s = 42) :method: GET
Table size: 42
Decoded header set:
:method: GET
D.1.4. 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.
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Decoded header set:
:method: GET
D.2. Request Examples without Huffman
This section shows several consecutive header sets, corresponding to
HTTP requests, on the same connection.
D.2.1. First request
Header set to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
Reference set: empty.
Hex dump of encoded data:
8287 8604 0f77 7777 2e65 7861 6d70 6c65 | .....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: /
04 | == 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
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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:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
D.2.2. 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:
1b08 6e6f 2d63 6163 6865 | ..no-cache
Decoding process:
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1b | == Literal indexed ==
| Indexed name (idx = 27)
| cache-control
08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | 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.2.3. 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:
8080 858c 8b84 000a 6375 7374 6f6d 2d6b | ........custom-k
6579 0c63 7573 746f 6d2d 7661 6c75 65 | ey.custom-value
Decoding process:
80 80 | == 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
00 | == 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):
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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.3. Request Examples with Huffman
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values.
D.3.1. First request
Header set to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
Reference set: empty.
Hex dump of encoded data:
8287 8604 8bdb 6d88 3e68 d1cb 1225 ba7f | ......m..h...%..
Decoding process:
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82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
04 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
8b | Literal value (len = 15)
| Huffman encoded:
db6d 883e 68d1 cb12 25ba 7f | .m..h...%..
| Decoded:
| 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:
: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:
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: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:
1b86 6365 4a13 98ff | ..ceJ...
Decoding process:
1b | == Literal indexed ==
| Indexed name (idx = 27)
| cache-control
86 | Literal value (len = 8)
| Huffman encoded:
6365 4a13 98ff | ceJ...
| 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:
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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:
8080 858c 8b84 0088 4eb0 8b74 9790 fa7f | ........N..t....
894e b08b 7497 9a17 a8ff | .N..t.....
Decoding process:
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80 80 | == 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
00 | == Literal indexed ==
88 | Literal name (len = 10)
| Huffman encoded:
4eb0 8b74 9790 fa7f | N..t....
| Decoded:
| custom-key
89 | Literal value (len = 12)
| Huffman encoded:
4eb0 8b74 979a 17a8 ff | N..t.....
| Decoded:
| 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:
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:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
D.4. 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.4.1. 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:
0803 3330 3218 0770 7269 7661 7465 221d | ..302..private".
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 5430 1768 | 20:13:21 GMT0.h
7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
6c65 2e63 6f6d | le.com
Decoding process:
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08 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 32 | 302
| -> :status: 302
18 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
22 | == Literal indexed ==
| Indexed name (idx = 34)
| 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
30 | == Literal indexed ==
| Indexed name (idx = 48)
| 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.4.2. 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.4.3. 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 031d 4d6f 6e2c 2032 3120 4f63 7420 | ....Mon, 21 Oct
3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
541d 0467 7a69 7084 8483 833a 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
03 | == 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
1d | == Literal indexed ==
| Indexed name (idx = 29)
| 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
3a | == Literal indexed ==
| Indexed name (idx = 58)
| 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.5. 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.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.
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Hex dump of encoded data:
0882 98a7 1885 73d5 cd11 1f22 98ef 6b3a | ......s...."..k:
7a0e 6e8f a263 d072 9a6e 8397 d869 bd87 | z.n..c.r.n...i..
3747 bbbf c730 90ce 3174 3d80 1b6d b107 | 7G...0..1t=..m..
cd1a 3962 44b7 4f | ..9bD.O
Decoding process:
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08 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 3)
| Huffman encoded:
98a7 | ..
| Decoded:
| 302
| -> :status: 302
18 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
85 | Literal value (len = 7)
| Huffman encoded:
73d5 cd11 1f | s....
| Decoded:
| private
| -> cache-control: private
22 | == Literal indexed ==
| Indexed name (idx = 34)
| date
98 | Literal value (len = 29)
| Huffman encoded:
ef6b 3a7a 0e6e 8fa2 63d0 729a 6e83 97d8 | .k:z.n..c.r.n...
69bd 8737 47bb bfc7 | i..7G...
| Decoded:
| Mon, 21 Oct 2013 20:13:21 \
| GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
30 | == Literal indexed ==
| Indexed name (idx = 48)
| location
90 | Literal value (len = 23)
| Huffman encoded:
ce31 743d 801b 6db1 07cd 1a39 6244 b74f | .1t=..m....9bD.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.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:
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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.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
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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 0398 ef6b 3a7a 0e6e 8fa2 63d0 729a | .....k:z.n..c.r.
6e83 97d8 69bd 873f 47bb bfc7 1d83 cbd5 | n...i..?G.......
4e84 8483 833a b3c5 adb7 7f87 6fc7 fbf7 | N....:......o...
fdbf bebf f3f7 f4fb 7ebb be9f 5f87 e37f | ............_...
efed faee fa7c 3f1d 5d1a 23ce 5464 36cd | .....|?.].#.Td6.
494b d5d1 cc5f 0535 969b | IK..._.5..
Decoding process:
84 | == Indexed - Remove ==
| idx = 4
| -> cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
03 | == Literal indexed ==
| Indexed name (idx = 3)
| date
98 | Literal value (len = 29)
| Huffman encoded:
ef6b 3a7a 0e6e 8fa2 63d0 729a 6e83 97d8 | .k:z.n..c.r.n...
69bd 873f 47bb bfc7 | i..?G...
| Decoded:
| Mon, 21 Oct 2013 20:13:22 \
| GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
1d | == Literal indexed ==
| Indexed name (idx = 29)
| content-encoding
83 | Literal value (len = 4)
| Huffman encoded:
cbd5 4e | ..N
| 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
3a | == Literal indexed ==
| Indexed name (idx = 58)
| set-cookie
b3 | Literal value (len = 56)
| Huffman encoded:
c5ad b77f 876f c7fb f7fd bfbe bff3 f7f4 | .....o..........
fb7e bbbe 9f5f 87e3 7fef edfa eefa 7c3f | ....._........|?
1d5d 1a23 ce54 6436 cd49 4bd5 d1cc 5f05 | .].#.Td6.IK..._.
3596 9b | 5..
| 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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