HPACK - Header Compression for HTTP/2
draft-ietf-httpbis-header-compression-09
The information below is for an old version of the document.
| Document | Type |
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| Authors | Roberto Peon , Herve Ruellan | ||
| Last updated | 2014-10-07 (Latest revision 2014-07-31) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Document shepherd | Mark Nottingham | ||
| Shepherd write-up | Show Last changed 2014-10-07 | ||
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draft-ietf-httpbis-header-compression-09
HTTPbis Working Group R. Peon
Internet-Draft Google, Inc
Intended status: Standards Track H. Ruellan
Expires: February 01, 2015 Canon CRF
July 31, 2014
HPACK - Header Compression for HTTP/2
draft-ietf-httpbis-header-compression-09
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 <https://
lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <http://tools.ietf.org/wg/
httpbis/>; that specific to HTTP/2 are at <http://http2.github.io/>.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 01, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. HPACK Overview . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
3. Compression Process Overview . . . . . . . . . . . . . . . . 5
3.1. Header List Ordering . . . . . . . . . . . . . . . . . . 6
3.2. Encoding and Decoding Contexts . . . . . . . . . . . . . 6
3.3. Indexing Tables . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. Static Table . . . . . . . . . . . . . . . . . . . . 7
3.3.2. Header Table . . . . . . . . . . . . . . . . . . . . 7
3.3.3. Index Address Space . . . . . . . . . . . . . . . . . 7
3.4. Header Field Representation . . . . . . . . . . . . . . . 8
4. Header Block Decoding . . . . . . . . . . . . . . . . . . . . 8
4.1. Header Block Processing . . . . . . . . . . . . . . . . . 8
4.2. Header Field Representation Processing . . . . . . . . . 9
5. Header Table Management . . . . . . . . . . . . . . . . . . . 9
5.1. Maximum Table Size . . . . . . . . . . . . . . . . . . . 9
5.2. Entry Eviction when Header Table Size Changes . . . . . . 10
5.3. Entry Eviction when Adding New Entries . . . . . . . . . 11
6. Primitive Type Representations . . . . . . . . . . . . . . . 11
6.1. Integer Representation . . . . . . . . . . . . . . . . . 11
6.2. String Literal Representation . . . . . . . . . . . . . . 12
7. Binary Format . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Indexed Header Field Representation . . . . . . . . . . . 13
7.2. Literal Header Field Representation . . . . . . . . . . . 14
7.2.1. Literal Header Field with Incremental Indexing . . . 14
7.2.2. Literal Header Field without Indexing . . . . . . . . 15
7.2.3. Literal Header Field never Indexed . . . . . . . . . 16
7.3. Header Table Size Update . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
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8.1. Probing Header Table State . . . . . . . . . . . . . . . 18
8.1.1. Applicability to HPACK and HTTP . . . . . . . . . . . 19
8.1.2. Mitigation . . . . . . . . . . . . . . . . . . . . . 19
8.1.3. Never Indexed Literals . . . . . . . . . . . . . . . 20
8.2. Static Huffman Encoding . . . . . . . . . . . . . . . . . 20
8.3. Memory Consumption . . . . . . . . . . . . . . . . . . . 20
8.4. Implementation Limits . . . . . . . . . . . . . . . . . . 21
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 23
A.1. Since draft-ietf-httpbis-header-compression-08 . . . . . 23
A.2. Since draft-ietf-httpbis-header-compression-07 . . . . . 23
A.3. Since draft-ietf-httpbis-header-compression-06 . . . . . 24
A.4. Since draft-ietf-httpbis-header-compression-05 . . . . . 24
A.5. Since draft-ietf-httpbis-header-compression-04 . . . . . 24
A.6. Since draft-ietf-httpbis-header-compression-03 . . . . . 25
A.7. Since draft-ietf-httpbis-header-compression-02 . . . . . 25
A.8. Since draft-ietf-httpbis-header-compression-01 . . . . . 25
A.9. Since draft-ietf-httpbis-header-compression-00 . . . . . 25
Appendix B. Static Table Definition . . . . . . . . . . . . . . 26
Appendix C. Huffman Code . . . . . . . . . . . . . . . . . . . . 28
Appendix D. Examples . . . . . . . . . . . . . . . . . . . . . . 34
D.1. Integer Representation Examples . . . . . . . . . . . . . 34
D.1.1. Example 1: Encoding 10 Using a 5-bit Prefix . . . . . 34
D.1.2. Example 2: Encoding 1337 Using a 5-bit Prefix . . . . 34
D.1.3. Example 3: Encoding 42 Starting at an Octet Boundary 35
D.2. Header Field Representation Examples . . . . . . . . . . 35
D.2.1. Literal Header Field with Indexing . . . . . . . . . 35
D.2.2. Literal Header Field without Indexing . . . . . . . . 36
D.2.3. Literal Header Field never Indexed . . . . . . . . . 37
D.2.4. Indexed Header Field . . . . . . . . . . . . . . . . 38
D.3. Request Examples without Huffman Coding . . . . . . . . . 38
D.3.1. First Request . . . . . . . . . . . . . . . . . . . . 38
D.3.2. Second Request . . . . . . . . . . . . . . . . . . . 39
D.3.3. Third Request . . . . . . . . . . . . . . . . . . . . 41
D.4. Request Examples with Huffman Coding . . . . . . . . . . 42
D.4.1. First Request . . . . . . . . . . . . . . . . . . . . 42
D.4.2. Second Request . . . . . . . . . . . . . . . . . . . 43
D.4.3. Third Request . . . . . . . . . . . . . . . . . . . . 44
D.5. Response Examples without Huffman Coding . . . . . . . . 46
D.5.1. First Response . . . . . . . . . . . . . . . . . . . 46
D.5.2. Second Response . . . . . . . . . . . . . . . . . . . 48
D.5.3. Third Response . . . . . . . . . . . . . . . . . . . 49
D.6. Response Examples with Huffman Coding . . . . . . . . . . 51
D.6.1. First Response . . . . . . . . . . . . . . . . . . . 51
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D.6.2. Second Response . . . . . . . . . . . . . . . . . . . 53
D.6.3. Third Response . . . . . . . . . . . . . . . . . . . 54
1. Introduction
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context of HTTP/2
[HTTP2].
2. HPACK Overview
In HTTP/1.1 (see [RFC7230]), header fields are encoded without any
form of compression. As web pages have grown to include dozens to
hundreds of requests, the redundant header fields in these requests
now measurably increase latency and unnecessarily consume bandwidth
(see [SPDY-DESC-1] and [SPDY-DESC-2]).
SPDY [SPDY] initially addressed this redundancy by compressing header
fields using the DEFLATE [DEFLATE] format, 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, limits vulnerability to
known security attacks, and which 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 as new values are encoded or decoded.
A list of header fields is treated as an ordered collection of name-
value pairs that can include duplicates. Names and values are
considered to be opaque sequences of octets. The order of header
fields is preserved after being compressed and decompressed.
In the encoded form, a header field is represented either literally
or as a reference to a name-value pair in a header table. A list of
header fields can therefore be encoded using a mixture of references
and literal values.
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 prescribed by the encoder,
reconstructing the list of header fields in the process. This
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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.
2.2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate. Hexadecimal literals are prefixed
with "0x" to distinguish them from decimal literals.
2.3. Terminology
This document uses the following terms:
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 3.3.2) is used to
associate stored header fields to index values. This table is
dynamic and specific to an encoding or decoding context.
Static Table: The static table (see Section 3.3.1) is used to
associate static header fields to index values. This table is
ordered, read-only, always accessible, and may be shared amongst
all encoding or decoding contexts.
Header List: A header list is an ordered collection of header fields
that are encoded jointly. It can contain duplicate header fields.
A complete list of key-value pairs contained in a HTTP request or
response is a header list.
Header Field Representation: A header field can be represented in
encoded form either as a literal or as an index (see Section 3.4).
Header Block: An ordered list of header field representations which,
when decoded, yields a complete header list.
3. Compression Process Overview
This specification does not describe a specific algorithm for an
encoder. Instead, it defines precisely how a decoder is expected to
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operate, allowing encoders to produce any encoding that this
definition permits.
3.1. Header List Ordering
The compression and decompression process preserve the ordering of
header fields inside the header list. An encoder SHOULD order header
field representations in the header block according to their ordering
in the original header list. A decoder SHOULD order header fields in
the decoded header list according to their ordering in the header
block.
In particular, representations for pseudo-header fields (see
Section 8.1.2.1 of [HTTP2]) MUST appear before representations for
regular header fields in a header block. In a decoded header list,
pseudo-header fields MUST appear before regular header fields.
3.2. Encoding and Decoding Contexts
To decompress header blocks, a decoder only needs to maintain a
header table (see Section 3.3.2) as a decoding context. No other
state information is needed.
An encoder that wishes to reference entries in the header table needs
to maintain a copy of the header table used by the decoder.
When used for bidirectional communication, such as in HTTP, the
encoding and decoding header tables maintained by an endpoint are
completely independent. Header fields are encoded without any
reference to the local decoding header table; and header fields are
decoded without reference to the local encoding header table.
3.3. Indexing Tables
HPACK uses two tables for associating header fields to indexes. The
static table (see Section 3.3.1) is predefined and contains common
header fields (most of them with an empty value). The header table
(see Section 3.3.2) is dynamic and can be used by the encoder to
index header fields repeated in the encoded header lists.
These two tables are combined into a single address space for
defining index values (see Section 3.3.3).
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3.3.1. Static Table
The static table consists of a predefined static list of header
fields. Its entries are defined in Appendix B.
3.3.2. Header Table
The header table consists of a list of header fields maintained in
first-in, first-out order. The first and newest entry in a header
table is always at index 1, and the oldest entry of a header table is
at the index corresponding to the number of entries in the header
table.
The header table is initially empty.
The header table can contain duplicate entries. Therefore, 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 5.1).
The header table is updated during the processing of a list of header
field representations (see Section 4.2).
3.3.3. Index Address Space
The static table and the header table are combined into a single
index address space.
Indices between 1 and the length of the static table (inclusive)
refer to elements in the static table (see Section 3.3.1).
Indices strictly greater than the length of the static table refer to
elements in the header table (see Section 3.3.2). The length of the
static table is subtracted to find the index into the header table.
Indices strictly greater than the sum of the lengths of both tables
MUST be treated as a decoding error.
For a static table size of s and a header table size of k, the
following diagram shows the entire valid index address space.
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<---------- Index Address Space ---------->
<-- Static Table --> <-- Header Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | s | |s+1| ... |s+k|
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Dropping Point
Index Address Space
3.4. Header Field Representation
An encoded header field can be represented either as a literal or as
an index.
A literal representation defines a header field by specifying its
name and value. The header field name can be represented literally
or as a reference to an entry in either the static table or the
header table. The header field value is represented literally.
Three different literal representations are provided:
o A literal representation that does not add the header field to the
header table (see Section 7.2.2).
o A literal representation that does not add the header field to the
header table, with the additional stipulation that this header
field always use a literal representation, in particular when re-
encoded by an intermediary (see Section 7.2.3).
o A literal representation that adds the header field as a new entry
at the beginning of the header table (see Section 7.2.1).
An indexed representation defines a header field as a reference to an
entry in either the static table or the header table (see
Section 7.1).
4. Header Block Decoding
4.1. Header Block Processing
A decoder processes an encoded header block sequentially to
reconstruct the original header list.
Once a header field is decoded and added to the reconstructed header
list, it cannot be removed from it. A header field added to the
header list can be safely passed to the upper processing layer.
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By passing decoded header fields to the upper processing layer, a
decoder can be implemented with minimal transitory memory commitment
in addition to the header table. The management of memory for
handling very large lists of header fields can therefore be deferred
to the upper processing layers.
4.2. Header Field Representation Processing
The processing of a header block to obtain a header list is defined
in this section. To ensure that the decoding will successfully
produce a header list, a decoder MUST obey the following rules.
All the header field representations contained in a header block are
processed in the order in which they appear, as specified below.
Details on the formatting of the various header field
representations, and some additional processing instructions are
found in Section 7.
An _indexed representation_ entails the following actions:
o The header field corresponding to the referenced entry in either
the static table or header table is added to the decoded header
list.
A _literal representation_ that is _not added_ to the header table
entails the following action:
o The header field is added to the decoded header list.
A _literal representation_ that is _added_ to the header table
entails the following actions:
o The header field is added to the decoded header list.
o The header field is inserted at the beginning of the header table.
5. Header Table Management
5.1. Maximum Table Size
To limit the memory requirements on the decoder side, the header
table is constrained in size.
The size of the header table is bounded by a maximum size defined by
the encoder. The size of the header table MUST always be lower than
or equal to this maximum size.
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By default, the maximum size of the header table is equal to the
value of the HTTP/2 setting parameter SETTINGS_HEADER_TABLE_SIZE
defined by the decoder (see Section 6.5.2 of [HTTP2]). The encoder
can change this maximum size (see Section 7.3), but it MUST stay
lower than or equal to the value of SETTINGS_HEADER_TABLE_SIZE.
After applying an updated value of the SETTINGS_HEADER_TABLE_SIZE
parameter that changes the maximum size of the header table used by
the encoder, the encoder MUST signal this change via an encoding
context update (see Section 7.3). This encoding context update MUST
occur at the beginning of the first header block following the
SETTINGS frame sent to acknowledge the application of the updated
settings (see Section 6.5.3 of [HTTP2]).
Several updates to the value of the SETTINGS_HEADER_TABLE_SIZE
parameter can occur between the sending of two header blocks. In the
case that the value of this parameter is changed more that once, if
one of its value is smaller than the new maximum size, the smallest
value for the parameter MUST be sent before the new maximum size,
using two encoding context updates. This ensures that the decoder is
able to perform eviction based on the decoder table size (see
Section 5.2).
This mechanism can be used with a SETTINGS_HEADER_TABLE_SIZE
parameter value of 0 to completely clear entries from the header
table.
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 6.2), its value's length in octets (see
Section 6.2), plus 32.
The size of an entry is calculated using the length of the name and
value without any Huffman encoding applied.
The additional 32 octets account for the overhead associated with an
entry. For example, an entry structure using two 64-bit pointers to
reference the name and the value of the entry, and two 64-bit
integers for counting the number of references to the name and value
would have 32 octets of overhead.
5.2. Entry Eviction when Header Table Size Changes
Whenever the maximum size for the header table is reduced, entries
are evicted from the end of the header table until the size of the
header table is less than or equal to the maximum size.
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5.3. Entry Eviction when Adding New Entries
Whenever a new entry is to be added to the header table, entries are
evicted from the end of the header table until the size of the header
table is less than or equal to (maximum size - new entry size), or
until the table is empty.
If the representation of the added entry references the name of an
entry in the header table, the referenced name is cached prior to
performing eviction to avoid having the name inadvertently evicted.
If the size of the new entry is less than or equal to the maximum
size, that entry is added to the table. It is not an error to
attempt to add an entry that is larger than the maximum size; an
attempt to add an entry larger than the entire table causes the table
to be emptied of all existing entries.
6. Primitive Type Representations
HPACK encoding uses two primitive types: unsigned variable length
integers, and strings of octets.
6.1. 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 <http://en.wikipedia.org/wiki/Variable-
length_quantity>). N is always between 1 and 8 bits. An integer
starting at an octet-boundary will have an 8-bit prefix.
The algorithm to represent an integer I is as follows:
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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
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.
6.2. 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 Huffman code (see [HUFFMAN]).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| H | String Length (7+) |
+---+---------------------------+
| String Data (Length octets) |
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+-------------------------------+
String Literal Representation
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 6.1).
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 code defined in Appendix C (see examples for requests in
Appendix D.4 and for responses in Appendix D.6). The encoded data is
the bitwise concatenation of the 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 code corresponding to the
EOS (end-of-string) symbol are used.
Upon decoding, an incomplete code at the end of the encoded data is
to be considered as padding and discarded. A padding strictly longer
than 7 bits MUST be treated as a decoding error. A padding not
corresponding to the most significant bits of the code for the EOS
symbol MUST be treated as a decoding error. A Huffman encoded string
literal containing the EOS symbol MUST be treated as a decoding
error.
7. Binary Format
This section describes the detailed format of each of the different
header field representations, plus the encoding context update
instruction.
7.1. Indexed Header Field Representation
An indexed header field representation identifies an entry in either
the static table or the header table (see Section 3.3).
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An indexed header field representation causes a header field to be
added to the decoded header list, as described in Section 4.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
Indexed Header Field
An indexed header field starts with the '1' 1-bit pattern, followed
by the index of the matching pair, represented as an integer with a
7-bit prefix (see Section 6.1).
The index value of 0 is not used. It MUST be treated as a decoding
error if found in an indexed header field representation.
7.2. Literal Header Field Representation
A literal header field representation contains a literal header field
value. Header field names are either provided as a literal or by
reference to an existing table entry, either from the static table or
the header table (see Section 3.3).
A literal representation causes a header field to be added to the
decoded header list, as described in Section 4.2.
7.2.1. Literal Header Field with Incremental Indexing
A literal header field with incremental indexing representation
results in adding a header field to the decoded header list and
inserting it as a new entry into 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 |
+---+---+-----------------------+
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| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field with Incremental Indexing - New Name
A literal header field with incremental indexing representation
starts with the '01' 2-bit pattern.
If the header field name matches the header field name of an entry
stored in the static table or the header table, the header field name
can be represented using the index of that entry. In this case, the
index of the entry is represented as an integer with a 6-bit prefix
(see Section 6.1). This value is always non-zero.
Otherwise, the header field name is represented as a literal string
(see Section 6.2). A value 0 is used in place of the 6-bit index,
followed by the header field name.
Either form of header field name representation is followed by the
header field value represented as a literal string (see Section 6.2).
7.2.2. Literal Header Field without Indexing
A literal header field without indexing representation results in
adding a header field to the decoded header list 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 |
+---+---+-----------------------+
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| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field without Indexing - New Name
A literal header field without indexing representation starts with
the '0000' 4-bit pattern.
If the header field name matches the header field name of an entry
stored in the static table or the header table, the header field name
can be represented using the index of that entry. In this case, the
index of the entry is represented as an integer with a 4-bit prefix
(see Section 6.1). This value is always non-zero.
Otherwise, the header field name is represented as a literal string
(see Section 6.2). A value 0 is used in place of the 4-bit index,
followed by the header field name.
Either form of header field name representation is followed by the
header field value represented as a literal string (see Section 6.2).
7.2.3. Literal Header Field never Indexed
A literal header field never indexed representation results in adding
a header field to the decoded header list without altering the header
table. Intermediaries MUST use the same representation for encoding
this header field.
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 |
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+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field never Indexed - New Name
A literal header field never indexed representation starts with the
'0001' 4-bit pattern.
When a header field is represented as a literal header field never
indexed, it MUST always be encoded with this specific literal
representation. In particular, when a peer sends a header field that
it received represented as a literal header field never indexed, it
MUST use the same representation to forward this header field.
This representation is intended for protecting header field values
that are not to be put at risk by compressing them (see Section 8.1
for more details).
The encoding of the representation is identical to the literal header
field without indexing (see Section 7.2.2).
7.3. Header Table Size Update
A header table size update signals a change to the size of the header
table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Max size (5+) |
+---+---------------------------+
Maximum Header Table Size Change
A header table size update starts with the '001' 3-bit pattern,
followed by the new maximum size, represented as an integer with a
5-bit prefix (see Section 6.1).
The new maximum size MUST be lower than or equal to the last value of
the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 6.5.2 of
[HTTP2]) received from the decoder and acknowledged by the encoder
(see Section 6.5.3 of [HTTP2]).
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Reducing the maximum size of the header table can cause entries to be
evicted (see Section 5.2).
8. Security Considerations
This section describes potential areas of security concern with
HPACK:
o Use of compression as a length-based oracle for verifying guesses
about secrets that are compressed into a shared compression
context.
o Denial of service resulting from exhausting processing or memory
capacity at a decoder.
8.1. Probing Header Table State
HPACK reduces the length of header field encodings by exploiting the
redundancy inherent in protocols like HTTP. The ultimate goal of
this is to reduce the amount of data that is required to send HTTP
requests or responses.
The compression context used to encode header fields can be probed by
an attacker that has the following capabilities: to define header
fields to be encoded and transmitted; and to observe the length of
those fields once they are encoded. This allows an attacker to
adaptively modify requests in order to confirm guesses about the
header table state. If a guess is compressed into a shorter length,
the attacker can observe the encoded length and infer that the guess
was correct.
This is possible because while TLS provides confidentiality
protection for content, it only provides a limited amount of
protection for the length of that content.
Note: Padding schemes only provide limited protection against an
attacker with these capabilities, potentially only forcing an
increased number of guesses to learn the length associated with a
given guess. Padding schemes also work directly against
compression by increasing the number of bits that are transmitted.
Attacks like CRIME [CRIME] demonstrated the existence of these
general attacker capabilities. The specific attack exploited the
fact that DEFLATE [DEFLATE] removes redundancy based on prefix
matching. This permitted the attacker to confirm guesses a character
at a time, reducing an exponential-time attack into a linear-time
attack.
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8.1.1. Applicability to HPACK and HTTP
HPACK mitigates but does not completely prevent attacks modelled on
CRIME [CRIME] by forcing a guess to match an entire header field
value, rather than individual characters. An attacker can only learn
whether a guess is correct or not, so is reduced to a brute force
guess for the header field values.
The viability of recovering specific header field values therefore
depends on the entropy of values. As a result, values with high
entropy are unlikely to be recovered successfully. However, values
with low entropy remain vulnerable.
Attacks of this nature are possible any time that two mutually
distrustful entities control requests or responses that are placed
onto a single HTTP/2 connection. If the shared HPACK compressor
permits one entity to add entries to the header table, and the other
to access those entries, then the state of the table can be learned.
Having requests or responses from mutually distrustful entities
occurs when an intermediary either:
o sends requests from multiple clients on a single connection toward
an origin server, or
o takes responses from multiple origin servers and places them on a
shared connection toward a client.
Web browsers also need to assume that requests made on the same
connection by different web origins [ORIGIN] are made by mutually
distrustful entities.
8.1.2. Mitigation
Users of HTTP that require confidentiality for header fields can use
values with entropy sufficient to make guessing infeasible. However,
this is impractical as a general solution because it forces all users
of HTTP to take steps to mitigate attacks. It would impose new
constraints on how HTTP is used.
Rather than impose constraints on users of HTTP, an implementation of
HPACK can instead constrain how compression is applied in order to
limit the potential for header table probing.
An ideal solution segregates access to the header table based on the
entity that is constructing header fields. Header field values that
are added to the table are attributed to an entity, and only the
entity that created an particular value can extract that value.
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To improve compression performance of this option, certain entries
might be tagged as being public. For example, a web browser might
make the values of the Accept-Encoding header field available in all
requests.
An encoder without good knowledge of the provenance of header fields
might instead introduce a penalty for bad guesses, such that attempts
to guess a header field value results in all values being removed
from consideration in all future requests, effectively preventing
further guesses.
Note: Simply removing values from the header table can be
ineffectual if the attacker has a reliable way of causing values
to be reinstalled. For example, a request to load an image in a
web browser typically includes the Cookie header field (a
potentially highly valued target for this sort of attack), and web
sites can easily force an image to be loaded, thereby refreshing
the entry in the header table.
This response might be made inversely proportional to the length of
the header field. Marking as inaccessible might occur for shorter
values more quickly or with higher probability than for longer
values.
Implementations might also choose to protect certain header fields
that are known to be highly valued, such as the Authorization or
Cookie header fields, by disabling or further limiting compression.
8.1.3. Never Indexed Literals
Refusing to generate an indexed representation for a header field is
only effective if compression is avoided on all hops. The never
indexed literal (see Section 7.2.3) can be used to signal to
intermediaries that a particular value was intentionally sent as a
literal. An intermediary MUST NOT re-encode a value that uses the
never indexed literal with a representation that would index it.
8.2. Static Huffman Encoding
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]).
8.3. Memory Consumption
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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
decoder using the value of the HTTP/2 setting parameter
SETTINGS_HEADER_TABLE_SIZE (see Section 6.5.2 of [HTTP2]). This
limit takes into account both the size of the data stored in the
header table, plus a small allowance for overhead.
A decoder can limit the amount of state memory used by setting an
appropriate value for the SETTINGS_HEADER_TABLE_SIZE parameter. An
encoder can limit the amount of state memory it uses by signalling
lower header table size than the decoder allows (see Section 7.3).
The amount of temporary memory consumed by an encoder or decoder can
be limited by processing header fields sequentially. An
implementation does not need to retain a complete list of header
fields. Note however that it might be necessary for an application
to retain a complete header list for other reasons; even though HPACK
does not force this to occur, application constraints might make this
necessary.
8.4. Implementation Limits
An implementation of HPACK needs to ensure that large values for
integers, long encoding for integers, or long string literals do not
create security weaknesses.
An implementation has to set a limit for the values it accepts for
integers, as well as for the encoded length (see Section 6.1). In
the same way, it has to set a limit to the length it accepts for
string literals (see Section 6.2).
9. Acknowledgements
This document includes substantial input from the following
individuals:
o Mike Bishop, Jeff Pinner, Julian Reschke, Martin Thomson
(substantial editorial contributions).
o Johnny Graettinger (Huffman code statistics).
10. References
10.1. Normative References
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[HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol version 2", draft-ietf-httpbis-http2-14
(work in progress), July 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels ", BCP 14, RFC 2119, March 1997.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing ", RFC
7230, June 2014.
10.2. Informative References
[CANONICAL]
Schwartz, E. and B. Kallick, "Generating a canonical
prefix encoding", Communications of the ACM Volume 7 Issue
3, pp. 166-169, March 1964, <https://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.
[HUFFMAN] 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, <https://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=4051119>.
[ORIGIN] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
[PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding Table
Information Leakage", April 2013, <http://www.pdl.cmu.edu/
PDL-FTP/associated/CMU-PDL-13-106.pdf>.
[SPDY-DESC-1]
Belshe, M., "IETF83: SPDY and What to Consider for HTTP/
2.0 ", March 2012, <https://www.ietf.org/proceedings/83/
slides/slides-83-httpbis-3>.
[SPDY-DESC-2]
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McManus, P., "SPDY: What I Like About You", September
2011, <https://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-08
o Removed the reference set.
o Removed header emission.
o Explicit handling of several SETTINGS_HEADER_TABLE_SIZE parameter
changes.
o Changed header set to header list, and forced ordering.
o Updated examples.
o Exchanged header and static table positions.
A.2. Since draft-ietf-httpbis-header-compression-07
o Removed old text on index value of 0.
o Added clarification for signalling of maximum table size after a
SETTINGS_HEADER_TABLE_SIZE update.
o Rewrote security considerations.
o Many editorial clarifications or improvements.
o Added convention section.
o Reworked document's outline.
o Updated static table. Entry 16 has now "gzip, deflate" for value.
o Updated Huffman table, using data set provided by Google.
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A.3. 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.4. 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.5. 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.6. 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.7. Since draft-ietf-httpbis-header-compression-02
o Corrected error in integer encoding pseudocode.
A.8. 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.9. 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 Definition
The static table (see Section 3.3.1) consists of a predefined and
unchangeable list of header fields.
The static table was created by listing the most common header fields
that are valid for messages exchanged inside a HTTP/2 connection.
For header fields with a few frequent values, an entry was added for
each of these frequent values. For other header fields, an entry was
added with an empty value.
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 | gzip, deflate |
| 17 | accept-language | |
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| 18 | accept-ranges | |
| 19 | accept | |
| 20 | access-control-allow-origin | |
| 21 | age | |
| 22 | allow | |
| 23 | authorization | |
| 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
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Table 1 gives the index of each entry in the static table.
Appendix C. Huffman Code
The following Huffman code is used when encoding string literals with
a Huffman coding (see Section 6.2).
This Huffman code was generated from statistics obtained on a large
sample of HTTP headers. It is a canonical Huffman code (see
[CANONICAL]) with some tweaking to ensure that no symbol has a unique
code length.
Each row in the table defines the code used to represent a symbol:
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, aligned on the most significant bit (MSB).
code as hex: The Huffman code for the symbol, represented as a
hexadecimal integer, aligned on the least significant bit (LSB).
len: The number of bits for the code representing the symbol.
As an example, the code for the symbol 47 (corresponding to the ASCII
character "/") consists in the 6 bits "0", "1", "1", "0", "0", "0".
This corresponds to the value 0x18 (in hexadecimal) encoded on 6
bits.
code
code as bits as hex len
sym aligned to MSB aligned in
to LSB bits
( 0) |11111111|11000 1ff8 [13]
( 1) |11111111|11111111|1011000 7fffd8 [23]
( 2) |11111111|11111111|11111110|0010 fffffe2 [28]
( 3) |11111111|11111111|11111110|0011 fffffe3 [28]
( 4) |11111111|11111111|11111110|0100 fffffe4 [28]
( 5) |11111111|11111111|11111110|0101 fffffe5 [28]
( 6) |11111111|11111111|11111110|0110 fffffe6 [28]
( 7) |11111111|11111111|11111110|0111 fffffe7 [28]
( 8) |11111111|11111111|11111110|1000 fffffe8 [28]
( 9) |11111111|11111111|11101010 ffffea [24]
( 10) |11111111|11111111|11111111|111100 3ffffffc [30]
( 11) |11111111|11111111|11111110|1001 fffffe9 [28]
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( 12) |11111111|11111111|11111110|1010 fffffea [28]
( 13) |11111111|11111111|11111111|111101 3ffffffd [30]
( 14) |11111111|11111111|11111110|1011 fffffeb [28]
( 15) |11111111|11111111|11111110|1100 fffffec [28]
( 16) |11111111|11111111|11111110|1101 fffffed [28]
( 17) |11111111|11111111|11111110|1110 fffffee [28]
( 18) |11111111|11111111|11111110|1111 fffffef [28]
( 19) |11111111|11111111|11111111|0000 ffffff0 [28]
( 20) |11111111|11111111|11111111|0001 ffffff1 [28]
( 21) |11111111|11111111|11111111|0010 ffffff2 [28]
( 22) |11111111|11111111|11111111|111110 3ffffffe [30]
( 23) |11111111|11111111|11111111|0011 ffffff3 [28]
( 24) |11111111|11111111|11111111|0100 ffffff4 [28]
( 25) |11111111|11111111|11111111|0101 ffffff5 [28]
( 26) |11111111|11111111|11111111|0110 ffffff6 [28]
( 27) |11111111|11111111|11111111|0111 ffffff7 [28]
( 28) |11111111|11111111|11111111|1000 ffffff8 [28]
( 29) |11111111|11111111|11111111|1001 ffffff9 [28]
( 30) |11111111|11111111|11111111|1010 ffffffa [28]
( 31) |11111111|11111111|11111111|1011 ffffffb [28]
' ' ( 32) |010100 14 [ 6]
'!' ( 33) |11111110|00 3f8 [10]
'"' ( 34) |11111110|01 3f9 [10]
'#' ( 35) |11111111|1010 ffa [12]
'$' ( 36) |11111111|11001 1ff9 [13]
'%' ( 37) |010101 15 [ 6]
'&' ( 38) |11111000 f8 [ 8]
''' ( 39) |11111111|010 7fa [11]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111110|11 3fb [10]
'*' ( 42) |11111001 f9 [ 8]
'+' ( 43) |11111111|011 7fb [11]
',' ( 44) |11111010 fa [ 8]
'-' ( 45) |010110 16 [ 6]
'.' ( 46) |010111 17 [ 6]
'/' ( 47) |011000 18 [ 6]
'0' ( 48) |00000 0 [ 5]
'1' ( 49) |00001 1 [ 5]
'2' ( 50) |00010 2 [ 5]
'3' ( 51) |011001 19 [ 6]
'4' ( 52) |011010 1a [ 6]
'5' ( 53) |011011 1b [ 6]
'6' ( 54) |011100 1c [ 6]
'7' ( 55) |011101 1d [ 6]
'8' ( 56) |011110 1e [ 6]
'9' ( 57) |011111 1f [ 6]
':' ( 58) |1011100 5c [ 7]
';' ( 59) |11111011 fb [ 8]
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'<' ( 60) |11111111|1111100 7ffc [15]
'=' ( 61) |100000 20 [ 6]
'>' ( 62) |11111111|1011 ffb [12]
'?' ( 63) |11111111|00 3fc [10]
'@' ( 64) |11111111|11010 1ffa [13]
'A' ( 65) |100001 21 [ 6]
'B' ( 66) |1011101 5d [ 7]
'C' ( 67) |1011110 5e [ 7]
'D' ( 68) |1011111 5f [ 7]
'E' ( 69) |1100000 60 [ 7]
'F' ( 70) |1100001 61 [ 7]
'G' ( 71) |1100010 62 [ 7]
'H' ( 72) |1100011 63 [ 7]
'I' ( 73) |1100100 64 [ 7]
'J' ( 74) |1100101 65 [ 7]
'K' ( 75) |1100110 66 [ 7]
'L' ( 76) |1100111 67 [ 7]
'M' ( 77) |1101000 68 [ 7]
'N' ( 78) |1101001 69 [ 7]
'O' ( 79) |1101010 6a [ 7]
'P' ( 80) |1101011 6b [ 7]
'Q' ( 81) |1101100 6c [ 7]
'R' ( 82) |1101101 6d [ 7]
'S' ( 83) |1101110 6e [ 7]
'T' ( 84) |1101111 6f [ 7]
'U' ( 85) |1110000 70 [ 7]
'V' ( 86) |1110001 71 [ 7]
'W' ( 87) |1110010 72 [ 7]
'X' ( 88) |11111100 fc [ 8]
'Y' ( 89) |1110011 73 [ 7]
'Z' ( 90) |11111101 fd [ 8]
'[' ( 91) |11111111|11011 1ffb [13]
'\' ( 92) |11111111|11111110|000 7fff0 [19]
']' ( 93) |11111111|11100 1ffc [13]
'^' ( 94) |11111111|111100 3ffc [14]
'_' ( 95) |100010 22 [ 6]
'`' ( 96) |11111111|1111101 7ffd [15]
'a' ( 97) |00011 3 [ 5]
'b' ( 98) |100011 23 [ 6]
'c' ( 99) |00100 4 [ 5]
'd' (100) |100100 24 [ 6]
'e' (101) |00101 5 [ 5]
'f' (102) |100101 25 [ 6]
'g' (103) |100110 26 [ 6]
'h' (104) |100111 27 [ 6]
'i' (105) |00110 6 [ 5]
'j' (106) |1110100 74 [ 7]
'k' (107) |1110101 75 [ 7]
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'l' (108) |101000 28 [ 6]
'm' (109) |101001 29 [ 6]
'n' (110) |101010 2a [ 6]
'o' (111) |00111 7 [ 5]
'p' (112) |101011 2b [ 6]
'q' (113) |1110110 76 [ 7]
'r' (114) |101100 2c [ 6]
's' (115) |01000 8 [ 5]
't' (116) |01001 9 [ 5]
'u' (117) |101101 2d [ 6]
'v' (118) |1110111 77 [ 7]
'w' (119) |1111000 78 [ 7]
'x' (120) |1111001 79 [ 7]
'y' (121) |1111010 7a [ 7]
'z' (122) |1111011 7b [ 7]
'{' (123) |11111111|1111110 7ffe [15]
'|' (124) |11111111|100 7fc [11]
'}' (125) |11111111|111101 3ffd [14]
'~' (126) |11111111|11101 1ffd [13]
(127) |11111111|11111111|11111111|1100 ffffffc [28]
(128) |11111111|11111110|0110 fffe6 [20]
(129) |11111111|11111111|010010 3fffd2 [22]
(130) |11111111|11111110|0111 fffe7 [20]
(131) |11111111|11111110|1000 fffe8 [20]
(132) |11111111|11111111|010011 3fffd3 [22]
(133) |11111111|11111111|010100 3fffd4 [22]
(134) |11111111|11111111|010101 3fffd5 [22]
(135) |11111111|11111111|1011001 7fffd9 [23]
(136) |11111111|11111111|010110 3fffd6 [22]
(137) |11111111|11111111|1011010 7fffda [23]
(138) |11111111|11111111|1011011 7fffdb [23]
(139) |11111111|11111111|1011100 7fffdc [23]
(140) |11111111|11111111|1011101 7fffdd [23]
(141) |11111111|11111111|1011110 7fffde [23]
(142) |11111111|11111111|11101011 ffffeb [24]
(143) |11111111|11111111|1011111 7fffdf [23]
(144) |11111111|11111111|11101100 ffffec [24]
(145) |11111111|11111111|11101101 ffffed [24]
(146) |11111111|11111111|010111 3fffd7 [22]
(147) |11111111|11111111|1100000 7fffe0 [23]
(148) |11111111|11111111|11101110 ffffee [24]
(149) |11111111|11111111|1100001 7fffe1 [23]
(150) |11111111|11111111|1100010 7fffe2 [23]
(151) |11111111|11111111|1100011 7fffe3 [23]
(152) |11111111|11111111|1100100 7fffe4 [23]
(153) |11111111|11111110|11100 1fffdc [21]
(154) |11111111|11111111|011000 3fffd8 [22]
(155) |11111111|11111111|1100101 7fffe5 [23]
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(156) |11111111|11111111|011001 3fffd9 [22]
(157) |11111111|11111111|1100110 7fffe6 [23]
(158) |11111111|11111111|1100111 7fffe7 [23]
(159) |11111111|11111111|11101111 ffffef [24]
(160) |11111111|11111111|011010 3fffda [22]
(161) |11111111|11111110|11101 1fffdd [21]
(162) |11111111|11111110|1001 fffe9 [20]
(163) |11111111|11111111|011011 3fffdb [22]
(164) |11111111|11111111|011100 3fffdc [22]
(165) |11111111|11111111|1101000 7fffe8 [23]
(166) |11111111|11111111|1101001 7fffe9 [23]
(167) |11111111|11111110|11110 1fffde [21]
(168) |11111111|11111111|1101010 7fffea [23]
(169) |11111111|11111111|011101 3fffdd [22]
(170) |11111111|11111111|011110 3fffde [22]
(171) |11111111|11111111|11110000 fffff0 [24]
(172) |11111111|11111110|11111 1fffdf [21]
(173) |11111111|11111111|011111 3fffdf [22]
(174) |11111111|11111111|1101011 7fffeb [23]
(175) |11111111|11111111|1101100 7fffec [23]
(176) |11111111|11111111|00000 1fffe0 [21]
(177) |11111111|11111111|00001 1fffe1 [21]
(178) |11111111|11111111|100000 3fffe0 [22]
(179) |11111111|11111111|00010 1fffe2 [21]
(180) |11111111|11111111|1101101 7fffed [23]
(181) |11111111|11111111|100001 3fffe1 [22]
(182) |11111111|11111111|1101110 7fffee [23]
(183) |11111111|11111111|1101111 7fffef [23]
(184) |11111111|11111110|1010 fffea [20]
(185) |11111111|11111111|100010 3fffe2 [22]
(186) |11111111|11111111|100011 3fffe3 [22]
(187) |11111111|11111111|100100 3fffe4 [22]
(188) |11111111|11111111|1110000 7ffff0 [23]
(189) |11111111|11111111|100101 3fffe5 [22]
(190) |11111111|11111111|100110 3fffe6 [22]
(191) |11111111|11111111|1110001 7ffff1 [23]
(192) |11111111|11111111|11111000|00 3ffffe0 [26]
(193) |11111111|11111111|11111000|01 3ffffe1 [26]
(194) |11111111|11111110|1011 fffeb [20]
(195) |11111111|11111110|001 7fff1 [19]
(196) |11111111|11111111|100111 3fffe7 [22]
(197) |11111111|11111111|1110010 7ffff2 [23]
(198) |11111111|11111111|101000 3fffe8 [22]
(199) |11111111|11111111|11110110|0 1ffffec [25]
(200) |11111111|11111111|11111000|10 3ffffe2 [26]
(201) |11111111|11111111|11111000|11 3ffffe3 [26]
(202) |11111111|11111111|11111001|00 3ffffe4 [26]
(203) |11111111|11111111|11111011|110 7ffffde [27]
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(204) |11111111|11111111|11111011|111 7ffffdf [27]
(205) |11111111|11111111|11111001|01 3ffffe5 [26]
(206) |11111111|11111111|11110001 fffff1 [24]
(207) |11111111|11111111|11110110|1 1ffffed [25]
(208) |11111111|11111110|010 7fff2 [19]
(209) |11111111|11111111|00011 1fffe3 [21]
(210) |11111111|11111111|11111001|10 3ffffe6 [26]
(211) |11111111|11111111|11111100|000 7ffffe0 [27]
(212) |11111111|11111111|11111100|001 7ffffe1 [27]
(213) |11111111|11111111|11111001|11 3ffffe7 [26]
(214) |11111111|11111111|11111100|010 7ffffe2 [27]
(215) |11111111|11111111|11110010 fffff2 [24]
(216) |11111111|11111111|00100 1fffe4 [21]
(217) |11111111|11111111|00101 1fffe5 [21]
(218) |11111111|11111111|11111010|00 3ffffe8 [26]
(219) |11111111|11111111|11111010|01 3ffffe9 [26]
(220) |11111111|11111111|11111111|1101 ffffffd [28]
(221) |11111111|11111111|11111100|011 7ffffe3 [27]
(222) |11111111|11111111|11111100|100 7ffffe4 [27]
(223) |11111111|11111111|11111100|101 7ffffe5 [27]
(224) |11111111|11111110|1100 fffec [20]
(225) |11111111|11111111|11110011 fffff3 [24]
(226) |11111111|11111110|1101 fffed [20]
(227) |11111111|11111111|00110 1fffe6 [21]
(228) |11111111|11111111|101001 3fffe9 [22]
(229) |11111111|11111111|00111 1fffe7 [21]
(230) |11111111|11111111|01000 1fffe8 [21]
(231) |11111111|11111111|1110011 7ffff3 [23]
(232) |11111111|11111111|101010 3fffea [22]
(233) |11111111|11111111|101011 3fffeb [22]
(234) |11111111|11111111|11110111|0 1ffffee [25]
(235) |11111111|11111111|11110111|1 1ffffef [25]
(236) |11111111|11111111|11110100 fffff4 [24]
(237) |11111111|11111111|11110101 fffff5 [24]
(238) |11111111|11111111|11111010|10 3ffffea [26]
(239) |11111111|11111111|1110100 7ffff4 [23]
(240) |11111111|11111111|11111010|11 3ffffeb [26]
(241) |11111111|11111111|11111100|110 7ffffe6 [27]
(242) |11111111|11111111|11111011|00 3ffffec [26]
(243) |11111111|11111111|11111011|01 3ffffed [26]
(244) |11111111|11111111|11111100|111 7ffffe7 [27]
(245) |11111111|11111111|11111101|000 7ffffe8 [27]
(246) |11111111|11111111|11111101|001 7ffffe9 [27]
(247) |11111111|11111111|11111101|010 7ffffea [27]
(248) |11111111|11111111|11111101|011 7ffffeb [27]
(249) |11111111|11111111|11111111|1110 ffffffe [28]
(250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27]
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(252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27]
(254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
Appendix D. Examples
A number of examples are worked through here, covering integer
encoding, header field representation, and the encoding of whole
lists 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 6.1).
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:
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I % 128 == 26
26 + 128 == 154
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. The header field is added to the header table.
Header list to encode:
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custom-key: custom-header
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 list:
custom-key: custom-header
D.2.2. Literal Header Field without Indexing
The header field representation uses an indexed name and a literal
value. The header field is not added to the header table.
Header list to encode:
:path: /sample/path
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 list:
:path: /sample/path
D.2.3. Literal Header Field never Indexed
The header field representation uses a literal name and a literal
value. The header field is not added to the header table, and must
use the same representation if re-encoded by an intermediary.
Header list to encode:
password: secret
Hex dump of encoded data:
1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre
74 | t
Decoding process:
10 | == Literal never indexed ==
08 | Literal name (len = 8)
7061 7373 776f 7264 | password
06 | Literal value (len = 6)
7365 6372 6574 | secret
| -> password: secret
Header table (after decoding): empty.
Decoded header list:
password: secret
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D.2.4. Indexed Header Field
The header field representation uses an indexed header field, from
the static table.
Header list to encode:
:method: GET
Hex dump of encoded data:
82 | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Header table (after decoding): empty.
Decoded header list:
:method: GET
D.3. Request Examples without Huffman Coding
This section shows several consecutive header lists, corresponding to
HTTP requests, on the same connection.
D.3.1. First Request
Header list to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
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Hex dump of encoded data:
8286 8441 0f77 7777 2e65 7861 6d70 6c65 | ...A.www.example
2e63 6f6d | .com
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
41 | == Literal indexed ==
| Indexed name (idx = 1)
| :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
Table size: 57
Decoded header list:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
D.3.2. Second Request
Header list to encode:
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:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
Hex dump of encoded data:
8286 84be 5808 6e6f 2d63 6163 6865 | ....X.no-cache
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
be | == Indexed - Add ==
| idx = 62
| -> :authority: www.example\
| .com
58 | == Literal indexed ==
| Indexed name (idx = 24)
| 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
Table size: 110
Decoded header list:
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:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
D.3.3. Third Request
Header list to encode:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
Hex dump of encoded data:
8287 85bf 400a 6375 7374 6f6d 2d6b 6579 | ....@.custom-key
0c63 7573 746f 6d2d 7661 6c75 65 | .custom-value
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: https
85 | == Indexed - Add ==
| idx = 5
| -> :path: /index.html
bf | == Indexed - Add ==
| idx = 63
| -> :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
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Header Table (after decoding):
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com
Table size: 164
Decoded header list:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
D.4. Request Examples with Huffman Coding
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values.
D.4.1. First Request
Header list to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
Hex dump of encoded data:
8286 8441 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...A......:k....
ff | .
Decoding process:
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82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
41 | == Literal indexed ==
| Indexed name (idx = 1)
| :authority
8c | Literal value (len = 12)
| Huffman encoded:
f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k.....
| Decoded:
| www.example.com
| -> :authority: www.example\
| .com
Header Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
Table size: 57
Decoded header list:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
D.4.2. Second Request
Header list to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
Hex dump of encoded data:
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8286 84be 5886 a8eb 1064 9cbf | ....X....d..
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
be | == Indexed - Add ==
| idx = 62
| -> :authority: www.example\
| .com
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
86 | Literal value (len = 6)
| Huffman encoded:
a8eb 1064 9cbf | ...d..
| 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
Table size: 110
Decoded header list:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
D.4.3. Third Request
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Header list to encode:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
Hex dump of encoded data:
8287 85bf 4088 25a8 49e9 5ba9 7d7f 8925 | ....@.%.I.[.}..%
a849 e95b b8e8 b4bf | .I.[....
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: https
85 | == Indexed - Add ==
| idx = 5
| -> :path: /index.html
bf | == Indexed - Add ==
| idx = 63
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
88 | Literal name (len = 8)
| Huffman encoded:
25a8 49e9 5ba9 7d7f | %.I.[.}.
| Decoded:
| custom-key
89 | Literal value (len = 9)
| Huffman encoded:
25a8 49e9 5bb8 e8b4 bf | %.I.[....
| 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 = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com
Table size: 164
Decoded header list:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
D.5. Response Examples without Huffman Coding
This section shows several consecutive header lists, corresponding to
HTTP responses, on the same connection. The HTTP/2 setting parameter
SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing
some evictions to occur.
D.5.1. First Response
Header list to encode:
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Hex dump of encoded data:
4803 3330 3258 0770 7269 7661 7465 611d | H.302X.privatea.
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 546e 1768 | 20:13:21 GMTn.h
7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
6c65 2e63 6f6d | le.com
Decoding process:
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48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 32 | 302
| -> :status: 302
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| 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
6e | == Literal indexed ==
| Indexed name (idx = 46)
| 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 list:
: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", "307") header field.
Header list to encode:
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Hex dump of encoded data:
4803 3330 37c1 c0bf | H.307...
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 37 | 307
| - evict: :status: 302
| -> :status: 307
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
c0 | == Indexed - Add ==
| idx = 64
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location: https://www.e\
| xample.com
Header Table (after decoding):
[ 1] (s = 42) :status: 307
[ 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
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Decoded header list:
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
D.5.3. Third Response
Several header fields are evicted from the header table during the
processing of this header list.
Header list 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
Hex dump of encoded data:
88c1 611d 4d6f 6e2c 2032 3120 4f63 7420 | ..a.Mon, 21 Oct
3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
54c0 5a04 677a 6970 7738 666f 6f3d 4153 | T.Z.gzipw8foo=AS
444a 4b48 514b 425a 584f 5157 454f 5049 | DJKHQKBZXOQWEOPI
5541 5851 5745 4f49 553b 206d 6178 2d61 | UAXQWEOIU; max-a
6765 3d33 3630 303b 2076 6572 7369 6f6e | ge=3600; version
3d31 | =1
Decoding process:
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88 | == Indexed - Add ==
| idx = 8
| -> :status: 200
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| 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
c0 | == Indexed - Add ==
| idx = 64
| -> location: https://www.e\
| xample.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
04 | Literal value (len = 4)
677a 6970 | gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
77 | == Literal indexed ==
| Indexed name (idx = 55)
| 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: 307
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1
Header Table (after decoding):
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[ 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 list:
: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
D.6. Response Examples with Huffman Coding
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values. The HTTP/2 setting
parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256
octets, causing some evictions to occur. The eviction mechanism uses
the length of the decoded literal values, so the same evictions
occurs as in the previous section.
D.6.1. First Response
Header list to encode:
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Hex dump of encoded data:
4882 6402 5885 aec3 771a 4b61 96d0 7abe | H.d.X...w.Ka..z.
9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f...
2d1b ff6e 919d 29ad 1718 63c7 8f0b 97c8 | -..n..)...c.....
e9ae 82ae 43d3 | ....C.
Decoding process:
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48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 2)
| Huffman encoded:
6402 | d.
| Decoded:
| 302
| -> :status: 302
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
85 | Literal value (len = 5)
| Huffman encoded:
aec3 771a 4b | ..w.K
| Decoded:
| private
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
96 | Literal value (len = 22)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e082 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:21 \
| GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
6e | == Literal indexed ==
| Indexed name (idx = 46)
| location
91 | Literal value (len = 17)
| Huffman encoded:
9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
d3 | .
| 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 list:
: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", "307") header field.
Header list to encode:
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
Hex dump of encoded data:
4883 640e ffc1 c0bf | H.d.....
Decoding process:
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48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
83 | Literal value (len = 3)
| Huffman encoded:
640e ff | d..
| Decoded:
| 307
| - evict: :status: 302
| -> :status: 307
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
c0 | == Indexed - Add ==
| idx = 64
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location: https://www.e\
| xample.com
Header Table (after decoding):
[ 1] (s = 42) :status: 307
[ 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 list:
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com
D.6.3. Third Response
Several header fields are evicted from the header table during the
processing of this header list.
Header list to encode:
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: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
Hex dump of encoded data:
88c1 6196 d07a be94 1054 d444 a820 0595 | ..a..z...T.D. ..
040b 8166 e084 a62d 1bff c05a 839b d9ab | ...f...-...Z....
77ad 94e7 821d d7f2 e6c7 b335 dfdf cd5b | w..........5...[
3960 d5af 2708 7f36 72c1 ab27 0fb5 291f | 9`..'..6r..'..).
9587 3160 65c0 03ed 4ee5 b106 3d50 07 | ..1`e...N...=P.
Decoding process:
88 | == Indexed - Add ==
| idx = 8
| -> :status: 200
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
96 | Literal value (len = 22)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e084 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:22 \
| GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location: https://www.e\
| xample.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
83 | Literal value (len = 3)
| Huffman encoded:
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9bd9 ab | ...
| Decoded:
| gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
77 | == Literal indexed ==
| Indexed name (idx = 55)
| set-cookie
ad | Literal value (len = 45)
| Huffman encoded:
94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P.
| Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ\
| WEOIU; max-age=3600; versi\
| on=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 307
| -> 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
Decoded header list:
: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
Authors' Addresses
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Roberto Peon
Google, Inc
EMail: fenix@google.com
Herve Ruellan
Canon CRF
EMail: herve.ruellan@crf.canon.fr
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