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IETF RFC 7541
Last modified on Friday, May 15th, 2015
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Internet Engineering Task Force (IETF) R. Peon
Request for Comments: 7541 Google, Inc
Category: Standards Track H. Ruellan
ISSN: 2070-1721 Canon CRF
May 2015
HPACK: Header Compression for HTTP/2
 Abstract
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields, to be used in HTTP/2.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7541.
Copyright Notice
Copyright (c) 2015 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
to this document. Code Components extracted from this document must
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.
Peon & Ruellan Standards Track PAGE 1 
RFC 7541 HPACK May 2015
Table of Contents
1. Introduction ....................................................4
1.1. Overview ...................................................4
1.2. Conventions ................................................5
1.3. Terminology ................................................5
2. Compression Process Overview ....................................6
2.1. Header List Ordering .......................................6
2.2. Encoding and Decoding Contexts .............................6
2.3. Indexing Tables ............................................6
2.3.1. Static Table ........................................6
2.3.2. Dynamic Table .......................................6
2.3.3. Index Address Space .................................7
2.4. Header Field Representation ................................8
3. Header Block Decoding ...........................................8
3.1. Header Block Processing ....................................8
3.2. Header Field Representation Processing .....................9
4. Dynamic Table Management ........................................9
4.1. Calculating Table Size ....................................10
4.2. Maximum Table Size ........................................10
4.3. Entry Eviction When Dynamic Table Size Changes ............11
4.4. Entry Eviction When Adding New Entries ....................11
5. Primitive Type Representations .................................11
5.1. Integer Representation ....................................11
5.2. String Literal Representation .............................13
6. Binary Format ..................................................14
6.1. Indexed Header Field Representation .......................14
6.2. Literal Header Field Representation .......................15
6.2.1. Literal Header Field with Incremental Indexing .....15
6.2.2. Literal Header Field without Indexing ..............16
6.2.3. Literal Header Field Never Indexed .................17
6.3. Dynamic Table Size Update .................................18
7. Security Considerations ........................................19
7.1. Probing Dynamic Table State ...............................19
7.1.1. Applicability to HPACK and HTTP ....................20
7.1.2. Mitigation .........................................20
7.1.3. Never-Indexed Literals .............................21
7.2. Static Huffman Encoding ...................................22
7.3. Memory Consumption ........................................22
7.4. Implementation Limits .....................................23
8. References .....................................................23
8.1. Normative References ......................................23
8.2. Informative References ....................................24
Appendix A. Static Table Definition ...............................25
Appendix B. Huffman Code ..........................................27
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RFC 7541 HPACK May 2015
Appendix C. Examples ..............................................33
C.1. Integer Representation Examples ............................33
C.1.1. Example 1: Encoding 10 Using a 5-Bit Prefix ............33
C.1.2. Example 2: Encoding 1337 Using a 5-Bit Prefix ..........33
C.1.3. Example 3: Encoding 42 Starting at an Octet Boundary ...34
C.2. Header Field Representation Examples .......................34
C.2.1. Literal Header Field with Indexing .....................34
C.2.2. Literal Header Field without Indexing ..................35
C.2.3. Literal Header Field Never Indexed .....................36
C.2.4. Indexed Header Field ...................................37
C.3. Request Examples without Huffman Coding ....................37
C.3.1. First Request ..........................................37
C.3.2. Second Request .........................................38
C.3.3. Third Request ..........................................39
C.4. Request Examples with Huffman Coding .......................41
C.4.1. First Request ..........................................41
C.4.2. Second Request .........................................42
C.4.3. Third Request ..........................................43
C.5. Response Examples without Huffman Coding ...................45
C.5.1. First Response .........................................45
C.5.2. Second Response ........................................46
C.5.3. Third Response .........................................47
C.6. Response Examples with Huffman Coding ......................49
C.6.1. First Response .........................................49
C.6.2. Second Response ........................................51
C.6.3. Third Response .........................................52
Acknowledgments ...................................................55
Authors' Addresses ................................................55
Peon & Ruellan Standards Track PAGE 3
RFC 7541 HPACK May 2015
1. Introduction
In HTTP/1.1 (see [RFC 7230]), header fields are not compressed. As
web pages have grown to require dozens to hundreds of requests, the
redundant header fields in these requests unnecessarily consume
bandwidth, measurably increasing latency.
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 (Compression Ratio Info-leak Made Easy) attack (see [CRIME]).
This specification defines HPACK, a new compressor that eliminates
redundant header fields, limits vulnerability to known security
attacks, and has a bounded memory requirement for use in constrained
environments. Potential security concerns for HPACK are described in
Section 7.
The HPACK format is intentionally simple and inflexible. Both
characteristics reduce the risk of interoperability or security
issues due to implementation error. No extensibility mechanisms are
defined; changes to the format are only possible by defining a
complete replacement.
1.1. Overview
The format defined in this specification treats a list of header
fields as an ordered collection of name-value pairs that can include
duplicate pairs. Names and values are considered to be opaque
sequences of octets, and the order of header fields is preserved
after being compressed and decompressed.
Encoding is informed by header field tables that map header fields to
indexed values. These header field tables can be incrementally
updated as new header fields are encoded or decoded.
In the encoded form, a header field is represented either literally
or as a reference to a header field in one of the header field
tables. Therefore, a list of header fields can be encoded using a
mixture of references and literal values.
Literal values are either encoded directly or use a static Huffman
code.
The encoder is responsible for deciding which header fields to insert
as new entries in the header field tables. The decoder executes the
modifications to the header field tables prescribed by the encoder,
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RFC 7541 HPACK May 2015
reconstructing the list of header fields in the process. This
enables decoders to remain simple and interoperate with a wide
variety of encoders.
Examples illustrating the use of these different mechanisms to
represent header fields are available in Appendix C.
1.2. 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 [RFC 2119].
All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate.
1.3. Terminology
This specification uses the following terms:
Header Field: A name-value pair. Both the name and value are
treated as opaque sequences of octets.
Dynamic Table: The dynamic table (see Section 2.3.2) is a table that
associates stored header fields with index values. This table is
dynamic and specific to an encoding or decoding context.
Static Table: The static table (see Section 2.3.1) is a table that
statically associates header fields that occur frequently with
index values. This table is ordered, read-only, always
accessible, and it 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 and can contain duplicate header fields.
A complete list of header fields contained in an HTTP/2 header
block 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 2.4).
Header Block: An ordered list of header field representations,
which, when decoded, yields a complete header list.
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2. Compression Process Overview
This specification does not describe a specific algorithm for an
encoder. Instead, it defines precisely how a decoder is expected to
operate, allowing encoders to produce any encoding that this
definition permits.
2.1. Header List Ordering
HPACK preserves the ordering of header fields inside the header list.
An encoder MUST order header field representations in the header
block according to their ordering in the original header list. A
decoder MUST order header fields in the decoded header list according
to their ordering in the header block.
2.2. Encoding and Decoding Contexts
To decompress header blocks, a decoder only needs to maintain a
dynamic table (see Section 2.3.2) as a decoding context. No other
dynamic state is needed.
When used for bidirectional communication, such as in HTTP, the
encoding and decoding dynamic tables maintained by an endpoint are
completely independent, i.e., the request and response dynamic tables
are separate.
2.3. Indexing Tables
HPACK uses two tables for associating header fields to indexes. The
static table (see Section 2.3.1) is predefined and contains common
header fields (most of them with an empty value). The dynamic table
(see Section 2.3.2) 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 2.3.3).
2.3.1. Static Table
The static table consists of a predefined static list of header
fields. Its entries are defined in Appendix A.
2.3.2. Dynamic Table
The dynamic table consists of a list of header fields maintained in
first-in, first-out order. The first and newest entry in a dynamic
table is at the lowest index, and the oldest entry of a dynamic table
is at the highest index.
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The dynamic table is initially empty. Entries are added as each
header block is decompressed.
The dynamic table can contain duplicate entries (i.e., entries with
the same name and same value). Therefore, duplicate entries MUST NOT
be treated as an error by a decoder.
The encoder decides how to update the dynamic table and as such can
control how much memory is used by the dynamic table. To limit the
memory requirements of the decoder, the dynamic table size is
strictly bounded (see Section 4.2).
The decoder updates the dynamic table during the processing of a list
of header field representations (see Section 3.2).
2.3.3. Index Address Space
The static table and the dynamic 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 2.3.1).
Indices strictly greater than the length of the static table refer to
elements in the dynamic table (see Section 2.3.2). The length of the
static table is subtracted to find the index into the dynamic 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 dynamic table size of k, the
following diagram shows the entire valid index address space.
<---------- Index Address Space ---------->
<-- Static Table --> <-- Dynamic Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | s | |s+1| ... |s+k|
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Dropping Point
Figure 1: Index Address Space
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2.4. Header Field Representation
An encoded header field can be represented either as an index or as a
literal.
An indexed representation defines a header field as a reference to an
entry in either the static table or the dynamic table (see
Section 6.1).
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
dynamic table. The header field value is represented literally.
Three different literal representations are defined:
o A literal representation that adds the header field as a new entry
at the beginning of the dynamic table (see Section 6.2.1).
o A literal representation that does not add the header field to the
dynamic table (see Section 6.2.2).
o A literal representation that does not add the header field to the
dynamic table, with the additional stipulation that this header
field always use a literal representation, in particular when re-
encoded by an intermediary (see Section 6.2.3). This
representation is intended for protecting header field values that
are not to be put at risk by compressing them (see Section 7.1.3
for more details).
The selection of one of these literal representations can be guided
by security considerations, in order to protect sensitive header
field values (see Section 7.1).
The literal representation of a header field name or of a header
field value can encode the sequence of octets either directly or
using a static Huffman code (see Section 5.2).
3. Header Block Decoding
3.1. Header Block Processing
A decoder processes a header block sequentially to reconstruct the
original header list.
A header block is the concatenation of header field representations.
The different possible header field representations are described in
Section 6.
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Once a header field is decoded and added to the reconstructed header
list, the header field cannot be removed. A header field added to
the header list can be safely passed to the application.
By passing the resulting header fields to the application, a decoder
can be implemented with minimal transitory memory commitment in
addition to the memory required for the dynamic table.
3.2. 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 6.
An _indexed representation_ entails the following actions:
o The header field corresponding to the referenced entry in either
the static table or dynamic table is appended to the decoded
header list.
A _literal representation_ that is _not added_ to the dynamic table
entails the following action:
o The header field is appended to the decoded header list.
A _literal representation_ that is _added_ to the dynamic table
entails the following actions:
o The header field is appended to the decoded header list.
o The header field is inserted at the beginning of the dynamic
table. This insertion could result in the eviction of previous
entries in the dynamic table (see Section 4.4).
4. Dynamic Table Management
To limit the memory requirements on the decoder side, the dynamic
table is constrained in size.
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RFC 7541 HPACK May 2015
4.1. Calculating Table Size
The size of the dynamic table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in octets (as
defined in Section 5.2), its value's length in octets, and 32.
The size of an entry is calculated using the length of its name and
value without any Huffman encoding applied.
Note: The additional 32 octets account for an estimated 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.
4.2. Maximum Table Size
Protocols that use HPACK determine the maximum size that the encoder
is permitted to use for the dynamic table. In HTTP/2, this value is
determined by the SETTINGS_HEADER_TABLE_SIZE setting (see
Section 6.5.2 of [HTTP2]).
An encoder can choose to use less capacity than this maximum size
(see Section 6.3), but the chosen size MUST stay lower than or equal
to the maximum set by the protocol.
A change in the maximum size of the dynamic table is signaled via a
dynamic table size update (see Section 6.3). This dynamic table size
update MUST occur at the beginning of the first header block
following the change to the dynamic table size. In HTTP/2, this
follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]).
Multiple updates to the maximum table size can occur between the
transmission of two header blocks. In the case that this size is
changed more than once in this interval, the smallest maximum table
size that occurs in that interval MUST be signaled in a dynamic table
size update. The final maximum size is always signaled, resulting in
at most two dynamic table size updates. This ensures that the
decoder is able to perform eviction based on reductions in dynamic
table size (see Section 4.3).
This mechanism can be used to completely clear entries from the
dynamic table by setting a maximum size of 0, which can subsequently
be restored.
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RFC 7541 HPACK May 2015
4.3. Entry Eviction When Dynamic Table Size Changes
Whenever the maximum size for the dynamic table is reduced, entries
are evicted from the end of the dynamic table until the size of the
dynamic table is less than or equal to the maximum size.
4.4. Entry Eviction When Adding New Entries
Before a new entry is added to the dynamic table, entries are evicted
from the end of the dynamic table until the size of the dynamic table
is less than or equal to (maximum size - new entry size) or until the
table is empty.
If the size of the new entry is less than or equal to the maximum
size, that entry is added to the table. It is not an error to
attempt to add an entry that is larger than the maximum size; an
attempt to add an entry larger than the maximum size causes the table
to be emptied of all existing entries and results in an empty table.
A new entry can reference the name of an entry in the dynamic table
that will be evicted when adding this new entry into the dynamic
table. Implementations are cautioned to avoid deleting the
referenced name if the referenced entry is evicted from the dynamic
table prior to inserting the new entry.
5. Primitive Type Representations
HPACK encoding uses two primitive types: unsigned variable-length
integers and strings of octets.
5.1. Integer Representation
Integers are used to represent name indexes, header field indexes, or
string lengths. An integer representation can start anywhere within
an octet. 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.
If the integer value is small enough, i.e., strictly less than 2^N-1,
it is encoded within the N-bit prefix.
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RFC 7541 HPACK May 2015
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | Value |
+---+---+---+-------------------+
Figure 2: Integer Value Encoded within the Prefix (Shown for N = 5)
Otherwise, all the bits of the prefix are set to 1, and the value,
decreased by 2^N-1, is encoded using a list of one or more octets.
The most significant bit of each octet is used as a continuation
flag: its value is set to 1 except for the last octet in the list.
The remaining bits of the octets are used to encode the decreased
value.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | 1 1 1 1 1 |
+---+---+---+-------------------+
| 1 | Value-(2^N-1) LSB |
+---+---------------------------+
...
+---+---------------------------+
| 0 | Value-(2^N-1) MSB |
+---+---------------------------+
Figure 3: Integer Value Encoded after the Prefix (Shown for N = 5)
Decoding the integer value from the list of octets starts by
reversing the order of the octets in the list. Then, for each octet,
its most significant bit is removed. The remaining bits of the
octets are concatenated, and the resulting value is increased by
2^N-1 to obtain the integer value.
The prefix size, N, is always between 1 and 8 bits. An integer
starting at an octet boundary will have an 8-bit prefix.
Pseudocode 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
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RFC 7541 HPACK May 2015
Pseudocode 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 C.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. Integer encodings that exceed implementation limits -- in
value or octet length -- MUST be treated as decoding errors.
Different limits can be set for each of the different uses of
integers, based on implementation constraints.
5.2. String Literal Representation
Header field names and header field values can be represented as
string literals. A string literal is encoded as a sequence of
octets, either by directly encoding the string literal'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) |
+-------------------------------+
Figure 4: String Literal Representation
A string literal 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 a 7-bit prefix (see
Section 5.1).
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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 that use Huffman encoding are encoded with the
Huffman code defined in Appendix B (see examples for requests in
Appendix C.4 and for responses in Appendix C.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 from being 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.
6. Binary Format
This section describes the detailed format of each of the different
header field representations and the dynamic table size update
instruction.
6.1. Indexed Header Field Representation
An indexed header field representation identifies an entry in either
the static table or the dynamic table (see Section 2.3).
An indexed header field representation causes a header field to be
added to the decoded header list, as described in Section 3.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
Figure 5: Indexed Header Field
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RFC 7541 HPACK May 2015
An indexed header field starts with the '1' 1-bit pattern, followed
by the index of the matching header field, represented as an integer
with a 7-bit prefix (see Section 5.1).
The index value of 0 is not used. It MUST be treated as a decoding
error if found in an indexed header field representation.
6.2. Literal Header Field Representation
A literal header field representation contains a literal header field
value. Header field names are provided either as a literal or by
reference to an existing table entry, either from the static table or
the dynamic table (see Section 2.3).
This specification defines three forms of literal header field
representations: with indexing, without indexing, and never indexed.
6.2.1. Literal Header Field with Incremental Indexing
A literal header field with incremental indexing representation
results in appending a header field to the decoded header list and
inserting it as a new entry into the dynamic table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Figure 6: Literal Header Field with Incremental Indexing -- Indexed
Name
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RFC 7541 HPACK May 2015
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) |
+-------------------------------+
Figure 7: 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 dynamic 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 5.1). This value is always non-zero.
Otherwise, the header field name is represented as a string literal
(see Section 5.2). 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 string literal (see Section 5.2).
6.2.2. Literal Header Field without Indexing
A literal header field without indexing representation results in
appending a header field to the decoded header list without altering
the dynamic table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Figure 8: Literal Header Field without Indexing -- Indexed Name
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RFC 7541 HPACK May 2015
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Figure 9: 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 dynamic 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 5.1). This value is always non-zero.
Otherwise, the header field name is represented as a string literal
(see Section 5.2). 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 string literal (see Section 5.2).
6.2.3. Literal Header Field Never Indexed
A literal header field never-indexed representation results in
appending a header field to the decoded header list without altering
the dynamic 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) |
+-------------------------------+
Figure 10: Literal Header Field Never Indexed -- Indexed Name
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RFC 7541 HPACK May 2015
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Figure 11: 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 7.1
for more details).
The encoding of the representation is identical to the literal header
field without indexing (see Section 6.2.2).
6.3. Dynamic Table Size Update
A dynamic table size update signals a change to the size of the
dynamic table.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Max size (5+) |
+---+---------------------------+
Figure 12: Maximum Dynamic Table Size Change
A dynamic 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 5.1).
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RFC 7541 HPACK May 2015
The new maximum size MUST be lower than or equal to the limit
determined by the protocol using HPACK. A value that exceeds this
limit MUST be treated as a decoding error. In HTTP/2, this limit is
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]).
Reducing the maximum size of the dynamic table can cause entries to
be evicted (see Section 4.3).
7. 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.
7.1. Probing Dynamic 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 who can both define header fields to be encoded and
transmitted and observe the length of those fields once they are
encoded. When an attacker can do both, they can adaptively modify
requests in order to confirm guesses about the dynamic table state.
If a guess is compressed into a shorter length, the attacker can
observe the encoded length and infer that the guess was correct.
This is possible even over the Transport Layer Security (TLS)
protocol (see [TLS12]), 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.
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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.
7.1.1. Applicability to HPACK and HTTP
HPACK mitigates but does not completely prevent attacks modeled on
CRIME [CRIME] by forcing a guess to match an entire header field
value rather than individual characters. Attackers can only learn
whether a guess is correct or not, so they are reduced to brute-force
guesses 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 dynamic 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.
7.1.2. 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.
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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 dynamic table probing.
An ideal solution segregates access to the dynamic 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 a particular value can extract that value.
To improve compression performance of this option, certain entries
might be tagged as being public. For example, a web browser might
make the values of the Accept-Encoding header field available in all
requests.
An encoder without good knowledge of the provenance of header fields
might instead introduce a penalty for a header field with many
different values, such that a large number of attempts to guess a
header field value results in the header field no longer being
compared to the dynamic table entries in future messages, effectively
preventing further guesses.
Note: Simply removing entries corresponding to the header field
from the dynamic table can be ineffectual if the attacker has a
reliable way of causing values to be reinstalled. For example, a
request to load an image in a web browser typically includes the
Cookie header field (a potentially highly valued target for this
sort of attack), and web sites can easily force an image to be
loaded, thereby refreshing the entry in the dynamic table.
This response might be made inversely proportional to the length of
the header field value. Marking a header field as not using the
dynamic table anymore might occur for shorter values more quickly or
with higher probability than for longer values.
7.1.3. Never-Indexed Literals
Implementations can also choose to protect sensitive header fields by
not compressing them and instead encoding their value as 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 6.2.3) can be used to signal to
intermediaries that a particular value was intentionally sent as a
literal.
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An intermediary MUST NOT re-encode a value that uses the never-
indexed literal representation with another representation that would
index it. If HPACK is used for re-encoding, the never-indexed
literal representation MUST be used.
The choice to use a never-indexed literal representation for a header
field depends on several factors. Since HPACK doesn't protect
against guessing an entire header field value, short or low-entropy
values are more readily recovered by an adversary. Therefore, an
encoder might choose not to index values with low entropy.
An encoder might also choose not to index values for header fields
that are considered to be highly valuable or sensitive to recovery,
such as the Cookie or Authorization header fields.
On the contrary, an encoder might prefer indexing values for header
fields that have little or no value if they were exposed. For
instance, a User-Agent header field does not commonly vary between
requests and is sent to any server. In that case, confirmation that
a particular User-Agent value has been used provides little value.
Note that these criteria for deciding to use a never-indexed literal
representation will evolve over time as new attacks are discovered.
7.2. Static Huffman Encoding
There is no currently known attack against a static Huffman encoding.
A study has shown that using a static Huffman encoding table created
an information leakage; however, this same study concluded that an
attacker could not take advantage of this information leakage to
recover any meaningful amount of information (see [PETAL]).
7.3. Memory Consumption
An attacker can try to cause an endpoint to exhaust its memory.
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 is limited by the
protocol using HPACK through the definition of the maximum size of
the dynamic table. In HTTP/2, this value is controlled by the
decoder through the 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 dynamic table, plus a small allowance
for overhead.
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A decoder can limit the amount of state memory used by setting an
appropriate value for the maximum size of the dynamic table. In
HTTP/2, this is realized by setting an appropriate value for the
SETTINGS_HEADER_TABLE_SIZE parameter. An encoder can limit the
amount of state memory it uses by signaling a lower dynamic table
size than the decoder allows (see Section 6.3).
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.
7.4. 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 5.1). In
the same way, it has to set a limit to the length it accepts for
string literals (see Section 5.2).
8. References
8.1. Normative References
[HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC 7540, May 2015,
<http://www.rfc-editor.org/info/RFC 7540>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax and
Routing", RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
Peon & Ruellan Standards Track PAGE 23
RFC 7541 HPACK May 2015
8.2. 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] Wikipedia, "CRIME", May 2015, <http://en.wikipedia.org/w/
index.php?title=CRIME&oldid=660948120>.
[DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format
Specification version 1.3", RFC 1951,
DOI 10.17487/RFC 1951, May 1996,
<http://www.rfc-editor.org/info/RFC 1951>.
[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, <http://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=4051119>.
[ORIGIN] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC 6454, December 2011,
<http://www.rfc-editor.org/info/RFC 6454>.
[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] Belshe, M. and R. Peon, "SPDY Protocol", Work in
Progress, draft-mbelshe-httpbis-spdy-00, February 2012.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
Peon & Ruellan Standards Track PAGE 24
RFC 7541 HPACK May 2015
Appendix A. Static Table Definition
The static table (see Section 2.3.1) consists in a predefined and
unchangeable list of header fields.
The static table was created from the most frequent header fields
used by popular web sites, with the addition of HTTP/2-specific
pseudo-header fields (see Section 8.1.2.1 of [HTTP2]). 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.
Table 1 lists the predefined header fields that make up the static
table and gives the index of each entry.
+-------+-----------------------------+---------------+
| 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 | |
| 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 | |
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| 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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RFC 7541 HPACK May 2015
Appendix B. Huffman Code
The following Huffman code is used when encoding string literals with
a Huffman coding (see Section 5.2).
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 in 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]
( 12) |11111111|11111111|11111110|1010 fffffea [28]
( 13) |11111111|11111111|11111111|111101 3ffffffd [30]
Peon & Ruellan Standards Track PAGE 27
RFC 7541 HPACK May 2015
( 14) |11111111|11111111|11111110|1011 fffffeb [28]
( 15) |11111111|11111111|11111110|1100 fffffec [28]
( 16) |11111111|11111111|11111110|1101 fffffed [28]
( 17) |11111111|11111111|11111110|1110 fffffee [28]
( 18) |11111111|11111111|11111110|1111 fffffef [28]
( 19) |11111111|11111111|11111111|0000 ffffff0 [28]
( 20) |11111111|11111111|11111111|0001 ffffff1 [28]
( 21) |11111111|11111111|11111111|0010 ffffff2 [28]
( 22) |11111111|11111111|11111111|111110 3ffffffe [30]
( 23) |11111111|11111111|11111111|0011 ffffff3 [28]
( 24) |11111111|11111111|11111111|0100 ffffff4 [28]
( 25) |11111111|11111111|11111111|0101 ffffff5 [28]
( 26) |11111111|11111111|11111111|0110 ffffff6 [28]
( 27) |11111111|11111111|11111111|0111 ffffff7 [28]
( 28) |11111111|11111111|11111111|1000 ffffff8 [28]
( 29) |11111111|11111111|11111111|1001 ffffff9 [28]
( 30) |11111111|11111111|11111111|1010 ffffffa [28]
( 31) |11111111|11111111|11111111|1011 ffffffb [28]
' ' ( 32) |010100 14 [ 6]
'!' ( 33) |11111110|00 3f8 [10]
'"' ( 34) |11111110|01 3f9 [10]
'#' ( 35) |11111111|1010 ffa [12]
'$' ( 36) |11111111|11001 1ff9 [13]
'%' ( 37) |010101 15 [ 6]
'&' ( 38) |11111000 f8 [ 8]
''' ( 39) |11111111|010 7fa [11]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111110|11 3fb [10]
'*' ( 42) |11111001 f9 [ 8]
'+' ( 43) |11111111|011 7fb [11]
',' ( 44) |11111010 fa [ 8]
'-' ( 45) |010110 16 [ 6]
'.' ( 46) |010111 17 [ 6]
'/' ( 47) |011000 18 [ 6]
'0' ( 48) |00000 0 [ 5]
'1' ( 49) |00001 1 [ 5]
'2' ( 50) |00010 2 [ 5]
'3' ( 51) |011001 19 [ 6]
'4' ( 52) |011010 1a [ 6]
'5' ( 53) |011011 1b [ 6]
'6' ( 54) |011100 1c [ 6]
'7' ( 55) |011101 1d [ 6]
'8' ( 56) |011110 1e [ 6]
'9' ( 57) |011111 1f [ 6]
':' ( 58) |1011100 5c [ 7]
';' ( 59) |11111011 fb [ 8]
'<' ( 60) |11111111|1111100 7ffc [15]
'=' ( 61) |100000 20 [ 6]
Peon & Ruellan Standards Track PAGE 28
RFC 7541 HPACK May 2015
'>' ( 62) |11111111|1011 ffb [12]
'?' ( 63) |11111111|00 3fc [10]
'@' ( 64) |11111111|11010 1ffa [13]
'A' ( 65) |100001 21 [ 6]
'B' ( 66) |1011101 5d [ 7]
'C' ( 67) |1011110 5e [ 7]
'D' ( 68) |1011111 5f [ 7]
'E' ( 69) |1100000 60 [ 7]
'F' ( 70) |1100001 61 [ 7]
'G' ( 71) |1100010 62 [ 7]
'H' ( 72) |1100011 63 [ 7]
'I' ( 73) |1100100 64 [ 7]
'J' ( 74) |1100101 65 [ 7]
'K' ( 75) |1100110 66 [ 7]
'L' ( 76) |1100111 67 [ 7]
'M' ( 77) |1101000 68 [ 7]
'N' ( 78) |1101001 69 [ 7]
'O' ( 79) |1101010 6a [ 7]
'P' ( 80) |1101011 6b [ 7]
'Q' ( 81) |1101100 6c [ 7]
'R' ( 82) |1101101 6d [ 7]
'S' ( 83) |1101110 6e [ 7]
'T' ( 84) |1101111 6f [ 7]
'U' ( 85) |1110000 70 [ 7]
'V' ( 86) |1110001 71 [ 7]
'W' ( 87) |1110010 72 [ 7]
'X' ( 88) |11111100 fc [ 8]
'Y' ( 89) |1110011 73 [ 7]
'Z' ( 90) |11111101 fd [ 8]
'[' ( 91) |11111111|11011 1ffb [13]
'\' ( 92) |11111111|11111110|000 7fff0 [19]
']' ( 93) |11111111|11100 1ffc [13]
'^' ( 94) |11111111|111100 3ffc [14]
'_' ( 95) |100010 22 [ 6]
'`' ( 96) |11111111|1111101 7ffd [15]
'a' ( 97) |00011 3 [ 5]
'b' ( 98) |100011 23 [ 6]
'c' ( 99) |00100 4 [ 5]
'd' (100) |100100 24 [ 6]
'e' (101) |00101 5 [ 5]
'f' (102) |100101 25 [ 6]
'g' (103) |100110 26 [ 6]
'h' (104) |100111 27 [ 6]
'i' (105) |00110 6 [ 5]
'j' (106) |1110100 74 [ 7]
'k' (107) |1110101 75 [ 7]
'l' (108) |101000 28 [ 6]
'm' (109) |101001 29 [ 6]
Peon & Ruellan Standards Track PAGE 29
RFC 7541 HPACK May 2015
'n' (110) |101010 2a [ 6]
'o' (111) |00111 7 [ 5]
'p' (112) |101011 2b [ 6]
'q' (113) |1110110 76 [ 7]
'r' (114) |101100 2c [ 6]
's' (115) |01000 8 [ 5]
't' (116) |01001 9 [ 5]
'u' (117) |101101 2d [ 6]
'v' (118) |1110111 77 [ 7]
'w' (119) |1111000 78 [ 7]
'x' (120) |1111001 79 [ 7]
'y' (121) |1111010 7a [ 7]
'z' (122) |1111011 7b [ 7]
'{' (123) |11111111|1111110 7ffe [15]
'|' (124) |11111111|100 7fc [11]
'}' (125) |11111111|111101 3ffd [14]
'~' (126) |11111111|11101 1ffd [13]
(127) |11111111|11111111|11111111|1100 ffffffc [28]
(128) |11111111|11111110|0110 fffe6 [20]
(129) |11111111|11111111|010010 3fffd2 [22]
(130) |11111111|11111110|0111 fffe7 [20]
(131) |11111111|11111110|1000 fffe8 [20]
(132) |11111111|11111111|010011 3fffd3 [22]
(133) |11111111|11111111|010100 3fffd4 [22]
(134) |11111111|11111111|010101 3fffd5 [22]
(135) |11111111|11111111|1011001 7fffd9 [23]
(136) |11111111|11111111|010110 3fffd6 [22]
(137) |11111111|11111111|1011010 7fffda [23]
(138) |11111111|11111111|1011011 7fffdb [23]
(139) |11111111|11111111|1011100 7fffdc [23]
(140) |11111111|11111111|1011101 7fffdd [23]
(141) |11111111|11111111|1011110 7fffde [23]
(142) |11111111|11111111|11101011 ffffeb [24]
(143) |11111111|11111111|1011111 7fffdf [23]
(144) |11111111|11111111|11101100 ffffec [24]
(145) |11111111|11111111|11101101 ffffed [24]
(146) |11111111|11111111|010111 3fffd7 [22]
(147) |11111111|11111111|1100000 7fffe0 [23]
(148) |11111111|11111111|11101110 ffffee [24]
(149) |11111111|11111111|1100001 7fffe1 [23]
(150) |11111111|11111111|1100010 7fffe2 [23]
(151) |11111111|11111111|1100011 7fffe3 [23]
(152) |11111111|11111111|1100100 7fffe4 [23]
(153) |11111111|11111110|11100 1fffdc [21]
(154) |11111111|11111111|011000 3fffd8 [22]
(155) |11111111|11111111|1100101 7fffe5 [23]
(156) |11111111|11111111|011001 3fffd9 [22]
(157) |11111111|11111111|1100110 7fffe6 [23]
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RFC 7541 HPACK May 2015
(158) |11111111|11111111|1100111 7fffe7 [23]
(159) |11111111|11111111|11101111 ffffef [24]
(160) |11111111|11111111|011010 3fffda [22]
(161) |11111111|11111110|11101 1fffdd [21]
(162) |11111111|11111110|1001 fffe9 [20]
(163) |11111111|11111111|011011 3fffdb [22]
(164) |11111111|11111111|011100 3fffdc [22]
(165) |11111111|11111111|1101000 7fffe8 [23]
(166) |11111111|11111111|1101001 7fffe9 [23]
(167) |11111111|11111110|11110 1fffde [21]
(168) |11111111|11111111|1101010 7fffea [23]
(169) |11111111|11111111|011101 3fffdd [22]
(170) |11111111|11111111|011110 3fffde [22]
(171) |11111111|11111111|11110000 fffff0 [24]
(172) |11111111|11111110|11111 1fffdf [21]
(173) |11111111|11111111|011111 3fffdf [22]
(174) |11111111|11111111|1101011 7fffeb [23]
(175) |11111111|11111111|1101100 7fffec [23]
(176) |11111111|11111111|00000 1fffe0 [21]
(177) |11111111|11111111|00001 1fffe1 [21]
(178) |11111111|11111111|100000 3fffe0 [22]
(179) |11111111|11111111|00010 1fffe2 [21]
(180) |11111111|11111111|1101101 7fffed [23]
(181) |11111111|11111111|100001 3fffe1 [22]
(182) |11111111|11111111|1101110 7fffee [23]
(183) |11111111|11111111|1101111 7fffef [23]
(184) |11111111|11111110|1010 fffea [20]
(185) |11111111|11111111|100010 3fffe2 [22]
(186) |11111111|11111111|100011 3fffe3 [22]
(187) |11111111|11111111|100100 3fffe4 [22]
(188) |11111111|11111111|1110000 7ffff0 [23]
(189) |11111111|11111111|100101 3fffe5 [22]
(190) |11111111|11111111|100110 3fffe6 [22]
(191) |11111111|11111111|1110001 7ffff1 [23]
(192) |11111111|11111111|11111000|00 3ffffe0 [26]
(193) |11111111|11111111|11111000|01 3ffffe1 [26]
(194) |11111111|11111110|1011 fffeb [20]
(195) |11111111|11111110|001 7fff1 [19]
(196) |11111111|11111111|100111 3fffe7 [22]
(197) |11111111|11111111|1110010 7ffff2 [23]
(198) |11111111|11111111|101000 3fffe8 [22]
(199) |11111111|11111111|11110110|0 1ffffec [25]
(200) |11111111|11111111|11111000|10 3ffffe2 [26]
(201) |11111111|11111111|11111000|11 3ffffe3 [26]
(202) |11111111|11111111|11111001|00 3ffffe4 [26]
(203) |11111111|11111111|11111011|110 7ffffde [27]
(204) |11111111|11111111|11111011|111 7ffffdf [27]
(205) |11111111|11111111|11111001|01 3ffffe5 [26]
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(206) |11111111|11111111|11110001 fffff1 [24]
(207) |11111111|11111111|11110110|1 1ffffed [25]
(208) |11111111|11111110|010 7fff2 [19]
(209) |11111111|11111111|00011 1fffe3 [21]
(210) |11111111|11111111|11111001|10 3ffffe6 [26]
(211) |11111111|11111111|11111100|000 7ffffe0 [27]
(212) |11111111|11111111|11111100|001 7ffffe1 [27]
(213) |11111111|11111111|11111001|11 3ffffe7 [26]
(214) |11111111|11111111|11111100|010 7ffffe2 [27]
(215) |11111111|11111111|11110010 fffff2 [24]
(216) |11111111|11111111|00100 1fffe4 [21]
(217) |11111111|11111111|00101 1fffe5 [21]
(218) |11111111|11111111|11111010|00 3ffffe8 [26]
(219) |11111111|11111111|11111010|01 3ffffe9 [26]
(220) |11111111|11111111|11111111|1101 ffffffd [28]
(221) |11111111|11111111|11111100|011 7ffffe3 [27]
(222) |11111111|11111111|11111100|100 7ffffe4 [27]
(223) |11111111|11111111|11111100|101 7ffffe5 [27]
(224) |11111111|11111110|1100 fffec [20]
(225) |11111111|11111111|11110011 fffff3 [24]
(226) |11111111|11111110|1101 fffed [20]
(227) |11111111|11111111|00110 1fffe6 [21]
(228) |11111111|11111111|101001 3fffe9 [22]
(229) |11111111|11111111|00111 1fffe7 [21]
(230) |11111111|11111111|01000 1fffe8 [21]
(231) |11111111|11111111|1110011 7ffff3 [23]
(232) |11111111|11111111|101010 3fffea [22]
(233) |11111111|11111111|101011 3fffeb [22]
(234) |11111111|11111111|11110111|0 1ffffee [25]
(235) |11111111|11111111|11110111|1 1ffffef [25]
(236) |11111111|11111111|11110100 fffff4 [24]
(237) |11111111|11111111|11110101 fffff5 [24]
(238) |11111111|11111111|11111010|10 3ffffea [26]
(239) |11111111|11111111|1110100 7ffff4 [23]
(240) |11111111|11111111|11111010|11 3ffffeb [26]
(241) |11111111|11111111|11111100|110 7ffffe6 [27]
(242) |11111111|11111111|11111011|00 3ffffec [26]
(243) |11111111|11111111|11111011|01 3ffffed [26]
(244) |11111111|11111111|11111100|111 7ffffe7 [27]
(245) |11111111|11111111|11111101|000 7ffffe8 [27]
(246) |11111111|11111111|11111101|001 7ffffe9 [27]
(247) |11111111|11111111|11111101|010 7ffffea [27]
(248) |11111111|11111111|11111101|011 7ffffeb [27]
(249) |11111111|11111111|11111111|1110 ffffffe [28]
(250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27]
(252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27]
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RFC 7541 HPACK May 2015
(254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
Appendix C. Examples
This appendix contains examples covering integer encoding, header
field representation, and the encoding of whole lists of header
fields for both requests and responses, with and without Huffman
coding.
C.1. Integer Representation Examples
This section shows the representation of integer values in detail
(see Section 5.1).
C.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
+---+---+---+---+---+---+---+---+
C.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, so the while loop
body executes:
I % 128 == 26
26 + 128 == 154
154 is encoded in 8 bits as: 10011010
I is set to 10 (1306 / 128 == 10)
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RFC 7541 HPACK May 2015
I is no longer greater than or equal to 128, so the while
loop terminates.
I, now 10, is encoded in 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
+---+---+---+---+---+---+---+---+
C.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
+---+---+---+---+---+---+---+---+
C.2. Header Field Representation Examples
This section shows several independent representation examples.
C.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 dynamic table.
Header list to encode:
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
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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-header
Dynamic Table (after decoding):
[ 1] (s = 55) custom-key: custom-header
Table size: 55
Decoded header list:
custom-key: custom-header
C.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 dynamic table.
Header list to encode:
:path: /sample/path
Hex dump of encoded data:
040c 2f73 616d 706c 652f 7061 7468 | ../sample/path
Decoding process:
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
Dynamic table (after decoding): empty.
Decoded header list:
:path: /sample/path
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C.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 dynamic 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
Dynamic table (after decoding): empty.
Decoded header list:
password: secret
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C.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
Dynamic table (after decoding): empty.
Decoded header list:
:method: GET
C.3. Request Examples without Huffman Coding
This section shows several consecutive header lists, corresponding to
HTTP requests, on the same connection.
C.3.1. First Request
Header list to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
Hex dump of encoded data:
8286 8441 0f77 7777 2e65 7861 6d70 6c65 | ...A.www.example
2e63 6f6d | .com
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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
Dynamic 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
C.3.2. Second Request
Header list to encode:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache
Hex dump of encoded data:
8286 84be 5808 6e6f 2d63 6163 6865 | ....X.no-cache
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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
Dynamic 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
C.3.3. Third Request
Header list to encode:
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value
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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-value
Dynamic 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
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C.4. Request Examples with Huffman Coding
This section shows the same examples as the previous section but uses
Huffman encoding for the literal values.
C.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:
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
Dynamic Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
Table size: 57
Peon & Ruellan Standards Track PAGE 41
RFC 7541 HPACK May 2015
Decoded header list:
:method: GET
:scheme: http
:path: /
:authority: www.example.com
C.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:
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
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Dynamic 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
C.4.3. 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 4088 25a8 49e9 5ba9 7d7f 8925 | ....@.%.I.[.}..%
a849 e95b b8e8 b4bf | .I.[....
Peon & Ruellan Standards Track PAGE 43
RFC 7541 HPACK May 2015
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-value
Dynamic 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
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RFC 7541 HPACK May 2015
C.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.
C.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:
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)
Peon & Ruellan Standards Track PAGE 45
RFC 7541 HPACK May 2015
| 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.example.com
Dynamic 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
C.5.2. Second Response
The (":status", "302") header field is evicted from the dynamic 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 ==
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RFC 7541 HPACK May 2015
| 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.example.com
Dynamic 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
C.5.3. Third Response
Several header fields are evicted from the dynamic 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
Peon & Ruellan Standards Track PAGE 47
RFC 7541 HPACK May 2015
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:
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:
| private
| -> date: Mon, 21 Oct 2013
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location:
| https://www.example.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
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| - evict: location:
| https://www.example.com
| - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ
| KBZXOQWEOPIUAXQWEOIU; ma
| x-age=3600; version=1
Dynamic 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
C.6. Response Examples with Huffman Coding
This section shows the same examples as the previous section but uses
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 occur
as in the previous section.
C.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.
Peon & Ruellan Standards Track PAGE 49
RFC 7541 HPACK May 2015
Decoding process:
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.example.com
Peon & Ruellan Standards Track PAGE 50
RFC 7541 HPACK May 2015
Dynamic 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
C.6.2. Second Response
The (":status", "302") header field is evicted from the dynamic 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:
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
Peon & Ruellan Standards Track PAGE 51
RFC 7541 HPACK May 2015
| -> date: Mon, 21 Oct 2013
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location:
| https://www.example.com
Dynamic 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
C.6.3. Third Response
Several header fields are evicted from the dynamic 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 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.
Peon & Ruellan Standards Track PAGE 52
RFC 7541 HPACK May 2015
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:
| private
| -> date: Mon, 21 Oct 2013
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location:
| https://www.example.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
83 | Literal value (len = 3)
| Huffman encoded:
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
Peon & Ruellan Standards Track PAGE 53
RFC 7541 HPACK May 2015
| on=1
| - evict: location:
| https://www.example.com
| - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ
| KBZXOQWEOPIUAXQWEOIU; ma
| x-age=3600; version=1
Dynamic 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
Peon & Ruellan Standards Track PAGE 54
RFC 7541 HPACK May 2015
Acknowledgments
This specification includes substantial input from the following
individuals:
o Mike Bishop, Jeff Pinner, Julian Reschke, and Martin Thomson
(substantial editorial contributions).
o Johnny Graettinger (Huffman code statistics).
Authors' Addresses
Roberto Peon
Google, Inc
EMail: fenix@google.com
Herve Ruellan
Canon CRF
EMail: herve.ruellan@crf.canon.fr
Peon & Ruellan Standards Track PAGE 55
RFC TOTAL SIZE: 117827 bytes
PUBLICATION DATE: Friday, May 15th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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