HPACK - Header Compression for HTTP/2Google, Incfenix@google.comCanon CRFherve.ruellan@crf.canon.fr
Applications
HTTPbisHTTPHeader
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context
of HTTP/2.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at .
Working Group information can be found at
; that specific to
HTTP/2 are at .
The changes in this draft are summarized in .
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields in the context
of HTTP/2 (see ).
In HTTP/1.1 (see ), 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 and ).
SPDY initially addressed this
redundancy by compressing header fields using the 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 ).
This document describes HPACK, a new compressor for header
fields which eliminates redundant header fields, is not
vulnerable to known security attacks, and which also has a
bounded memory requirement for use in constrained environments.
The HTTP header field encoding defined in this document
is based on a header table that maps name-value pairs to
index values. The header table is incrementally updated
during the HTTP/2 connection.
A set of header fields is treated as an unordered collection
of name-value pairs. Names and values are considered to be
opaque sequences of octets. The order of header fields is
not guaranteed to be preserved after being compressed and
decompressed.
As two consecutive sets of header fields often have header
fields in common, each set is coded as a difference from
the previous set. The goal is to only encode the changes
(header fields present in one of the sets that are absent
from the other) between the two sets of header fields.
A header field is represented either literally or as a
reference to a name-value pair in the header table. A set
of header fields is stored as a set of references to
entries in the header table (possibly keeping only a
subset of it, as some header fields may be missing a
corresponding entry in the header table). Differences
between consecutive sets of header fields are encoded as
changes to the set of references.
The encoder is responsible for deciding which header
fields to insert as new entries in the header table. The
decoder executes the modifications to the header table and
reference set prescribed by the encoder, reconstructing
the set of header fields in the process. This enables
decoders to remain simple and understand a wide variety of
encoders.
Examples illustrating the use of these different
mechanisms to represent header fields are available in
.
The encoding and decoding of header fields relies on some
components and concepts:
A name-value pair. Both the name and value are
treated as opaque sequences of octets.
The header table (see ) is a component used
to associate stored header fields to index values.
The static table (see ) is a component used
to associate static header fields to index values.
This data is ordered, read-only, always
accessible, and may be shared amongst all encoding
contexts.
The reference set (see ) is a component
containing an unordered set of references to
entries in the header table. This is used for the
differential encoding of a new header set.
A header set is an unordered group of
header fields that are encoded jointly. A complete
set of key-value pairs contained in a HTTP
request or response is a header set.
A header field can be represented in encoded form
either as a literal or as an index (see ).
The entire set of encoded header field
representations which, when decoded, yield a
complete header set.
When decoding a set of header field
representations, some operations emit a header
field (see ).
Emitted header fields are added to the current
header set and cannot be removed.
The set of mutable structures used within an encoding
context include a header table and a reference set.
Everything else is either immutable or conceptual.
HTTP messages are exchanged between a client and a
server in both directions. The encoding of header
fields in each direction is independent from the
other direction. There is a single encoding context
for each direction used to encode all header fields
sent in that direction.
A header table consists of a list of header fields
maintained in first-in, first-out order.
The first and newest entry in a header table is always
at index 1, and the oldest entry of a header table is
at the index len(header table).
The header table is initially empty.
There is typically no need for the header table to
contain duplicate entries. However, duplicate entries
MUST NOT be treated as an error by a decoder.
The encoder decides how to update the header table and
as such can control how much memory is used by the
header table. To limit the memory requirements of
the decoder, the header table size is strictly
bounded (see ).
The header table is updated during the processing of
a set of header field representations (see ).
A reference set is an unordered set of references to
entries of the header table.
The reference set is initially empty.
The reference set is updated during the processing of
a set of header field representations (see ).
The reference set enables differential encoding,
whereby only differences between the previous header
set and the current header set need to be encoded. The
use of differential encoding is optional for any
header set.
When an entry is evicted from the header table, if it
was referenced from the reference set, its reference
is removed from the reference set.
To limit the memory requirements on the decoder side
for handling the reference set, only entries within
the header table can be contained in the reference
set. To still allow entries from the static table to
take advantage of the differential encoding, when a
header field is represented as a reference to an entry
of the static table, this entry is inserted into the
header table (see ).
An encoded header field can be represented either as a
literal or as an index.
A literal representation defines a new header
field. The header field name is represented
either literally or as a reference to an entry
of the header table. The header field value is
represented literally.
Three different literal representations are
provided:
A literal representation that does not
add the header field to the header
table (see ).
A literal representation that does not
add the header field to the header
table and require that this header
field always use a literal
representation, in particular when
re-encoded by an intermediary (see
).
A literal representation that adds the
header field as a new entry at the
beginning of the header table (see
).
The indexed representation defines a header
field as a reference to an entry in either the
header table or the static table (see ).
Indices between 1 and len(header table),
inclusive, refer to elements in the header
table, with index 1 referring to the beginning
of the table.
Indices between len(header table) + 1 and
len(header table) + len(static table),
inclusive, refer to elements in the static
table, where the index len(header table) + 1
refers to the first entry in the static table.
Any other indices MUST be treated as a
decoding error.
<-- Header Table --> <-- Static Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | k | |k+1| ... | n |
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Drop Point
]]>
The emission of a header field is the process of
marking a header field as belonging to the current
header set. Once a header has been emitted, it cannot
be removed from the current header set.
On the decoding side, an emitted header field can be
safely passed to the upper processing layer as part of
the current header set. The decoder MAY pass the
emitted header fields to the upper processing layer in
any order.
By emitting header fields instead of emitting header
sets, the decoder can be implemented in a streaming
way, and as such has only to keep in memory the header
table and the reference set. This bounds the amount of
memory used by the decoder, even in presence of a very
large set of header fields. The management of memory
for handling very large sets of header fields can
therefore be deferred to the upper processing layers.
The processing of a header block to obtain a header set is
defined in this section. To ensure that the decoding will
successfully produce a header set, a decoder MUST obey the
following rules.
All the header field representations contained in a
header block are processed in the order in which they
are presented, as specified below.
An indexed representation with an index
value of 0 entails one of the following actions,
depending on what is encoded next:
The reference set is emptied.The maximum size of the header table is
updated.
An indexed representation corresponding
to an entry present in the
reference set entails the following actions:
The entry is removed from the reference
set.
An indexed representation corresponding
to an entry not present in the
reference set entails the following actions:
If referencing an element of the static table:
The header field corresponding to the
referenced entry is emitted.The referenced static entry is inserted
at the beginning of the header
table.A reference to this new header table
entry is added to the reference set,
except if this new entry didn't fit
in the header table.If referencing an element of the header table:
The header field corresponding to the
referenced entry is emitted.The referenced header table entry is
added to the reference set.
A literal representation that is
not added to the header table entails
the following action:
The header field is emitted.
A literal representation that is
added to the header table entails
the following actions:
The header field is emitted.The header field is inserted at the beginning
of the header table.A reference to the new entry is added to the
reference set (except if this new entry didn't
fit in the header table).
Once all the representations contained in a header
block have been processed, the header fields
referenced in the reference set which have not
previously been emitted during this processing are
emitted.
Once all of the header field representations have been
processed, and the remaining items in the reference
set have been emitted, the header set is complete.
To limit the memory requirements on the decoder side,
the size of the header table is bounded. The size
of the header table MUST stay lower than or equal to its
maximum size.
By default, the maximum size of the header table is
equal to the value of the HTTP/2 setting
SETTINGS_HEADER_TABLE_SIZE defined by the decoder (see
). The encoder can change this
maximum size (see ), but it must
stay lower than or equal to the value of
SETTINGS_HEADER_TABLE_SIZE.
The size of the header table is the sum of the
size of its entries.
The size of an entry is the sum of its name's length
in octets (as defined in ), of its
value's length in octets () and of
32 octets.
The lengths are measured on the non-encoded entry
name and entry value (for the case when a Huffman
encoding is used to transmit string values).
The 32 octets are an accounting for the entry
structure overhead. For example, an entry structure
using two 64-bits pointers to reference the name and
the value and the entry, and two 64-bits integer for
counting the number of references to these name and
value would use 32 octets.
Whenever an entry is evicted from the header table,
any reference to that entry contained by the reference
set is removed.
Whenever the maximum size for the header table is made
smaller, entries are evicted from the end of the
header table until the size of the header table is
less than or equal to the maximum size.
The eviction of an entry from the header
table causes the index of the entries in the static
table to be reduced by one.
Whenever a new entry is to be added to the table, any
name referenced by the representation of this new
entry is cached, and then entries are evicted from the
end of the header table until the size of the header
table is less than or equal to (maximum size - new
entry size), or until the table is empty.
If the size of the new entry is less than or equal to
the maximum size, that entry is added to the table. It
is not an error to attempt to add an entry
that is larger than the maximum size.
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
2N-1), it is encoded within the N-bit
prefix. Otherwise all the bits of the prefix are set
to 1 and the value is encoded using an unsigned
variable length integer representation (see ).
N is always between 1 and 8 bits. An integer starting
at an octet-boundary will have an 8-bit prefix.
The algorithm to represent an integer I is as follows:
= 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:
Examples illustrating the encoding of integers are
available in .
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.
Header field names and header field values can be
represented as literal string. A literal string is
encoded as a sequence of octets, either by directly
encoding the literal string's octets, or by using a
canonical Huffman encoding
.
A literal string representation contains the following
fields:
A one bit flag, H, indicating whether or not the
octets of the string are Huffman encoded.
The number of octets used to encode the string
literal, encoded as an integer with 7-bit prefix
(see ).
The encoded data of the string literal. If H is
'0', then the encoded data is the raw octets of
the string literal. If H is '1', then the encoded
data is the Huffman encoding of the string
literal.
String literals which use Huffman encoding are encoded
with the Huffman codes defined in (see examples inRequest
Examples with Huffman and in
Response Examples with Huffman ). The
encoded data is the bitwise concatenation of the
Huffman codes corresponding to each octet of the
string literal.
As the Huffman encoded data doesn't always end at an
octet boundary, some padding is inserted after it up
to the next octet boundary. To prevent this padding to
be misinterpreted as part of the string literal, the
most significant bits of the EOS (end-of-string) entry
in the Huffman table are used.
Upon decoding, an incomplete Huffman code at the end
of the encoded data is to be considered as padding and
discarded. A padding strictly longer than 7 bits MUST
be treated as a decoding error. A padding not
corresponding to the most significant bits of the EOS
entry MUST be treated as a decoding error. A Huffman
encoded string literal containing the EOS entry MUST
be treated as a decoding error.
An indexed header field representation either identifies an
entry in the header table or static table. The processing
of an indexed header field representation is described in
.
This representation starts with the '1' 1-bit pattern,
followed by the index of the matching pair, represented as
an integer with a 7-bit prefix.
The index value of 0 is not used. It MUST be treated as a
decoding error if found in an indexed header field
representation.
Literal header field representations contain a literal
header field value. Header field names are either
provided as a literal or by reference to an existing
header table or static table entry.
Literal representations all result in the emission of a
header field when decoded.
A literal header field with incremental indexing adds
a new entry to the header table.
This representation starts with the '01' 2-bit
pattern.
If the header field name matches the header field name
of a (name, value) pair stored in the Header Table or
Static Table, the header field name can be represented
using the index of that entry. In this case, the index
of the entry, index (which is strictly greater than 0),
is represented as an integer with a 6-bit prefix (see
).
Otherwise, the header field name is represented as a
literal. The value 0 is represented on 6 bits followed
by the header field name (see ).
The header field name representation is followed by
the header field value represented as a literal string
as described in .
A literal header field without indexing causes the
emission of a header field without altering the header
table.
The literal header field without indexing
representation starts with the '0000' 4-bit pattern.
If the header field name matches the header field name
of a (name, value) pair stored in the Header Table or
Static Table, the header field name can be represented
using the index of that entry. In this case, the index
of the entry, index (which is strictly greater than 0),
is represented as an integer with a 6-bit prefix (see
).
Otherwise, the header field name is represented as a
literal. The value 0 is represented on 4 bits followed
by the header field name (see ).
The header field name representation is followed by
the header field value represented as a literal string
as described in .
A literal header field never indexed causes the
emission of a header field without altering the header
table.
The 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 same representation. In particular, when a
peer sends a header field that it received represented
as a literal header field never indexed, it MUST use
the same representation to forward this header field.
This representation is intended for protecting header
field values that are not to be put at risk by
compressing them (see for more
details).
The encoding of the representation is the same as for
the literal header field without indexing
representation (see ).
An encoding context update causes the immediate application of a
change to the encoding context.
An encoding context update starts with the '001' 3-bit
pattern.
It is followed by a flag specifying the type of the
change, and by any data necessary to describe the change
itself.
The flag bit being set to '1' signals that the reference
set is emptied. The remaining bits are set to '0'.
The flag bit being set to '0' signals that a change to the
maximum size of the header table. This new maximum size
MUST be lower than or equal to the value of the setting
SETTINGS_HEADER_TABLE_SIZE (see ).
The new maximum size is encoded as an integer with a 4-bit
prefix.
Change in the maximum size of the header table can trigger
entry evictions (see ).
Compression can create a weak point allowing an attacker
to recover secret data. For example, the CRIME attack (see
) took advantage of the DEFLATE
mechanism (see ) of SPDY (see
) to efficiently probe the
compression context. The full-text compression mechanism
of DEFLATE allowed the attacker to learn some information
from each failed attempt at guessing the secret.
For this reason, HPACK provides only limited compression
mechanisms in the form of an indexing table and of a
static Huffman encoding.
The indexing table can still provide information to an
attacker that would be able to probe the compression
context. However, this information is limited to the
knowledge of whether the attacker's guess is correct or
not.
Still, an attacker could take advantage of this limited
information for breaking low-entropy secrets using a
brute-force attack. A server usually has some protections
against such brute-force attack. Here, the attack would
target the client, where it would be harder to detect. The
attack would be even more dangerous if the attacker is
able to prevent the traffic generated by its brute-force
attack from reaching the server.
To offer some protection against such type of attacks,
HPACK enables an endpoint to indicate that a header field
must never be compressed, across any hop up to the other
endpoint (see ). An endpoint
MUST use this feature to prevent the compression of any
header field whose value contains a secret which could be
put at risk by a brute-force attack.
For optimal processing, a sensitive value (for example a
cookie) needs to have an entropy high enough to not be
endangered by a brute-force attack, in order to take
advantage of HPACK indexing.
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 ).
An attacker can try to cause an endpoint to exhaust its
memory. HPACK is designed to limit both the peak and state
amounts of memory allocated by an endpoint.
The amount of memory used by the compressor state is
limited by the value of the setting
SETTINGS_HEADER_TABLE_SIZE. This limitation takes into
account both the size of the data stored in the header
table, and the overhead required by the table structure
itself.
For the decoding side, an endpoint can limit the amount of
state memory used by setting an appropriate value for
SETTINGS_HEADER_TABLE_SIZE. For the encoding side, the
endpoint can limit the amount of state memory it uses by
defining a header table maximum size lower than the value
of SETTINGS_HEADER_TABLE_SIZE defined by its peer (see
).
The amount of temporary memory consumed is linked to the
set of header fields emitted or received. However, this
amount of temporary memory can be limited by processing
these header fields in a streaming manner.
An implementation of HPACK needs to ensure that large
values for integers, long encoding for integers, or long
string literal do not create security weaknesses.
An implementation has to set a limit for the values it
accepts for integers, as well as for the encoded length
(see ). In the
same way, it has to set a limit to the length it accepts
for string literals (see ).
This document includes substantial editorial contributions
from the following individuals: Mike Bishop, Jeff Pinner,
Julian Reschke, Martin Thomson.
Hypertext Transfer Protocol version 2TwistGoogleMozilla
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deSPDY ProtocolTwistGoogleDEFLATE Compressed Data Format Specification version 1.3Aladdin EnterprisesThe CRIME AttackIETF83: SPDY and What to Consider for HTTP/2.0SPDY: What I Like About YouA Method for the Construction of Minimum Redundancy CodesGenerating a canonical prefix encodingPETAL: Preset Encoding Table Information
Leakage
Updated format to include literal headers that must
never be compressed.
Updated security considerations.
Moved integer encoding examples to the appendix.
Updated Huffman table.
Updated static header table (adding and removing
status values).
Updated examples.
Regenerated examples.
Only one Huffman table for requests and responses.
Added maximum size for header table, independent of
SETTINGS_HEADER_TABLE_SIZE.
Added pseudo-code for integer decoding.
Improved examples (removing unnecessary removals).
Updated examples: take into account changes in the
spec, and show more features.
Use 'octet' everywhere instead of having both 'byte'
and 'octet'.
Added reference set emptying.
Editorial changes and clarifications.
Added "host" header to the static table.
Ordering for list of values (either NULL- or
comma-separated).
A large number of editorial changes; changed the
description of evicting/adding new entries.
Removed substitution indexing
Changed 'initial headers' to 'static headers', as per
issue #258
Merged 'request' and 'response' static headers, as per
issue #259
Changed text to indicate that new headers are added at
index 0 and expire from the largest index, as per
issue #233
Corrected error in integer encoding pseudocode.
Refactored of Header Encoding Section: split
definitions and processing rule.
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.
Backward incompatible change: modified initial
header list, as per issue #188.
Added example of 32 octets entry structure (issue
#191).
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.
Fixed bug/omission in integer representation algorithm.Changed the document title.Header matching text rewritten.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" sectionCorrected erroneous statement about what index can
be contained in one octetAdded descriptions of opcodesRemoved security claims from introduction.
The static table consists of an unchangeable ordered list of
(name, value) pairs. The first entry in the table is always
represented by the index len(header table) + 1, and the last entry
in the table is represented by the index len(header
table) + len(static table).
The following table lists the pre-defined header fields that
make-up the static table.
IndexHeader NameHeader Value1:authority2:methodGET3:methodPOST4:path/5:path/index.html6:schemehttp7:schemehttps8:status2009:status20410:status20611:status30412:status40013:status40414:status50015accept-charset16accept-encoding17accept-language18accept-ranges19accept20access-control-allow-origin21age22allow23authorization24cache-control25content-disposition26content-encoding27content-language28content-length29content-location30content-range31content-type32cookie33date34etag35expect36expires37from38host39if-match40if-modified-since41if-none-match42if-range43if-unmodified-since44last-modified45link46location47max-forwards48proxy-authenticate49proxy-authorization50range51referer52refresh53retry-after54server55set-cookie56strict-transport-security57transfer-encoding58user-agent59vary60via61www-authenticate
The table give the index of each entry in the static table.
The full index of each entry, to be used for encoding a
reference to this entry, is computed by adding the number of
entries in the header table to this index.
The following codes are used when encoding string
literals with an Huffman coding (see ).
Each row in the table specifies one Huffman code:
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.
The Huffman code for the symbol represented as a
base-2 integer.
The Huffman code for the symbol, represented as a
hexadecimal integer, aligned on the least significant
bit.
The number of bits for the Huffman code of the symbol.
As an example, the Huffman code for the symbol 48
(corresponding to the ASCII character "0") consists in the 5
bits "0", "0", "1", "0", "1". This corresponds to the value 5
encoded on 5 bits.
' ( 62) |11111111|1111101 7ffd [15]
'?' ( 63) |11111111|01 3fd [10]
'@' ( 64) |11111111|1111110 7ffe [15]
'A' ( 65) |1100111 67 [ 7]
'B' ( 66) |11101101| ed [ 8]
'C' ( 67) |11101110| ee [ 8]
'D' ( 68) |1101000 68 [ 7]
'E' ( 69) |11101111| ef [ 8]
'F' ( 70) |1101001 69 [ 7]
'G' ( 71) |1101010 6a [ 7]
'H' ( 72) |11111001|0 1f2 [ 9]
'I' ( 73) |11110000| f0 [ 8]
'J' ( 74) |11111001|1 1f3 [ 9]
'K' ( 75) |11111010|0 1f4 [ 9]
'L' ( 76) |11111010|1 1f5 [ 9]
'M' ( 77) |1101011 6b [ 7]
'N' ( 78) |1101100 6c [ 7]
'O' ( 79) |11110001| f1 [ 8]
'P' ( 80) |11110010| f2 [ 8]
'Q' ( 81) |11111011|0 1f6 [ 9]
'R' ( 82) |11111011|1 1f7 [ 9]
'S' ( 83) |1101101 6d [ 7]
'T' ( 84) |101000 28 [ 6]
'U' ( 85) |11110011| f3 [ 8]
'V' ( 86) |11111100|0 1f8 [ 9]
'W' ( 87) |11111100|1 1f9 [ 9]
'X' ( 88) |11110100| f4 [ 8]
'Y' ( 89) |11111101|0 1fa [ 9]
'Z' ( 90) |11111101|1 1fb [ 9]
'[' ( 91) |11111111|100 7fc [11]
'\' ( 92) |11111111|11111111|11110110|10 3ffffda [26]
']' ( 93) |11111111|101 7fd [11]
'^' ( 94) |11111111|111101 3ffd [14]
'_' ( 95) |1101110 6e [ 7]
'`' ( 96) |11111111|11111111|10 3fffe [18]
'a' ( 97) |01001 9 [ 5]
'b' ( 98) |1101111 6f [ 7]
'c' ( 99) |01010 a [ 5]
'd' (100) |101001 29 [ 6]
'e' (101) |01011 b [ 5]
'f' (102) |1110000 70 [ 7]
'g' (103) |101010 2a [ 6]
'h' (104) |101011 2b [ 6]
'i' (105) |01100 c [ 5]
'j' (106) |11110101| f5 [ 8]
'k' (107) |11110110| f6 [ 8]
'l' (108) |101100 2c [ 6]
'm' (109) |101101 2d [ 6]
'n' (110) |101110 2e [ 6]
'o' (111) |01101 d [ 5]
'p' (112) |101111 2f [ 6]
'q' (113) |11111110|0 1fc [ 9]
'r' (114) |110000 30 [ 6]
's' (115) |110001 31 [ 6]
't' (116) |01110 e [ 5]
'u' (117) |1110001 71 [ 7]
'v' (118) |1110010 72 [ 7]
'w' (119) |1110011 73 [ 7]
'x' (120) |1110100 74 [ 7]
'y' (121) |1110101 75 [ 7]
'z' (122) |11110111| f7 [ 8]
'{' (123) |11111111|11111110|1 1fffd [17]
'|' (124) |11111111|1100 ffc [12]
'}' (125) |11111111|11111111|0 1fffe [17]
'~' (126) |11111111|1101 ffd [12]
(127) |11111111|11111111|11110110|11 3ffffdb [26]
(128) |11111111|11111111|11110111|00 3ffffdc [26]
(129) |11111111|11111111|11110111|01 3ffffdd [26]
(130) |11111111|11111111|11110111|10 3ffffde [26]
(131) |11111111|11111111|11110111|11 3ffffdf [26]
(132) |11111111|11111111|11111000|00 3ffffe0 [26]
(133) |11111111|11111111|11111000|01 3ffffe1 [26]
(134) |11111111|11111111|11111000|10 3ffffe2 [26]
(135) |11111111|11111111|11111000|11 3ffffe3 [26]
(136) |11111111|11111111|11111001|00 3ffffe4 [26]
(137) |11111111|11111111|11111001|01 3ffffe5 [26]
(138) |11111111|11111111|11111001|10 3ffffe6 [26]
(139) |11111111|11111111|11111001|11 3ffffe7 [26]
(140) |11111111|11111111|11111010|00 3ffffe8 [26]
(141) |11111111|11111111|11111010|01 3ffffe9 [26]
(142) |11111111|11111111|11111010|10 3ffffea [26]
(143) |11111111|11111111|11111010|11 3ffffeb [26]
(144) |11111111|11111111|11111011|00 3ffffec [26]
(145) |11111111|11111111|11111011|01 3ffffed [26]
(146) |11111111|11111111|11111011|10 3ffffee [26]
(147) |11111111|11111111|11111011|11 3ffffef [26]
(148) |11111111|11111111|11111100|00 3fffff0 [26]
(149) |11111111|11111111|11111100|01 3fffff1 [26]
(150) |11111111|11111111|11111100|10 3fffff2 [26]
(151) |11111111|11111111|11111100|11 3fffff3 [26]
(152) |11111111|11111111|11111101|00 3fffff4 [26]
(153) |11111111|11111111|11111101|01 3fffff5 [26]
(154) |11111111|11111111|11111101|10 3fffff6 [26]
(155) |11111111|11111111|11111101|11 3fffff7 [26]
(156) |11111111|11111111|11111110|00 3fffff8 [26]
(157) |11111111|11111111|11111110|01 3fffff9 [26]
(158) |11111111|11111111|11111110|10 3fffffa [26]
(159) |11111111|11111111|11111110|11 3fffffb [26]
(160) |11111111|11111111|11111111|00 3fffffc [26]
(161) |11111111|11111111|11111111|01 3fffffd [26]
(162) |11111111|11111111|11111111|10 3fffffe [26]
(163) |11111111|11111111|11111111|11 3ffffff [26]
(164) |11111111|11111111|11000000|0 1ffff80 [25]
(165) |11111111|11111111|11000000|1 1ffff81 [25]
(166) |11111111|11111111|11000001|0 1ffff82 [25]
(167) |11111111|11111111|11000001|1 1ffff83 [25]
(168) |11111111|11111111|11000010|0 1ffff84 [25]
(169) |11111111|11111111|11000010|1 1ffff85 [25]
(170) |11111111|11111111|11000011|0 1ffff86 [25]
(171) |11111111|11111111|11000011|1 1ffff87 [25]
(172) |11111111|11111111|11000100|0 1ffff88 [25]
(173) |11111111|11111111|11000100|1 1ffff89 [25]
(174) |11111111|11111111|11000101|0 1ffff8a [25]
(175) |11111111|11111111|11000101|1 1ffff8b [25]
(176) |11111111|11111111|11000110|0 1ffff8c [25]
(177) |11111111|11111111|11000110|1 1ffff8d [25]
(178) |11111111|11111111|11000111|0 1ffff8e [25]
(179) |11111111|11111111|11000111|1 1ffff8f [25]
(180) |11111111|11111111|11001000|0 1ffff90 [25]
(181) |11111111|11111111|11001000|1 1ffff91 [25]
(182) |11111111|11111111|11001001|0 1ffff92 [25]
(183) |11111111|11111111|11001001|1 1ffff93 [25]
(184) |11111111|11111111|11001010|0 1ffff94 [25]
(185) |11111111|11111111|11001010|1 1ffff95 [25]
(186) |11111111|11111111|11001011|0 1ffff96 [25]
(187) |11111111|11111111|11001011|1 1ffff97 [25]
(188) |11111111|11111111|11001100|0 1ffff98 [25]
(189) |11111111|11111111|11001100|1 1ffff99 [25]
(190) |11111111|11111111|11001101|0 1ffff9a [25]
(191) |11111111|11111111|11001101|1 1ffff9b [25]
(192) |11111111|11111111|11001110|0 1ffff9c [25]
(193) |11111111|11111111|11001110|1 1ffff9d [25]
(194) |11111111|11111111|11001111|0 1ffff9e [25]
(195) |11111111|11111111|11001111|1 1ffff9f [25]
(196) |11111111|11111111|11010000|0 1ffffa0 [25]
(197) |11111111|11111111|11010000|1 1ffffa1 [25]
(198) |11111111|11111111|11010001|0 1ffffa2 [25]
(199) |11111111|11111111|11010001|1 1ffffa3 [25]
(200) |11111111|11111111|11010010|0 1ffffa4 [25]
(201) |11111111|11111111|11010010|1 1ffffa5 [25]
(202) |11111111|11111111|11010011|0 1ffffa6 [25]
(203) |11111111|11111111|11010011|1 1ffffa7 [25]
(204) |11111111|11111111|11010100|0 1ffffa8 [25]
(205) |11111111|11111111|11010100|1 1ffffa9 [25]
(206) |11111111|11111111|11010101|0 1ffffaa [25]
(207) |11111111|11111111|11010101|1 1ffffab [25]
(208) |11111111|11111111|11010110|0 1ffffac [25]
(209) |11111111|11111111|11010110|1 1ffffad [25]
(210) |11111111|11111111|11010111|0 1ffffae [25]
(211) |11111111|11111111|11010111|1 1ffffaf [25]
(212) |11111111|11111111|11011000|0 1ffffb0 [25]
(213) |11111111|11111111|11011000|1 1ffffb1 [25]
(214) |11111111|11111111|11011001|0 1ffffb2 [25]
(215) |11111111|11111111|11011001|1 1ffffb3 [25]
(216) |11111111|11111111|11011010|0 1ffffb4 [25]
(217) |11111111|11111111|11011010|1 1ffffb5 [25]
(218) |11111111|11111111|11011011|0 1ffffb6 [25]
(219) |11111111|11111111|11011011|1 1ffffb7 [25]
(220) |11111111|11111111|11011100|0 1ffffb8 [25]
(221) |11111111|11111111|11011100|1 1ffffb9 [25]
(222) |11111111|11111111|11011101|0 1ffffba [25]
(223) |11111111|11111111|11011101|1 1ffffbb [25]
(224) |11111111|11111111|11011110|0 1ffffbc [25]
(225) |11111111|11111111|11011110|1 1ffffbd [25]
(226) |11111111|11111111|11011111|0 1ffffbe [25]
(227) |11111111|11111111|11011111|1 1ffffbf [25]
(228) |11111111|11111111|11100000|0 1ffffc0 [25]
(229) |11111111|11111111|11100000|1 1ffffc1 [25]
(230) |11111111|11111111|11100001|0 1ffffc2 [25]
(231) |11111111|11111111|11100001|1 1ffffc3 [25]
(232) |11111111|11111111|11100010|0 1ffffc4 [25]
(233) |11111111|11111111|11100010|1 1ffffc5 [25]
(234) |11111111|11111111|11100011|0 1ffffc6 [25]
(235) |11111111|11111111|11100011|1 1ffffc7 [25]
(236) |11111111|11111111|11100100|0 1ffffc8 [25]
(237) |11111111|11111111|11100100|1 1ffffc9 [25]
(238) |11111111|11111111|11100101|0 1ffffca [25]
(239) |11111111|11111111|11100101|1 1ffffcb [25]
(240) |11111111|11111111|11100110|0 1ffffcc [25]
(241) |11111111|11111111|11100110|1 1ffffcd [25]
(242) |11111111|11111111|11100111|0 1ffffce [25]
(243) |11111111|11111111|11100111|1 1ffffcf [25]
(244) |11111111|11111111|11101000|0 1ffffd0 [25]
(245) |11111111|11111111|11101000|1 1ffffd1 [25]
(246) |11111111|11111111|11101001|0 1ffffd2 [25]
(247) |11111111|11111111|11101001|1 1ffffd3 [25]
(248) |11111111|11111111|11101010|0 1ffffd4 [25]
(249) |11111111|11111111|11101010|1 1ffffd5 [25]
(250) |11111111|11111111|11101011|0 1ffffd6 [25]
(251) |11111111|11111111|11101011|1 1ffffd7 [25]
(252) |11111111|11111111|11101100|0 1ffffd8 [25]
(253) |11111111|11111111|11101100|1 1ffffd9 [25]
(254) |11111111|11111111|11101101|0 1ffffda [25]
(255) |11111111|11111111|11101101|1 1ffffdb [25]
EOS (256) |11111111|11111111|11101110|0 1ffffdc [25]
]]>
A number of examples are worked through here, covering integer
encoding, header field representation, and the encoding of
whole sets of header fields, for both requests and responses,
and with and without Huffman coding.
This section shows the representation of integer values in
details (see ).
The value 10 is to be encoded with a 5-bit prefix.
10 is less than 31 (25 -
1) and is represented using the 5-bit
prefix.
The value I=1337 is to be encoded with a 5-bit
prefix.
1337 is greater than 31 (25 -
1).
The 5-bit prefix is filled with its
max value (31).I = 1337 - (25 - 1) =
1306.I (1306) is greater than or equal
to 128, the while loop body
executes:I % 128 == 2626 + 128 == 154154 is encoded in 8 bits as:
10011010I 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.=128, encode(154), I=1306/128
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done
+---+---+---+---+---+---+---+---+
]]>
The value 42 is to be encoded starting at an
octet-boundary. This implies that a 8-bit prefix is
used.
42 is less than 255 (28 -
1) and is represented using the 8-bit
prefix.
This section shows several independent representation examples.
The header field representation uses a literal name and a literal value.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
custom-key: custom-head\
| er]]>Header Table (after decoding):Decoded header set:
The header field representation uses an indexed name and a literal value.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:path: /sample/path]]>
Header table (after decoding): empty.
Decoded header set:
The header field representation uses an indexed header field, from
the static table. Upon using it, the static table entry is copied into
the header table.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:method: GET]]>Header Table (after decoding):Decoded header set:
The header field representation uses an indexed header field, from
the static table. In this example, the SETTINGS_HEADER_TABLE_SIZE is set to 0,
therefore, the entry is not copied into the header table.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:method: GET]]>
Header table (after decoding): empty.
Decoded header set:
This section shows several consecutive header sets, corresponding
to HTTP requests, on the same connection.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
44 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
0f | Literal value (len = 15)
7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
| -> :authority: www.example\
| .com]]>Header Table (after decoding):Decoded header set:
This request takes advantage of the differential encoding of header sets.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
cache-control: no-cache]]>Header Table (after decoding):Decoded header set:
This request has not enough headers in common with the previous request
to take advantage of the differential encoding. Therefore, the reference set
is emptied before encoding the header fields.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
:method: GET
8c | == Indexed - Add ==
| idx = 12
| -> :scheme: https
8b | == Indexed - Add ==
| idx = 11
| -> :path: /index.html
84 | == Indexed - Add ==
| idx = 4
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0c | Literal value (len = 12)
6375 7374 6f6d 2d76 616c 7565 | custom-value
| -> custom-key: custom-valu\
| e]]>Header Table (after decoding):Decoded header set:
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
44 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
8c | Literal value (len = 15)
| Huffman encoded:
e7cf 9beb e89b 6fb1 6fa9 b6ff | ......o.o...
| Decoded:
| www.example.com
| -> :authority: www.example\
| .com]]>Header Table (after decoding):Decoded header set:
This request takes advantage of the differential encoding of header sets.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
cache-control: no-cache]]>Header Table (after decoding):Decoded header set:
This request has not enough headers in common with the previous request
to take advantage of the differential encoding. Therefore, the reference set
is emptied before encoding the header fields.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
:method: GET
8c | == Indexed - Add ==
| idx = 12
| -> :scheme: https
8b | == Indexed - Add ==
| idx = 11
| -> :path: /index.html
84 | == Indexed - Add ==
| idx = 4
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
88 | Literal name (len = 10)
| Huffman encoded:
571c 5cdb 737b 2faf | W.\.s{/.
| Decoded:
| custom-key
89 | Literal value (len = 12)
| Huffman encoded:
571c 5cdb 7372 4d9c 57 | W.\.srM.W
| Decoded:
| custom-value
| -> custom-key: custom-valu\
| e]]>Header Table (after decoding):Decoded header set:
This section shows several consecutive header sets, corresponding
to HTTP responses, on the same connection. SETTINGS_HEADER_TABLE_SIZE is set
to the value of 256 octets, causing some evictions to occur.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
71 | == Literal indexed ==
| Indexed name (idx = 49)
| location
17 | Literal value (len = 23)
6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
706c 652e 636f 6d | ple.com
| -> location: https://www.e\
| xample.com]]>Header Table (after decoding):Decoded header set:
The (":status", "302") header field is evicted from the header
table to free space to allow adding the (":status", "200") header
field, copied from the static table into the header table.
The (":status", "302") header field doesn't need to be
removed from the reference set as it is evicted from the header
table.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
:status: 200]]>Header Table (after decoding):Decoded header set:
Several header fields are evicted from the header table during the
processing of this header set. Before evicting a header belonging to the
reference set, it is emitted, by coding it twice as an Indexed Representation.
The first representation removes the header field from the reference set, the
second one adds it again to the reference set, also emitting it.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
5e | == Literal indexed ==
| Indexed name (idx = 30)
| content-encoding
04 | Literal value (len = 4)
677a 6970 | gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
84 | == Indexed - Remove ==
| idx = 4
| -> location: https://www.e\
| xample.com
84 | == Indexed - Add ==
| idx = 4
| -> location: https://www.e\
| xample.com
83 | == Indexed - Remove ==
| idx = 3
| -> :status: 200
83 | == Indexed - Add ==
| idx = 3
| -> :status: 200
7b | == Literal indexed ==
| Indexed name (idx = 59)
| set-cookie
38 | Literal value (len = 56)
666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
6572 7369 6f6e 3d31 | ersion=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 200
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1]]>Header Table (after decoding):Decoded header set:
This section shows the same examples as the previous section, but
using Huffman encoding for the literal values. The eviction mechanism uses the
length of the decoded literal values, so the same evictions occurs as in the
previous section.
Header set to encode:
Reference set: empty.
Hex dump of encoded data:Decoding process:
:status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
85 | Literal value (len = 7)
| Huffman encoded:
bf06 724b 97 | ..rK.
| Decoded:
| private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
93 | Literal value (len = 29)
| Huffman encoded:
d6db b298 84de 2a71 8805 0620 9851 3109 | ......*q... .Q1.
b56b a3 | .k.
| Decoded:
| Mon, 21 Oct 2013 20:13:21 \
| GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
71 | == Literal indexed ==
| Indexed name (idx = 49)
| location
91 | Literal value (len = 23)
| Huffman encoded:
adce bf19 8e7e 7cf9 bebe 89b6 fb16 fa9b | ......|.........
6f | o
| Decoded:
| https://www.example.com
| -> location: https://www.e\
| xample.com]]>Header Table (after decoding):Decoded header set:
The (":status", "302") header field is evicted from the header
table to free space to allow adding the (":status", "200") header
field, copied from the static table into the header table.
The (":status", "302") header field doesn't need to be
removed from the reference set as it is evicted from the header
table.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
:status: 200]]>Header Table (after decoding):Decoded header set:
Several header fields are evicted from the header table during the
processing of this header set. Before evicting a header belonging to the
reference set, it is emitted, by coding it twice as an Indexed Representation.
The first representation removes the header field from the reference set, the
second one adds it again to the reference set, also emitting it.
Header set to encode:Reference set:Hex dump of encoded data:Decoding process:
cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
93 | Literal value (len = 29)
| Huffman encoded:
d6db b298 84de 2a71 8805 0620 9851 3111 | ......*q... .Q1.
b56b a3 | .k.
| Decoded:
| Mon, 21 Oct 2013 20:13:22 \
| GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
5e | == Literal indexed ==
| Indexed name (idx = 30)
| content-encoding
84 | Literal value (len = 4)
| Huffman encoded:
abdd 97ff | ....
| Decoded:
| gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
84 | == Indexed - Remove ==
| idx = 4
| -> location: https://www.e\
| xample.com
84 | == Indexed - Add ==
| idx = 4
| -> location: https://www.e\
| xample.com
83 | == Indexed - Remove ==
| idx = 3
| -> :status: 200
83 | == Indexed - Add ==
| idx = 3
| -> :status: 200
7b | == Literal indexed ==
| Indexed name (idx = 59)
| set-cookie
b1 | Literal value (len = 56)
| Huffman encoded:
e0d6 cf9f 6e8f 9fd3 e5f6 fa76 fefd 3c7e | ....n......v....
df9e ff1f 2f0f 3cfe 9f6f cf7f 8f87 9f61 | ..../....o.....a
ad4f 4cc9 a973 a220 0ec3 725e 18b1 b74e | .OL..s. ..r^...N
3f | ?
| Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ\
| WEOIU; max-age=3600; versi\
| on=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 200
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1]]>Header Table (after decoding):Decoded header set: