HTTPbis Working Group R. Peon
Internet-Draft Google, Inc
Intended status: Informational H. Ruellan
Expires: January 10, 2014 Canon CRF
July 09, 2013
HTTP/2.0 Header Compression
draft-ietf-httpbis-header-compression-01
Abstract
This document describes a format adapted to efficiently represent
HTTP headers in the context of HTTP/2.0.
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Header Encoding . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Encoding Components . . . . . . . . . . . . . . . . . . . 3
3.2. Header Table . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Header Representation . . . . . . . . . . . . . . . . . . 5
3.3.1. Literal Representation . . . . . . . . . . . . . . . 5
3.3.2. Indexed Representation . . . . . . . . . . . . . . . 6
3.4. Differential Coding . . . . . . . . . . . . . . . . . . . 6
4. Detailed Format . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Header Blocks . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Low-level representations . . . . . . . . . . . . . . . . 7
4.2.1. Integer representation . . . . . . . . . . . . . . . 7
4.2.2. String literal representation . . . . . . . . . . . . 9
4.3. Indexed Header Representation . . . . . . . . . . . . . . 9
4.4. Literal Header Representation . . . . . . . . . . . . . . 10
4.4.1. Literal Header without Indexing . . . . . . . . . . . 10
4.4.2. Literal Header with Incremental Indexing . . . . . . 10
4.4.3. Literal Header with Substitution Indexing . . . . . . 11
5. Parameter Negotiation . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Informative References . . . . . . . . . . . . . . . . . . . 13
Appendix A. Initial header names . . . . . . . . . . . . . . . . 13
A.1. Requests . . . . . . . . . . . . . . . . . . . . . . . . 14
A.2. Responses . . . . . . . . . . . . . . . . . . . . . . . . 15
Appendix B. Example . . . . . . . . . . . . . . . . . . . . . . 16
B.1. First header set . . . . . . . . . . . . . . . . . . . . 16
B.2. Second header set . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
This document describes a format adapted to efficiently represent
HTTP headers in the context of HTTP/2.0.
2. Overview
In HTTP/1.X, HTTP headers, which are necessary for the functioning of
the protocol, are transmitted with no transformations.
Unfortunately, the amount of redundancy in both the keys and the
values of these headers is astonishingly high, and is the cause of
increased latency on lower bandwidth links. This indicates that an
alternate encoding for headers would be beneficial to latency, and
that is what is proposed here. As shown by SPDY [SPDY], Deflate
compresses HTTP very effectively. However, the use of a compression
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scheme which allows for arbitrary matches against the previously
encoded data (such as Deflate) exposes users to security issues. In
particular, the compression of sensitive data, together with other
data controlled by an attacker, may lead to leakage of that sensitive
data, even when the resultant bytes are transmitted over an encrypted
channel. Another consideration is that processing and memory costs
of a compressor such as Deflate may also be too high for some classes
of devices, for example when doing forward or reverse proxying.
2.1. Outline
The HTTP header representation described in this document is based on
indexing tables that store (name, value) pairs, called header tables
in the remainder of this document. This scheme is believed to be
safe for all known attacks against the compression context today.
Header tables are incrementally updated during the whole HTTP/2.0
session. Two independent header tables are used during a HTTP/2.0
session, one for HTTP request headers and one for HTTP response
headers.
The encoder is responsible for deciding which headers to insert as
(name, value) pairs in the header table. The decoder then does
exactly what the encoder prescribes, ending in a state that exactly
matches the encoder's state. This enables decoders to remain simple
and understand a wide variety of encoders.
A header may be represented as a literal or as an index. If
represented as a literal, the representation specifies whether this
header is used to update the indexing table. The different
representations are described in Section 3.3.
A set of headers is coded as a difference from the previous set of
headers.
An example illustrating the use these different mechanisms to
represent headers is available in Appendix B.
3. Header Encoding
3.1. Encoding Components
The encoding and decoding of headers relies on a few components.
First, a header table (see Section 3.2) is used to associate headers
to index values. Second, a set of headers is encoded as a difference
from the previous reference set of headers (see Section 3.4).
As messages are exchanged in two directions, from client to server
and from server to client, there are two sets of components: one for
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each direction. All the headers sent in messages from the client to
the server are encoded (and decoded) using one set of components.
All the headers sent in messages from the server to the client
(including headers contained in PUSH_PROMISE frame) are encoded using
the other set of compotents.
3.2. Header Table
A header table consists of an ordered list of (name, value) pairs. A
pair is either inserted at the end of the table or replaces an
existing pair depending on the chosen representation. A pair can be
represented as an index which is its position in the table, starting
with 0 for the first entry.
An input header name matches the header name of a (name, value) pair
stored in the Header Table if they are equal using a character-based,
_case sensitive_ comparison. An input header value matches the
header value of a (name, value) pair stored in the Header Table if
they are equal using a character-based, _case sensitive_ comparison.
An input header (name, value) pair matches a pair in the Header Table
if both the name and value are matching as per above.
Generally, the header table will not contain duplicate header (name,
value) entries. However, implementations MUST be prepared to accept
duplicates without signaling an error. If duplicates are added to
the table, they MUST be treated as distinct entries with their own
index positions.
The header table is progressively updated based on headers
represented as literal (as defined in Section 3.3.1). Two update
mechanisms are defined:
o Incremental indexing: the represented header is inserted at the
end of the header table as a (name, value) pair. The inserted
pair index is set to the next free index in the table: it is equal
to the number of headers in the table before its insertion.
o Substitution indexing: the represented header contains an index to
an existing (name, value) pair. The existing pair value is
replaced by the pair representing the new header.
Incremental and substitution indexing are optional. If none of them
is selected in a header representation, the header table is not
updated. In particular, no update happens on the header table when
processing an indexed representation.
The header table size can be bounded so as to limit the memory
requirements (see the SETTINGS_MAX_BUFFER_SIZE in Section 5). The
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header table size is defined as the sum of the size of each entry of
the table. The size of an entry is the sum of the length in bytes
(as defined in Section 4.2.2) of its name, of value's length in bytes
and of 32 bytes (for accounting for the entry structure overhead).
The header table size MUST NOT exceed this limit.
Before adding a new entry to the header table or changing an existing
one, a check has to be performed to ensure that the change will not
cause the table to grow in size beyond the SETTINGS_MAX_BUFFER_SIZE
limit. If necessary, one or more items from the beginning of the
table are removed until there is enough free space available to make
the modification. Dropping an entry from the beginning of the table
causes the index positions of the remaining entries in the table to
be decremented by 1. [[Feedback is needed on this automatic eviction
strategy. ]]
When using substitution indexing, it is possible that the existing
item being replaced might be one of the items removed when performing
the necessary size adjustment. In such cases, the substituted value
being added to the header table is inserted at the beginning of the
header table (at index position #0) and the index positions of the
other remaining entries in the table are incremented by 1.
To optimize the representation of the headers exchanged at the
beginning of an HTTP/2.0 session, the header table is initialized
with common headers. Two lists of initial headers are provided in
Appendix A. One is for messages sent from a client to a server, the
other is for messages sent from a server to a client.
3.3. Header Representation
3.3.1. Literal Representation
The literal representation defines a new header. A literal header is
represented as:
o A header name, with two possible representations:
* A literal string, as described in Section 4.2.2.
* A index in the header table referencing the name of the
corresponding header. The index is represented as an integer,
as described in Section 4.2.1.
o The header value, represented as a literal string, as described in
Section 4.2.2.
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3.3.2. Indexed Representation
The indexed representation defines a header as a match to a (name,
value) pair in the header table. An indexed header is represented
as:
o An integer representing the index of the matching (name, value)
pair, as described in Section 4.2.1.
3.4. Differential Coding
A set of headers is encoded as a difference from the previous
reference set of headers. The initial reference set of headers is
the empty set.
An indexed representation toggles the presence of the header in the
current set of headers. If the header corresponding to the indexed
representation was not in the set, it is added to the set. If the
header index was in the set, it is removed from it.
A literal representation adds a header to the current set of headers.
To ensure a correct decoding of a set of headers, the following steps
or equivalent ones MUST be executed by the decoder.
First, upon starting the decoding of a new set of headers, the
reference set of headers is interpreted into the working set of
headers: for each header in the reference set, an entry is added to
the working set, containing the header name, its value, and its
current index in the header table.
Then, the header representations are processed in their order of
occurrence in the frame.
For an indexed representation, the decoder checks whether the index
is present in the working set. If true, the corresponding entry is
removed from the working set. If several entries correspond to this
encoded index, all these entries are removed from the working set.
If the index is not present in the working set, it is used to
retrieve the corresponding header from the header table, and a new
entry is added to the working set representing this header.
For a literal representation, a new entry is added to the working set
representing this header. If the literal representation specifies
that the header is to be indexed, the header is added accordingly to
the header table, and its index is included in the entry in the
working set. Otherwise, the entry in the working set contains an
undefined index.
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When all the header representations have been processed, the working
set contains all the headers of the set of headers.
The new reference set of headers is computed by removing from the
working set all the headers that are not present in the header table.
It should be noted that during the decoding of the header
representations, the same index may be associated to different
headers in the working set and in the header table.
4. Detailed Format
4.1. Header Blocks
A header block consists of a set of header fields, which are name-
value pairs. Each header field is encoded using one of the header
representation.
4.2. Low-level representations
4.2.1. Integer representation
Integers are used to represent name indexes, pair indexes or string
lengths. The integer representation keeps byte-alignment as much as
possible as this allows various processing optimizations as well as
efficient use of DEFLATE. For that purpose, an integer
representation always finishes at the end of a byte.
An integer is represented in two parts: a prefix that fills the
current byte and an optional list of bytes that are used if the
integer value does not fit in 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 byte. If the value is
small enough (strictly less than 2^N-1), it is encoded within the
N-bit prefix. Otherwise all the bits of the prefix are set to 1 and
the value is encoded using an unsigned variable length integer [1]
representation.
The algorithm to represent an integer I is as follows:
1. If I < 2^N - 1, encode I on N bits
2. Else, encode 2^N - 1 on N bits and do the following steps:
3.
1. Set I to (I - (2^N - 1)) and Q to 1
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2. While Q > 0
3.
1. Compute Q and R, quotient and remainder of I divided by
2^7
2. If Q is strictly greater than 0, write one 1 bit;
otherwise, write one 0 bit
3. Encode R on the next 7 bits
4. I = Q
4.2.1.1. Example 1: Encoding 10 using a 5-bit prefix
The value 10 is to be encoded with a 5-bit prefix.
o 10 is less than 31 (= 2^5 - 1) and is represented using the 5-bit
prefix.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits
+---+---+---+---+---+---+---+---+
4.2.1.2. Example 2: Encoding 1337 using a 5-bit prefix
The value I=1337 is to be encoded with a 5-bit prefix.
o 1337 is greater than 31 (= 2^5 - 1).
o
* The 5-bit prefix is filled with its max value (31).
o The value to represent on next bytes is I = 1337 - (2^5 - 1) =
1306.
o
* 1306 = 128*10 + 26, i.e. Q=10 and R=26.
* Q is greater than 1, bit 8 is set to 1.
* The remainder R=26 is encoded on next 7 bits.
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* I is replaced by the quotient Q=10.
o The value to represent on next bytes is I = 10.
o
* 10 = 128*0 + 10, i.e. Q=0 and R=10.
* Q is equal to 0, bit 16 is set to 0.
* The remainder R=10 is encoded on next 7 bits.
* I is replaced by the quotient Q=0.
o The process ends.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | Q>=1, R=26
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | Q=0 , R=10
+---+---+---+---+---+---+---+---+
4.2.2. String literal representation
Literal strings can represent header names or header values. They
are encoded in two parts:
1. The string length, defined as the number of bytes needed to store
its UTF-8 representation, is represented as an integer with a
zero bits prefix. If the string length is strictly less than
128, it is represented as one byte.
2. The string value represented as a list of UTF-8 characters.
4.3. Indexed Header Representation
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
Indexed Header
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.
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4.4. Literal Header Representation
4.4.1. Literal Header without Indexing
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 1 | Index (5+) |
+---+---+---+-------------------+
| Value Length (8+) |
+-------------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header without Indexing - Indexed Name
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 1 | 0 |
+---+---+---+-------------------+
| Name Length (8+) |
+-------------------------------+
| Name String (Length octets) |
+-------------------------------+
| Value Length (8+) |
+-------------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header without Indexing - New Name
This representation, which does not involve updating the header
table, starts with the '011' 3-bit pattern.
If the header name matches the header name of a (name, value) pair
stored in the Header Table, the index of the pair increased by one
(index + 1) is represented as an integer with a 5-bit prefix. Note
that if the index is strictly below 31, one byte is used.
If the header name does not match a header name entry, the value 0 is
represented on 5 bits followed by the header name, represented as a
literal string.
Header name representation is followed by the header value
represented as a literal string as described in Section 4.2.2.
4.4.2. Literal Header with Incremental Indexing
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 | Index (5+) |
+---+---+---+-------------------+
| Value Length (8+) |
+-------------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header with Incremental Indexing - Indexed Name
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 | 0 |
+---+---+---+-------------------+
| Name Length (8+) |
+-------------------------------+
| Name String (Length octets) |
+-------------------------------+
| Value Length (8+) |
+-------------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header with Incremental Indexing - New Name
This representation starts with the '010' 3-bit pattern.
If the header name matches the header name of a (name, value) pair
stored in the Header Table, the index of the pair increased by one
(index + 1) is represented as an integer with a 5-bit prefix. Note
that if the index is strictly below 31, one byte is used.
If the header name does not match a header name entry, the value 0 is
represented on 5 bits followed by the header name, represented as a
literal string.
Header name representation is followed by the header value
represented as a literal string as described in Section 4.2.2.
4.4.3. Literal Header with Substitution Indexing
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Index (6+) |
+---+---+-----------------------+
| Substituted Index (8+) |
+-------------------------------+
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| Value Length (8+) |
+-------------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header with Substitution Indexing - Indexed Name
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 |
+---+---+-----------------------+
| Name Length (8+) |
+-------------------------------+
| Name String (Length octets) |
+-------------------------------+
| Substituted Index (8+) |
+-------------------------------+
| Value Length (8+) |
+-------------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header with Substitution Indexing - New Name
This representation starts with the '00' 2-bit pattern.
If the header name matches the header name of a (name, value) pair
stored in the Header Table, the index of the pair increased by one
(index + 1) is represented as an integer with a 6-bit prefix. Note
that if the index is strictly below 62, one byte is used.
If the header name does not match a header name entry, the value 0 is
represented on 6 bits followed by the header name, represented as a
literal string.
The index of the substituted (name, value) pair is inserted after the
header name representation as a 0-bit prefix integer.
The index of the substituted pair MUST correspond to a position in
the header table containing a non-void entry. An index for the
substituted pair that corresponds to empty position in the header
table MUST be treated as an error.
This index is followed by the header value represented as a literal
string as described in Section 4.2.2.
5. Parameter Negotiation
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A few parameters can be used to accomodate client and server
processing and memory requirements. [[These settings are currently
not supported as they have not been integrated in the main
specification. Therefore, the maximum buffer size for the header
table is fixed at 4096 bytes. ]]
SETTINGS_MAX_BUFFER_SIZE: Allows the sender to inform the remote
endpoint of the maximum size it accepts for the header table.
The default value is 4096 bytes.
[[Is this default value OK? Do we need a maximum size? Do we
want to allow infinite buffer?]]
When the remote endpoint receives a SETTINGS frame containing a
SETTINGS_MAX_BUFFER_SIZE setting with a value smaller than the one
currently in use, it MUST send as soon as possible a HEADER frame
with a stream identifier of 0x0 containing a value smaller than or
equal to the received setting value.
[[This changes slightly the behaviour of the HEADERS frame, which
should be updated as follows: ]]
A HEADER frame with a stream identifier of 0x0 indicates that the
sender has reduced the maximum size of the header table. The new
maximum size of the header table is encoded on 32-bit. The
decoder MUST reduce its own header table by dropping entries from
it until the size of the header table is lower than or equal to
the transmitted maximum size.
6. Security Considerations
TODO?
7. IANA Considerations
This memo includes no request to IANA.
8. Informative References
[SPDY] Belshe, M. and R. Peon, "SPDY Protocol", February 2012,
<http://tools.ietf.org/html/draft-mbelshe-httpbis-spdy>.
Appendix A. Initial header names
[[The tables in this section should be updated based on statistical
analysis of header names frequency and specific HTTP 2.0 header rules
(like removal of some headers). ]]
[[These tables are not adapted for headers contained in PUSH_PROMISE
frames. Either the tables can be merged, or the table for responses
can be updated. ]]
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A.1. Requests
The following table lists the pre-defined headers that make-up the
initial header table user to represent requests sent from a client to
a server.
+-------+---------------------+--------------+
| Index | Header Name | Header Value |
+-------+---------------------+--------------+
| 0 | :scheme | http |
| 1 | :scheme | https |
| 2 | :host | |
| 3 | :path | / |
| 4 | :method | GET |
| 5 | accept | |
| 6 | accept-charset | |
| 7 | accept-encoding | |
| 8 | accept-language | |
| 9 | cookie | |
| 10 | if-modified-since | |
| 11 | keep-alive | |
| 12 | user-agent | |
| 13 | proxy-connection | |
| 14 | referer | |
| 15 | accept-datetime | |
| 16 | authorization | |
| 17 | allow | |
| 18 | cache-control | |
| 19 | connection | |
| 20 | content-length | |
| 21 | content-md5 | |
| 22 | content-type | |
| 23 | date | |
| 24 | expect | |
| 25 | from | |
| 26 | if-match | |
| 27 | if-none-match | |
| 28 | if-range | |
| 29 | if-unmodified-since | |
| 30 | max-forwards | |
| 31 | pragma | |
| 32 | proxy-authorization | |
| 33 | range | |
| 34 | te | |
| 35 | upgrade | |
| 36 | via | |
| 37 | warning | |
+-------+---------------------+--------------+
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Table 1
A.2. Responses
The following table lists the pre-defined headers that make-up the
initial header table used to represent responses sent from a server
to a client. The same header table is also used to represent request
headers sent from a server to a client in a PUSH_PROMISE frame.
+-------+-----------------------------+--------------+
| Index | Header Name | Header Value |
+-------+-----------------------------+--------------+
| 0 | :status | 200 |
| 1 | age | |
| 2 | cache-control | |
| 3 | content-length | |
| 4 | content-type | |
| 5 | date | |
| 6 | etag | |
| 7 | expires | |
| 8 | last-modified | |
| 9 | server | |
| 10 | set-cookie | |
| 11 | vary | |
| 12 | via | |
| 13 | access-control-allow-origin | |
| 14 | accept-ranges | |
| 15 | allow | |
| 16 | connection | |
| 17 | content-disposition | |
| 18 | content-encoding | |
| 19 | content-language | |
| 20 | content-location | |
| 21 | content-md5 | |
| 22 | content-range | |
| 23 | link | |
| 24 | location | |
| 25 | p3p | |
| 26 | pragma | |
| 27 | proxy-authenticate | |
| 28 | refresh | |
| 29 | retry-after | |
| 30 | strict-transport-security | |
| 31 | trailer | |
| 32 | transfer-encoding | |
| 33 | warning | |
| 34 | www-authenticate | |
+-------+-----------------------------+--------------+
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Table 2
Appendix B. Example
Here is an example that illustrates different representations and how
tables are updated. [[This section needs to be updated to integrate
differential coding.]]
B.1. First header set
The first header set to represent is the following:
:path: /my-example/index.html
user-agent: my-user-agent
x-my-header: first
The header table is empty, all headers are represented as literal
headers with indexing. The 'x-my-header' header name is not in the
header name table and is encoded literally. This gives the following
representation:
0x44 (literal header with incremental indexing, name index = 3)
0x16 (header value string length = 22)
/my-example/index.html
0x4D (literal header with incremental indexing, name index = 12)
0x0D (header value string length = 13)
my-user-agent
0x40 (literal header with incremental indexing, new name)
0x0B (header name string length = 11)
x-my-header
0x05 (header value string length = 5)
first
The header table is as follows after the processing of these headers:
Header table
+---------+----------------+---------------------------+
| Index | Header Name | Header Value |
+---------+----------------+---------------------------+
| 0 | :scheme | http |
+---------+----------------+---------------------------+
| 1 | :scheme | https |
+---------+----------------+---------------------------+
| ... | ... | ... |
+---------+----------------+---------------------------+
| 37 | warning | |
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+---------+----------------+---------------------------+
| 38 | :path | /my-example/index.html | added header
+---------+----------------+---------------------------+
| 39 | user-agent | my-user-agent | added header
+---------+----------------+---------------------------+
| 40 | x-my-header | first | added header
+---------+----------------+---------------------------+
As all the headers in the first header set are indexed in the header
table, all are kept in the reference set of headers, which is:
Reference Set:
:path, /my-example/index.html
user-agent, my-user-agent
x-my-header, first
B.2. Second header set
The second header set to represent is the following:
:path: /my-example/resources/script.js
user-agent: my-user-agent
x-my-header: second
Comparing this second header set to the reference set, the first and
third headers are from the reference set are not present in this
second header set and must be removed. In addition, in this new set,
the first and third headers have to be encoded. The path header is
represented as a literal header with substitution indexing. The x
-my-header will be represented as a literal header with incremental
indexing.
0xa6 (indexed header, index = 38: removal from reference set)
0xa8 (indexed header, index = 40: removal from reference set)
0x04 (literal header, substitution indexing, name index = 3)
0x26 (replaced entry index = 38)
0x1f (header value string length = 31)
/my-example/resources/script.js
0x5f 0x0a (literal header, incremental indexing, name index = 40)
0x06 (header value string length = 6)
second
The header table is updated as follow:
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Header table
+---------+----------------+---------------------------+
| Index | Header Name | Header Value |
+---------+----------------+---------------------------+
| 0 | :scheme | http |
+---------+----------------+---------------------------+
| 1 | :scheme | https |
+---------+----------------+---------------------------+
| ... | ... | ... |
+---------+----------------+---------------------------+
| 37 | warning | |
+---------+----------------+---------------------------+
| 38 | :path | /my-example/resources/ | replaced
| | | script.js | header
+---------+----------------+---------------------------+
| 39 | user-agent | my-user-agent |
+---------+----------------+---------------------------+
| 40 | x-my-header | first |
+---------+----------------+---------------------------+
| 41 | x-my-header | second | added header
+---------+----------------+---------------------------+
All the headers in this second header set are indexed in the header
table, therefore, all are kept in the reference set of headers, which
becomes:
Reference Set:
:path, /my-example/resources/script.js
user-agent, my-user-agent
x-my-header, second
Authors' Addresses
Roberto Peon
Google, Inc
EMail: fenix@google.com
Herve Ruellan
Canon CRF
EMail: herve.ruellan@crf.canon.fr
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