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HTTPbis Working Group                                            R. Peon
Internet-Draft                                               Google, Inc
Intended status: Informational                                H. Ruellan
Expires: February 28, 2014                                     Canon CRF
                                                         August 27, 2013


                HPACK - Header Compression for HTTP/2.0
                draft-ietf-httpbis-header-compression-03

Abstract

   This document describes HPACK, a format adapted to efficiently
   represent HTTP headers in the context of HTTP/2.0.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 28, 2014.

Copyright Notice

   Copyright (c) 2013 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.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Outline  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Header Encoding  . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Encoding Concepts  . . . . . . . . . . . . . . . . . . . .  3
       3.1.1.  Encoding Context . . . . . . . . . . . . . . . . . . .  4
       3.1.2.  Header Table . . . . . . . . . . . . . . . . . . . . .  4
       3.1.3.  Reference Set  . . . . . . . . . . . . . . . . . . . .  5
       3.1.4.  Header set . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.5.  Header Representation  . . . . . . . . . . . . . . . .  6
       3.1.6.  Header Emission  . . . . . . . . . . . . . . . . . . .  6
     3.2.  Header Set Processing  . . . . . . . . . . . . . . . . . .  7
       3.2.1.  Header Representation Processing . . . . . . . . . . .  7
       3.2.2.  Reference Set Emission . . . . . . . . . . . . . . . .  7
       3.2.3.  Header Set Completion  . . . . . . . . . . . . . . . .  8
       3.2.4.  Header Table Management  . . . . . . . . . . . . . . .  8
   4.  Detailed Format  . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Low-level representations  . . . . . . . . . . . . . . . .  8
       4.1.1.  Integer representation . . . . . . . . . . . . . . . .  8
       4.1.2.  Header Name Representation . . . . . . . . . . . . . . 10
       4.1.3.  Header Value Representation  . . . . . . . . . . . . . 11
     4.2.  Indexed Header Representation  . . . . . . . . . . . . . . 11
     4.3.  Literal Header Representation  . . . . . . . . . . . . . . 11
       4.3.1.  Literal Header without Indexing  . . . . . . . . . . . 11
       4.3.2.  Literal Header with Incremental Indexing . . . . . . . 12
       4.3.3.  Literal Header with Substitution Indexing  . . . . . . 13
   5.  Parameter Negotiation  . . . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication . . . . . . . . . . . . . . . . . . . . . 17
     A.1.  Since draft-ietf-httpbis-header-compression-01 . . . . . . 17
     A.2.  Since draft-ietf-httpbis-header-compression-01 . . . . . . 17
   Appendix B.  Initial Header Tables . . . . . . . . . . . . . . . . 18
     B.1.  Requests . . . . . . . . . . . . . . . . . . . . . . . . . 18
     B.2.  Responses  . . . . . . . . . . . . . . . . . . . . . . . . 19
   Appendix C.  Example . . . . . . . . . . . . . . . . . . . . . . . 20
     C.1.  First header set . . . . . . . . . . . . . . . . . . . . . 20
     C.2.  Second header set  . . . . . . . . . . . . . . . . . . . . 22







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1.  Introduction

   This document describes HPACK, a format adapted to efficiently
   represent HTTP headers in the context of HTTP/2.0.

2.  Overview

   In HTTP/1.X, headers are sent without any form of compression.  As
   web pages have grown to include dozens to hundreds of requests, the
   redundant headers in these requests now pose a problem of measurable
   latency and unnecessary bandwidth. 1 [PERF1] 2 [PERF2]

   SPDY [SPDY] initially addressed this redundancy by compressing
   headers with Deflate, which proved very effective at eliminating the
   redundant headers.  However, that aproach exposed a security risk as
   demonstrated by the CRIME [CRIME].

   In this document, we propose a new header compressor which eliminates
   the redundant headers, is not vulnerable to the CRIME style attack,
   and which also has a bounded memory cost for use in small constrained
   environments.

2.1.  Outline

   The HTTP header encoding described in this document is based on a
   header table that map (name, value) pairs to index values.  Header
   tables are incrementally updated during the HTTP/2.0 session.

   The encoder is responsible for deciding which headers to insert as
   new entries 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.

   As two consecutive sets of headers often have headers in common, each
   set of headers is coded as a difference from the previous set of
   headers.  The goal is to only encode the changes (headers present in
   one of the set and not in the other) between the two sets of headers.

   An example illustrating the use of these different mechanisms to
   represent headers is available in Appendix C.

3.  Header Encoding

3.1.  Encoding Concepts

   The encoding and decoding of headers relies on some components and
   concepts.  The set of components used form an encoding context.



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   Header Table:  The header table (see Section 3.1.2) is a component
      used to associate headers to index values.

   Reference Set:  The reference set (see Section 3.1.3) is a component
      containing a group of headers used as a reference for the
      differential encoding of a new set of headers.

   Header Set:  A header set (see Section 3.1.4) is a group of headers
      that are encoded jointly.  A complete set of key-value pairs as
      encoded in an HTTP request or response is a header set.

   Header Representation:  A header can be represented in encoded form
      either as a literal or as an index (see Section 3.1.5).  The
      indexed representation is based on the header table.

   Header Emission:  When decoding a set of headers, some operations
      emit a header (see Section 3.1.6).  An emitted header is added to
      the set of headers that form the HTTP request or response.  Once
      emitted, a header can't be removed from the set of headers.

3.1.1.  Encoding Context

   The set of components used to encode or decode a header set form an
   encoding context: an encoding context contains a header table and a
   reference set.

   Using HTTP, messages are exchanged between a client and a server in
   both direction.  To keep the encoding of headers in each direction
   independent from the other direction, there is one encoding context
   for each direction.

   The headers contained in a PUSH_PROMISE frame sent by a server to a
   client are encoded within the same context as the headers contained
   in the HEADERS frame corresponding to a response sent from the server
   to the client.

3.1.2.  Header Table

   A header table consists of an ordered list of (name, value) pairs.
   The first entry of a header table is assigned the index 0.

   A header can be represented by an entry from the header table.
   Rather than encoding a literal value for the header field name and
   value, the encoder can select an entry from the header table.

   Literal header names MUST be translated to lowercase before encoding
   and transmission.  This enables an encoder to perform direct bitwise
   comparisons on names and values when determining if an entry already



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   exists in the header table.

   There is no need for the header table to contain duplicate entries.
   However, duplicate entries MUST NOT be treated as an error by a
   decoder.

   Initially, a header table contains a list of common headers.  Two
   initial lists of header are provided in Appendix B.  One list is for
   headers transmitted from a client to a server, the other for the
   reverse direction.

   A header table is modified by either adding a new entry at the end of
   the table, or by replacing an existing entry.

   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 on the decoder side, the header table size is
   bounded (see the SETTINGS_HEADER_TABLE_SIZE in Section 5).

   The size of an entry is the sum of its name's length in bytes (as
   defined in Section 4.1.2), of its value's length in bytes
   (Section 4.1.3) and of 32 bytes.  The 32 bytes 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 bytes.

   The size of a header table is the sum of the size of its entries.

3.1.3.  Reference Set

   A reference set is defined as an unordered set of references to
   entries of the header table.

   The initial reference set is the empty set.

   The reference set is updated during the processing of a set of
   headers.

   Using the differential encoding, a header that is not present in the
   reference set can be encoded either with an indexed representation
   (if the header is present in the header table), or with a literal
   representation (if the header is not present in the header table).

   A header that is to be removed from the reference set is encoded with
   an indexed representation.





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3.1.4.  Header set

   A header set is a group of header fields that are encoded as a whole.
   Each header field is a (name, value) pair.

   A header set is encoded using an ordered list of zero or more header
   representations.  All the header representations describing a header
   set a grouped into a header block.

3.1.5.  Header Representation

   A header can be represented either as a literal or as an index.

   Literal Representation:  A literal representation defines a new
      header.  The header name is represented either literally or as a
      reference to an entry of the header table.  The header value is
      represented literally.

      Three different literal representations are provided:

      *  A literal representation that does not add the header to the
         header table (see Section 4.3.1).

      *  A literal representation that adds the header at the end of the
         header table (see Section 4.3.2).

      *  A literal representation that uses the header to replace an
         existing entry of the header table (see Section 4.3.3).

   Indexed Representation:  The indexed representation defines a header
      as a reference in the header table (see Section 4.2).

3.1.6.  Header Emission

   The emission of header is the process of adding a header to the
   current set of headers.  Once an header is emitted, it can't be
   removed from the current set of headers.

   The concept of header emission allows a decoder to know when it can
   pass a header safely to a higher level on the receiver side.  This
   allows a decoder to be implemented in a streaming way, and as such to
   only keep in memory the header table and the reference set.  With
   such an implementation, the amount of memory used by the decoder is
   bounded, even in presence of a very large set of headers.  The
   management of memory for handling very large sets of headers can
   therefore be deferred to the application, which may be able to emit
   the header to the wire and thus free up memory quickly.




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3.2.  Header Set Processing

   The processing of an encoded header set to obtain a list of headers
   is defined in this section.  To ensure a correct decoding of a header
   set, a decoder MUST obey the following rules.

3.2.1.  Header Representation Processing

   All the header representations contained in a header block are
   processed in the order in which they are presented, as specified
   below.

   An _indexed representation_ corresponding to an entry _not present_
   in the reference set entails the following actions:

   o  The header corresponding to the entry is emitted.

   o  The entry is added to the reference set.

   An _indexed representation_ corresponding to an entry _present_ in
   the reference set entails the following actions:

   o  The entry is removed from the reference set.

   A _literal representation_ that is _not added_ to the header table
   entails the following action:

   o  The header is emitted.

   A _literal representation_ that is _added_ to the header table
   entails the following actions:

   o  The header is emitted.

   o  The header is added to the header table, at the location defined
      by the representation.

   o  The new entry is added to the reference set.

3.2.2.  Reference Set Emission

   Once all the representations contained in a header block have been
   processed, the headers that are in common with the previous header
   set are emitted, during the reference set emission.

   For the reference set emission, each header contained in the
   reference set that has not been emitted during the processing of the
   header block is emitted.



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3.2.3.  Header Set Completion

   Once all of the header representations have been processed, and the
   remaining items in the reference set have been emitted, the header
   set is complete.

3.2.4.  Header Table Management

   The header table can be modified by either adding a new entry to it
   or by replacing an existing one.  Before doing such a modification,
   it has to be ensured that the header table size will stay lower than
   or equal to the SETTINGS_HEADER_TABLE_SIZE limit (see Section 5).  To
   achieve this, repeatedly, the first entry of the header table is
   removed, until enough space is available for the modification.

   A consequence of removing one or more entries at the beginning of the
   header table is that the remaining entries are renumbered.  The first
   entry of the header table is always associated to the index 0.

   When the modification of the header table is the replacement of an
   existing entry, the replaced entry is the one indicated in the
   literal representation before any entry is removed from the header
   table.  If the entry to be replaced is removed from the header table
   when performing the size adjustment, the replacement entry is
   inserted at the beginning of the header table.

   The addition of a new entry with a size greater than the
   SETTINGS_HEADER_TABLE_SIZE limit causes all the entries from the
   header table to be dropped and the new entry not to be added to the
   header table.  The replacement of an existing entry with a new entry
   with a size greater than the SETTINGS_HEADER_TABLE_SIZE has the same
   consequences.

4.  Detailed Format

4.1.  Low-level representations

4.1.1.  Integer representation

   Integers are used to represent name indexes, pair indexes or string
   lengths.  To allow for optimized processing, an integer
   representation always finishes at the end of 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.




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   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:

   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

4.1.1.1.  Example 1: Encoding 10 using a 5-bit prefix

   The value 10 is to be encoded with a 5-bit prefix.

   o  10 is less than 31 (= 2^5 - 1) and is represented using the 5-bit
      prefix.


     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | X | X | X | 0 | 1 | 0 | 1 | 0 |   10 stored on 5 bits
   +---+---+---+---+---+---+---+---+

4.1.1.2.  Example 2: Encoding 1337 using a 5-bit prefix

   The value I=1337 is to be encoded with a 5-bit prefix.

      1337 is greater than 31 (= 2^5 - 1).



         The 5-bit prefix is filled with its max value (31).

      I = 1337 - (2^5 - 1) = 1306.



         I (1306) is greater than or equal to 128, the while loop body
         executes:





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            I % 128 == 26

            26 + 128 == 154

            154 is encoded in 8 bits as: 10011010

            I is set to 10 (1306 / 128 == 10)

            I is no longer greater than or equal to 128, the while loop
            terminates.

         I, now 10, is encoded on 8 bits as: 00001010

      The process ends.


  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| X | X | X | 1 | 1 | 1 | 1 | 1 |   Prefix = 31, I = 1306
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 |   1306>=128, encode(154), I = 1306/128
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 |   10<128, encode(10), done
+---+---+---+---+---+---+---+---+

4.1.2.  Header Name Representation

   Header names are sequences of ASCII characters that MUST conform to
   the following header-name ABNF construction:

     LOWERALPHA = %x61-7A
     header-char = "!" / "#" / "$" / "%" / "&" / "'" /
                   "*" / "+" / "-" / "." / "^" / "_" /
                   "`" / "|" / "~" / DIGIT / LOWERALPHA
     header-name = [":"] 1*header-char

   They are encoded in two parts:

   1.  The length of the text, defined as the number of octets of
       storage required to store the text, represented as a variable-
       length-quantity (Section 4.1.1).

   2.  The specific sequence of ASCII octets








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4.1.3.  Header Value Representation

   Header values are encoded as sequences of UTF-8 encoded text.  They
   are encoded in two parts:

   1.  The length of the text, defined as the number of octets of
       storage required to store the text, represented as a variable-
       length-quantity (Section 4.1.1).

   2.  The specific sequence of octets representing the UTF-8 text.

   Invalid UTF-8 octet sequences, "over-long" UTF-8 encodings, and UTF-8
   octets that represent invalid Unicode Codepoints MUST NOT be used.

4.2.  Indexed Header Representation

   An indexed header representation identifies an entry in the header
   table.  The entry is emitted and added to the reference set if it is
   not currently in the reference set.  The entry is removed from the
   reference set if it is present in the reference set.

     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.

4.3.  Literal Header Representation

   Literal header representations contain a literal header field value.
   Header field names are either provided as a literal or by reference
   to an existing header table entry.

   Literal representations all result in the emission of a header when
   decoded.

4.3.1.  Literal Header without Indexing

   An literal header without indexing causes the emission of a header
   without altering the header table.






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     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 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 (Section 4.1.2).

   Header name representation is followed by the header value
   represented as a literal string as described in Section 4.1.3.

4.3.2.  Literal Header with Incremental Indexing

   A literal header with incremental indexing adds a new entry to the
   header table.







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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 (Section 4.1.2).

   Header name representation is followed by the header value
   represented as a literal string as described in Section 4.1.3.

4.3.3.  Literal Header with Substitution Indexing

   A literal header with substitution indexing replaces an existing
   header table entry.





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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 |      Index (6+)       |
   +---+---+-----------------------+
   |    Substituted Index (8+)     |
   +-------------------------------+
   |       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 63, 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 (Section 4.1.2).

   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



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   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 4.1.3.

5.  Parameter Negotiation

   A few parameters can be used to accommodate client and server
   processing and memory requirements. [[anchor3: 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_HEADER_TABLE_SIZE (TBD):  Allows the sender to inform the
      remote endpoint of the maximum size it accepts for the header
      table.
      The default value is 4096 bytes.
      [[anchor4: 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_HEADER_TABLE_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.
      [[anchor5: 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

   This compressor exists to solve security issues present in stream
   compressors such as DEFLATE whereby the compression context can be
   efficiently probed to reveal secrets.  A conformant implementation of
   this specification should be fairly safe against that kind of attack,
   as the reaping of any information from the compression context
   requires more work than guessing and verifying the plaintext data
   directly with the server.  As with any secret, however, the longer
   the length of the secret, the more difficult the secret is to guess.
   It is inadvisable to have short cookies that are relied upon to
   remain secret for any duration of time.

   A proper security-conscious implementation will also need to prevent



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   timing attacks by ensuring that the amount of time it takes to do
   string comparisons is always a function of the total length of the
   strings, and not a function of the number of matched characters.

   Another common security problem is when the remote endpoint
   successfully causes the local endpoint to exhaust its memory.  This
   compressor attempts to deal with the most obvious ways that this
   could occur by limiting both the peak and the steady-state amount of
   memory consumed in the compressor state, by providing ways for the
   application to consume/flush the emitted headers in small chunks, and
   by considering overhead in the state size calculation.  Implementors
   must still be careful in the creation of APIs to an implementation of
   this compressor by ensuring that header keys and values are either
   emitted as a stream, or that the compression implementation have a
   limit on the maximum size of a key or value.  Failure to implement
   these kinds of safeguards may still result in a scenario where the
   local endpoint exhausts its memory.

7.  IANA Considerations

   This document registers the SETTINGS_HEADER_TABLE_SIZE setting in the
   "HTTP/2.0 Settings" registry established by [HTTP2].  The assigned
   code for this setting is TBD.

8.  References

8.1.  Normative References

   [HTTP2]  Belshe, M., Peon, R., Thomson, M., and A. Melnikov,
            "Hypertext Transfer Protocol version 2.0",
            draft-ietf-httpbis-http2-06 (work in progress),
            February 2013.

8.2.  Informative References

   [CRIME]  Rizzo, J. and T. Duong, "The Crime Attack", September 2012,
            <https://docs.google.com/a/twist.com/presentation/d/
            11eBmGiHbYcHR9gL5nDyZChu_-lCa2GizeuOfaLU2HOU/
            edit#slide=id.g1eb6c1b5_3_6>.

   [PERF1]  Belshe, M., "IETF83: SPDY and What to Consider for
            HTTP/2.0", March 2012, <http://www.ietf.org/proceedings/83/
            slides/slides-83-httpbis-3>.

   [PERF2]  McManus, P., "SPDY What I Like About You", September 2011, <
            http://bitsup.blogspot.com/2011/09/
            spdy-what-i-like-about-you.html>.




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   [SPDY]   Belshe, M. and R. Peon, "SPDY Protocol", February 2012,
            <http://tools.ietf.org/html/draft-mbelshe-httpbis-spdy>.

URIs

   [1]  <http://en.wikipedia.org/wiki/Variable-length_quantity>

Appendix A.  Change Log (to be removed by RFC Editor before publication

A.1.  Since draft-ietf-httpbis-header-compression-01

   o  Refactored of Header Encoding Section: split definitions and
      processing rule.

   o  Backward incompatible change: Updated reference set management as
      per issue #214.  This changes how the interaction between the
      reference set and eviction works.  This also changes the working
      of the reference set in some specific cases.

   o  Backward incompatible change: modified initial header list, as per
      issue #188.

   o  Added example of 32 bytes entry structure (issue #191).

   o  Added Header Set Completion section.  Reflowed some text.
      Clarified some writing which was akward.  Added text about
      duplicate header entry encoding.  Clarified some language w.r.t
      Header Set. Changed x-my-header to mynewheader.  Added text in the
      HeaderEmission section indicating that the application may also be
      able to free up memory more quickly.  Added information in
      Security Considerations section.

A.2.  Since draft-ietf-httpbis-header-compression-01

      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" section





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      Corrected erroneous statement about what index can be contained in
      one byte

      Added descriptions of opcodes

      Removed security claims from introduction.

Appendix B.  Initial Header Tables

   [[anchor11: 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).]]
   [[anchor12: 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.]]

B.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.






























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              +-------+---------------------+--------------+
              | 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    | user-agent          |              |
              | 12    | referer             |              |
              | 13    | authorization       |              |
              | 14    | allow               |              |
              | 15    | cache-control       |              |
              | 16    | connection          |              |
              | 17    | content-length      |              |
              | 18    | content-type        |              |
              | 19    | date                |              |
              | 20    | expect              |              |
              | 21    | from                |              |
              | 22    | if-match            |              |
              | 23    | if-none-match       |              |
              | 24    | if-range            |              |
              | 25    | if-unmodified-since |              |
              | 26    | max-forwards        |              |
              | 27    | proxy-authorization |              |
              | 28    | range               |              |
              | 29    | via                 |              |
              +-------+---------------------+--------------+

                Table 1: Initial Header Table for Requests

B.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.








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          +-------+-----------------------------+--------------+
          | 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-range               |              |
          | 22    | link                        |              |
          | 23    | location                    |              |
          | 24    | proxy-authenticate          |              |
          | 25    | refresh                     |              |
          | 26    | retry-after                 |              |
          | 27    | strict-transport-security   |              |
          | 28    | transfer-encoding           |              |
          | 29    | www-authenticate            |              |
          +-------+-----------------------------+--------------+

                Table 2: Initial Header Table for Responses

Appendix C.  Example

   Here is an example that illustrates different representations and how
   tables are updated. [[anchor15: This section needs to be updated to
   better reflect the new processing of header fields, and include more
   examples.]]

C.1.  First header set

   The first header set to represent is the following:




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   :path: /my-example/index.html
   user-agent: my-user-agent
   mynewheader: first

   The header table is empty, all headers are represented as literal
   headers with indexing.  The 'mynewheader' 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)
   mynewheader
   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        |                           |
   +---------+----------------+---------------------------+
   |   38    | :path          | /my-example/index.html    | added header
   +---------+----------------+---------------------------+
   |   39    | user-agent     | my-user-agent             | added header
   +---------+----------------+---------------------------+
   |   40    | mynewheader    | 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



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   mynewheader, first

C.2.  Second header set

   The second header set to represent is the following:

   :path: /my-example/resources/script.js
   user-agent: my-user-agent
   mynewheader: 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
   mynewheader 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    | mynewheader    | first                     |
   +---------+----------------+---------------------------+
   |   41    | mynewheader    | 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
   mynewheader, 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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