





Internet Engineering Task Force (IETF)                           R. Peon
Request for Comments: 7541                                   Google, Inc
Category: Standards Track                                     H. Ruellan
ISSN: 2070-1721                                                Canon CRF
                                                                May 2015


                  HPACK: Header Compression for HTTP/2

Abstract

   This specification defines HPACK, a compression format for
   efficiently representing HTTP header fields, to be used in HTTP/2.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7541.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.









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Table of Contents

   1. Introduction ....................................................4
      1.1. Overview ...................................................4
      1.2. Conventions ................................................5
      1.3. Terminology ................................................5
   2. Compression Process Overview ....................................6
      2.1. Header List Ordering .......................................6
      2.2. Encoding and Decoding Contexts .............................6
      2.3. Indexing Tables ............................................6
           2.3.1. Static Table ........................................6
           2.3.2. Dynamic Table .......................................6
           2.3.3. Index Address Space .................................7
      2.4. Header Field Representation ................................8
   3. Header Block Decoding ...........................................8
      3.1. Header Block Processing ....................................8
      3.2. Header Field Representation Processing .....................9
   4. Dynamic Table Management ........................................9
      4.1. Calculating Table Size ....................................10
      4.2. Maximum Table Size ........................................10
      4.3. Entry Eviction When Dynamic Table Size Changes ............11
      4.4. Entry Eviction When Adding New Entries ....................11
   5. Primitive Type Representations .................................11
      5.1. Integer Representation ....................................11
      5.2. String Literal Representation .............................13
   6. Binary Format ..................................................14
      6.1. Indexed Header Field Representation .......................14
      6.2. Literal Header Field Representation .......................15
           6.2.1. Literal Header Field with Incremental Indexing .....15
           6.2.2. Literal Header Field without Indexing ..............16
           6.2.3. Literal Header Field Never Indexed .................17
      6.3. Dynamic Table Size Update .................................18
   7. Security Considerations ........................................19
      7.1. Probing Dynamic Table State ...............................19
           7.1.1. Applicability to HPACK and HTTP ....................20
           7.1.2. Mitigation .........................................20
           7.1.3. Never-Indexed Literals .............................21
      7.2. Static Huffman Encoding ...................................22
      7.3. Memory Consumption ........................................22
      7.4. Implementation Limits .....................................23
   8. References .....................................................23
      8.1. Normative References ......................................23
      8.2. Informative References ....................................24
   Appendix A. Static Table Definition ...............................25
   Appendix B. Huffman Code ..........................................27






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   Appendix C. Examples ..............................................33
     C.1. Integer Representation Examples ............................33
       C.1.1. Example 1: Encoding 10 Using a 5-Bit Prefix ............33
       C.1.2. Example 2: Encoding 1337 Using a 5-Bit Prefix ..........33
       C.1.3. Example 3: Encoding 42 Starting at an Octet Boundary ...34
     C.2. Header Field Representation Examples .......................34
       C.2.1. Literal Header Field with Indexing .....................34
       C.2.2. Literal Header Field without Indexing ..................35
       C.2.3. Literal Header Field Never Indexed .....................36
       C.2.4. Indexed Header Field ...................................37
     C.3. Request Examples without Huffman Coding ....................37
       C.3.1. First Request ..........................................37
       C.3.2. Second Request .........................................38
       C.3.3. Third Request ..........................................39
     C.4. Request Examples with Huffman Coding .......................41
       C.4.1. First Request ..........................................41
       C.4.2. Second Request .........................................42
       C.4.3. Third Request ..........................................43
     C.5. Response Examples without Huffman Coding ...................45
       C.5.1. First Response .........................................45
       C.5.2. Second Response ........................................46
       C.5.3. Third Response .........................................47
     C.6. Response Examples with Huffman Coding ......................49
       C.6.1. First Response .........................................49
       C.6.2. Second Response ........................................51
       C.6.3. Third Response .........................................52
   Acknowledgments ...................................................55
   Authors' Addresses ................................................55























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1.  Introduction

   In HTTP/1.1 (see [RFC7230]), header fields are not compressed.  As
   web pages have grown to require dozens to hundreds of requests, the
   redundant header fields in these requests unnecessarily consume
   bandwidth, measurably increasing latency.

   SPDY [SPDY] initially addressed this redundancy by compressing header
   fields using the DEFLATE [DEFLATE] format, which proved very
   effective at efficiently representing the redundant header fields.
   However, that approach exposed a security risk as demonstrated by the
   CRIME (Compression Ratio Info-leak Made Easy) attack (see [CRIME]).

   This specification defines HPACK, a new compressor that eliminates
   redundant header fields, limits vulnerability to known security
   attacks, and has a bounded memory requirement for use in constrained
   environments.  Potential security concerns for HPACK are described in
   Section 7.

   The HPACK format is intentionally simple and inflexible.  Both
   characteristics reduce the risk of interoperability or security
   issues due to implementation error.  No extensibility mechanisms are
   defined; changes to the format are only possible by defining a
   complete replacement.

1.1.  Overview

   The format defined in this specification treats a list of header
   fields as an ordered collection of name-value pairs that can include
   duplicate pairs.  Names and values are considered to be opaque
   sequences of octets, and the order of header fields is preserved
   after being compressed and decompressed.

   Encoding is informed by header field tables that map header fields to
   indexed values.  These header field tables can be incrementally
   updated as new header fields are encoded or decoded.

   In the encoded form, a header field is represented either literally
   or as a reference to a header field in one of the header field
   tables.  Therefore, a list of header fields can be encoded using a
   mixture of references and literal values.

   Literal values are either encoded directly or use a static Huffman
   code.

   The encoder is responsible for deciding which header fields to insert
   as new entries in the header field tables.  The decoder executes the
   modifications to the header field tables prescribed by the encoder,



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   reconstructing the list of header fields in the process.  This
   enables decoders to remain simple and interoperate with a wide
   variety of encoders.

   Examples illustrating the use of these different mechanisms to
   represent header fields are available in Appendix C.

1.2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   All numeric values are in network byte order.  Values are unsigned
   unless otherwise indicated.  Literal values are provided in decimal
   or hexadecimal as appropriate.

1.3.  Terminology

   This specification uses the following terms:

   Header Field:  A name-value pair.  Both the name and value are
      treated as opaque sequences of octets.

   Dynamic Table:  The dynamic table (see Section 2.3.2) is a table that
      associates stored header fields with index values.  This table is
      dynamic and specific to an encoding or decoding context.

   Static Table:  The static table (see Section 2.3.1) is a table that
      statically associates header fields that occur frequently with
      index values.  This table is ordered, read-only, always
      accessible, and it may be shared amongst all encoding or decoding
      contexts.

   Header List:  A header list is an ordered collection of header fields
      that are encoded jointly and can contain duplicate header fields.
      A complete list of header fields contained in an HTTP/2 header
      block is a header list.

   Header Field Representation:  A header field can be represented in
      encoded form either as a literal or as an index (see Section 2.4).

   Header Block:  An ordered list of header field representations,
      which, when decoded, yields a complete header list.







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2.  Compression Process Overview

   This specification does not describe a specific algorithm for an
   encoder.  Instead, it defines precisely how a decoder is expected to
   operate, allowing encoders to produce any encoding that this
   definition permits.

2.1.  Header List Ordering

   HPACK preserves the ordering of header fields inside the header list.
   An encoder MUST order header field representations in the header
   block according to their ordering in the original header list.  A
   decoder MUST order header fields in the decoded header list according
   to their ordering in the header block.

2.2.  Encoding and Decoding Contexts

   To decompress header blocks, a decoder only needs to maintain a
   dynamic table (see Section 2.3.2) as a decoding context.  No other
   dynamic state is needed.

   When used for bidirectional communication, such as in HTTP, the
   encoding and decoding dynamic tables maintained by an endpoint are
   completely independent, i.e., the request and response dynamic tables
   are separate.

2.3.  Indexing Tables

   HPACK uses two tables for associating header fields to indexes.  The
   static table (see Section 2.3.1) is predefined and contains common
   header fields (most of them with an empty value).  The dynamic table
   (see Section 2.3.2) is dynamic and can be used by the encoder to
   index header fields repeated in the encoded header lists.

   These two tables are combined into a single address space for
   defining index values (see Section 2.3.3).

2.3.1.  Static Table

   The static table consists of a predefined static list of header
   fields.  Its entries are defined in Appendix A.

2.3.2.  Dynamic Table

   The dynamic table consists of a list of header fields maintained in
   first-in, first-out order.  The first and newest entry in a dynamic
   table is at the lowest index, and the oldest entry of a dynamic table
   is at the highest index.



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   The dynamic table is initially empty.  Entries are added as each
   header block is decompressed.

   The dynamic table can contain duplicate entries (i.e., entries with
   the same name and same value).  Therefore, duplicate entries MUST NOT
   be treated as an error by a decoder.

   The encoder decides how to update the dynamic table and as such can
   control how much memory is used by the dynamic table.  To limit the
   memory requirements of the decoder, the dynamic table size is
   strictly bounded (see Section 4.2).

   The decoder updates the dynamic table during the processing of a list
   of header field representations (see Section 3.2).

2.3.3.  Index Address Space

   The static table and the dynamic table are combined into a single
   index address space.

   Indices between 1 and the length of the static table (inclusive)
   refer to elements in the static table (see Section 2.3.1).

   Indices strictly greater than the length of the static table refer to
   elements in the dynamic table (see Section 2.3.2).  The length of the
   static table is subtracted to find the index into the dynamic table.

   Indices strictly greater than the sum of the lengths of both tables
   MUST be treated as a decoding error.

   For a static table size of s and a dynamic table size of k, the
   following diagram shows the entire valid index address space.

           <----------  Index Address Space ---------->
           <-- Static  Table -->  <-- Dynamic Table -->
           +---+-----------+---+  +---+-----------+---+
           | 1 |    ...    | s |  |s+1|    ...    |s+k|
           +---+-----------+---+  +---+-----------+---+
                                  ^                   |
                                  |                   V
                           Insertion Point      Dropping Point

                       Figure 1: Index Address Space








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2.4.  Header Field Representation

   An encoded header field can be represented either as an index or as a
   literal.

   An indexed representation defines a header field as a reference to an
   entry in either the static table or the dynamic table (see
   Section 6.1).

   A literal representation defines a header field by specifying its
   name and value.  The header field name can be represented literally
   or as a reference to an entry in either the static table or the
   dynamic table.  The header field value is represented literally.

   Three different literal representations are defined:

   o  A literal representation that adds the header field as a new entry
      at the beginning of the dynamic table (see Section 6.2.1).

   o  A literal representation that does not add the header field to the
      dynamic table (see Section 6.2.2).

   o  A literal representation that does not add the header field to the
      dynamic table, with the additional stipulation that this header
      field always use a literal representation, in particular when re-
      encoded by an intermediary (see Section 6.2.3).  This
      representation is intended for protecting header field values that
      are not to be put at risk by compressing them (see Section 7.1.3
      for more details).

   The selection of one of these literal representations can be guided
   by security considerations, in order to protect sensitive header
   field values (see Section 7.1).

   The literal representation of a header field name or of a header
   field value can encode the sequence of octets either directly or
   using a static Huffman code (see Section 5.2).

3.  Header Block Decoding

3.1.  Header Block Processing

   A decoder processes a header block sequentially to reconstruct the
   original header list.

   A header block is the concatenation of header field representations.
   The different possible header field representations are described in
   Section 6.



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   Once a header field is decoded and added to the reconstructed header
   list, the header field cannot be removed.  A header field added to
   the header list can be safely passed to the application.

   By passing the resulting header fields to the application, a decoder
   can be implemented with minimal transitory memory commitment in
   addition to the memory required for the dynamic table.

3.2.  Header Field Representation Processing

   The processing of a header block to obtain a header list is defined
   in this section.  To ensure that the decoding will successfully
   produce a header list, a decoder MUST obey the following rules.

   All the header field representations contained in a header block are
   processed in the order in which they appear, as specified below.
   Details on the formatting of the various header field representations
   and some additional processing instructions are found in Section 6.

   An _indexed representation_ entails the following actions:

   o  The header field corresponding to the referenced entry in either
      the static table or dynamic table is appended to the decoded
      header list.

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

   o  The header field is appended to the decoded header list.

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

   o  The header field is appended to the decoded header list.

   o  The header field is inserted at the beginning of the dynamic
      table.  This insertion could result in the eviction of previous
      entries in the dynamic table (see Section 4.4).

4.  Dynamic Table Management

   To limit the memory requirements on the decoder side, the dynamic
   table is constrained in size.








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4.1.  Calculating Table Size

   The size of the dynamic table is the sum of the size of its entries.

   The size of an entry is the sum of its name's length in octets (as
   defined in Section 5.2), its value's length in octets, and 32.

   The size of an entry is calculated using the length of its name and
   value without any Huffman encoding applied.

      Note: The additional 32 octets account for an estimated overhead
      associated with an entry.  For example, an entry structure using
      two 64-bit pointers to reference the name and the value of the
      entry and two 64-bit integers for counting the number of
      references to the name and value would have 32 octets of overhead.

4.2.  Maximum Table Size

   Protocols that use HPACK determine the maximum size that the encoder
   is permitted to use for the dynamic table.  In HTTP/2, this value is
   determined by the SETTINGS_HEADER_TABLE_SIZE setting (see
   Section 6.5.2 of [HTTP2]).

   An encoder can choose to use less capacity than this maximum size
   (see Section 6.3), but the chosen size MUST stay lower than or equal
   to the maximum set by the protocol.

   A change in the maximum size of the dynamic table is signaled via a
   dynamic table size update (see Section 6.3).  This dynamic table size
   update MUST occur at the beginning of the first header block
   following the change to the dynamic table size.  In HTTP/2, this
   follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]).

   Multiple updates to the maximum table size can occur between the
   transmission of two header blocks.  In the case that this size is
   changed more than once in this interval, the smallest maximum table
   size that occurs in that interval MUST be signaled in a dynamic table
   size update.  The final maximum size is always signaled, resulting in
   at most two dynamic table size updates.  This ensures that the
   decoder is able to perform eviction based on reductions in dynamic
   table size (see Section 4.3).

   This mechanism can be used to completely clear entries from the
   dynamic table by setting a maximum size of 0, which can subsequently
   be restored.






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4.3.  Entry Eviction When Dynamic Table Size Changes

   Whenever the maximum size for the dynamic table is reduced, entries
   are evicted from the end of the dynamic table until the size of the
   dynamic table is less than or equal to the maximum size.

4.4.  Entry Eviction When Adding New Entries

   Before a new entry is added to the dynamic table, entries are evicted
   from the end of the dynamic table until the size of the dynamic table
   is less than or equal to (maximum size - new entry size) or until the
   table is empty.

   If the size of the new entry is less than or equal to the maximum
   size, that entry is added to the table.  It is not an error to
   attempt to add an entry that is larger than the maximum size; an
   attempt to add an entry larger than the maximum size causes the table
   to be emptied of all existing entries and results in an empty table.

   A new entry can reference the name of an entry in the dynamic table
   that will be evicted when adding this new entry into the dynamic
   table.  Implementations are cautioned to avoid deleting the
   referenced name if the referenced entry is evicted from the dynamic
   table prior to inserting the new entry.

5.  Primitive Type Representations

   HPACK encoding uses two primitive types: unsigned variable-length
   integers and strings of octets.

5.1.  Integer Representation

   Integers are used to represent name indexes, header field indexes, or
   string lengths.  An integer representation can start anywhere within
   an octet.  To allow for optimized processing, an integer
   representation always finishes at the end of an octet.

   An integer is represented in two parts: a prefix that fills the
   current octet and an optional list of octets that are used if the
   integer value does not fit within the prefix.  The number of bits of
   the prefix (called N) is a parameter of the integer representation.

   If the integer value is small enough, i.e., strictly less than 2^N-1,
   it is encoded within the N-bit prefix.







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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | ? | ? | ? |       Value       |
   +---+---+---+-------------------+

    Figure 2: Integer Value Encoded within the Prefix (Shown for N = 5)

   Otherwise, all the bits of the prefix are set to 1, and the value,
   decreased by 2^N-1, is encoded using a list of one or more octets.
   The most significant bit of each octet is used as a continuation
   flag: its value is set to 1 except for the last octet in the list.
   The remaining bits of the octets are used to encode the decreased
   value.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | ? | ? | ? | 1   1   1   1   1 |
   +---+---+---+-------------------+
   | 1 |    Value-(2^N-1) LSB      |
   +---+---------------------------+
                  ...
   +---+---------------------------+
   | 0 |    Value-(2^N-1) MSB      |
   +---+---------------------------+

    Figure 3: Integer Value Encoded after the Prefix (Shown for N = 5)

   Decoding the integer value from the list of octets starts by
   reversing the order of the octets in the list.  Then, for each octet,
   its most significant bit is removed.  The remaining bits of the
   octets are concatenated, and the resulting value is increased by
   2^N-1 to obtain the integer value.

   The prefix size, N, is always between 1 and 8 bits.  An integer
   starting at an octet boundary will have an 8-bit prefix.

   Pseudocode to represent an integer I is as follows:

   if I < 2^N - 1, encode I on N bits
   else
       encode (2^N - 1) on N bits
       I = I - (2^N - 1)
       while I >= 128
            encode (I % 128 + 128) on 8 bits
            I = I / 128
       encode I on 8 bits





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   Pseudocode to decode an integer I is as follows:

   decode I from the next N bits
   if I < 2^N - 1, return I
   else
       M = 0
       repeat
           B = next octet
           I = I + (B & 127) * 2^M
           M = M + 7
       while B & 128 == 128
       return I

   Examples illustrating the encoding of integers are available in
   Appendix C.1.

   This integer representation allows for values of indefinite size.  It
   is also possible for an encoder to send a large number of zero
   values, which can waste octets and could be used to overflow integer
   values.  Integer encodings that exceed implementation limits -- in
   value or octet length -- MUST be treated as decoding errors.
   Different limits can be set for each of the different uses of
   integers, based on implementation constraints.

5.2.  String Literal Representation

   Header field names and header field values can be represented as
   string literals.  A string literal is encoded as a sequence of
   octets, either by directly encoding the string literal's octets or by
   using a Huffman code (see [HUFFMAN]).

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | H |    String Length (7+)     |
   +---+---------------------------+
   |  String Data (Length octets)  |
   +-------------------------------+

                  Figure 4: String Literal Representation

   A string literal representation contains the following fields:

   H: A one-bit flag, H, indicating whether or not the octets of the
      string are Huffman encoded.

   String Length:  The number of octets used to encode the string
      literal, encoded as an integer with a 7-bit prefix (see
      Section 5.1).



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   String Data:  The encoded data of the string literal.  If H is '0',
      then the encoded data is the raw octets of the string literal.  If
      H is '1', then the encoded data is the Huffman encoding of the
      string literal.

   String literals that use Huffman encoding are encoded with the
   Huffman code defined in Appendix B (see examples for requests in
   Appendix C.4 and for responses in Appendix C.6).  The encoded data is
   the bitwise concatenation of the codes corresponding to each octet of
   the string literal.

   As the Huffman-encoded data doesn't always end at an octet boundary,
   some padding is inserted after it, up to the next octet boundary.  To
   prevent this padding from being misinterpreted as part of the string
   literal, the most significant bits of the code corresponding to the
   EOS (end-of-string) symbol are used.

   Upon decoding, an incomplete code at the end of the encoded data is
   to be considered as padding and discarded.  A padding strictly longer
   than 7 bits MUST be treated as a decoding error.  A padding not
   corresponding to the most significant bits of the code for the EOS
   symbol MUST be treated as a decoding error.  A Huffman-encoded string
   literal containing the EOS symbol MUST be treated as a decoding
   error.

6.  Binary Format

   This section describes the detailed format of each of the different
   header field representations and the dynamic table size update
   instruction.

6.1.  Indexed Header Field Representation

   An indexed header field representation identifies an entry in either
   the static table or the dynamic table (see Section 2.3).

   An indexed header field representation causes a header field to be
   added to the decoded header list, as described in Section 3.2.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 |        Index (7+)         |
   +---+---------------------------+

                      Figure 5: Indexed Header Field






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   An indexed header field starts with the '1' 1-bit pattern, followed
   by the index of the matching header field, represented as an integer
   with a 7-bit prefix (see Section 5.1).

   The index value of 0 is not used.  It MUST be treated as a decoding
   error if found in an indexed header field representation.

6.2.  Literal Header Field Representation

   A literal header field representation contains a literal header field
   value.  Header field names are provided either as a literal or by
   reference to an existing table entry, either from the static table or
   the dynamic table (see Section 2.3).

   This specification defines three forms of literal header field
   representations: with indexing, without indexing, and never indexed.

6.2.1.  Literal Header Field with Incremental Indexing

   A literal header field with incremental indexing representation
   results in appending a header field to the decoded header list and
   inserting it as a new entry into the dynamic table.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 |      Index (6+)       |
   +---+---+-----------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

    Figure 6: Literal Header Field with Incremental Indexing -- Indexed
                                   Name

















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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 |           0           |
   +---+---+-----------------------+
   | H |     Name Length (7+)      |
   +---+---------------------------+
   |  Name String (Length octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

   Figure 7: Literal Header Field with Incremental Indexing -- New Name

   A literal header field with incremental indexing representation
   starts with the '01' 2-bit pattern.

   If the header field name matches the header field name of an entry
   stored in the static table or the dynamic table, the header field
   name can be represented using the index of that entry.  In this case,
   the index of the entry is represented as an integer with a 6-bit
   prefix (see Section 5.1).  This value is always non-zero.

   Otherwise, the header field name is represented as a string literal
   (see Section 5.2).  A value 0 is used in place of the 6-bit index,
   followed by the header field name.

   Either form of header field name representation is followed by the
   header field value represented as a string literal (see Section 5.2).

6.2.2.  Literal Header Field without Indexing

   A literal header field without indexing representation results in
   appending a header field to the decoded header list without altering
   the dynamic table.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 | 0 |  Index (4+)   |
   +---+---+-----------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

      Figure 8: Literal Header Field without Indexing -- Indexed Name




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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 | 0 |       0       |
   +---+---+-----------------------+
   | H |     Name Length (7+)      |
   +---+---------------------------+
   |  Name String (Length octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

        Figure 9: Literal Header Field without Indexing -- New Name

   A literal header field without indexing representation starts with
   the '0000' 4-bit pattern.

   If the header field name matches the header field name of an entry
   stored in the static table or the dynamic table, the header field
   name can be represented using the index of that entry.  In this case,
   the index of the entry is represented as an integer with a 4-bit
   prefix (see Section 5.1).  This value is always non-zero.

   Otherwise, the header field name is represented as a string literal
   (see Section 5.2).  A value 0 is used in place of the 4-bit index,
   followed by the header field name.

   Either form of header field name representation is followed by the
   header field value represented as a string literal (see Section 5.2).

6.2.3.  Literal Header Field Never Indexed

   A literal header field never-indexed representation results in
   appending a header field to the decoded header list without altering
   the dynamic table.  Intermediaries MUST use the same representation
   for encoding this header field.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 | 1 |  Index (4+)   |
   +---+---+-----------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

       Figure 10: Literal Header Field Never Indexed -- Indexed Name



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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 | 1 |       0       |
   +---+---+-----------------------+
   | H |     Name Length (7+)      |
   +---+---------------------------+
   |  Name String (Length octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

         Figure 11: Literal Header Field Never Indexed -- New Name

   A literal header field never-indexed representation starts with the
   '0001' 4-bit pattern.

   When a header field is represented as a literal header field never
   indexed, it MUST always be encoded with this specific literal
   representation.  In particular, when a peer sends a header field that
   it received represented as a literal header field never indexed, it
   MUST use the same representation to forward this header field.

   This representation is intended for protecting header field values
   that are not to be put at risk by compressing them (see Section 7.1
   for more details).

   The encoding of the representation is identical to the literal header
   field without indexing (see Section 6.2.2).

6.3.  Dynamic Table Size Update

   A dynamic table size update signals a change to the size of the
   dynamic table.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 1 |   Max size (5+)   |
   +---+---------------------------+

               Figure 12: Maximum Dynamic Table Size Change

   A dynamic table size update starts with the '001' 3-bit pattern,
   followed by the new maximum size, represented as an integer with a
   5-bit prefix (see Section 5.1).





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   The new maximum size MUST be lower than or equal to the limit
   determined by the protocol using HPACK.  A value that exceeds this
   limit MUST be treated as a decoding error.  In HTTP/2, this limit is
   the last value of the SETTINGS_HEADER_TABLE_SIZE parameter (see
   Section 6.5.2 of [HTTP2]) received from the decoder and acknowledged
   by the encoder (see Section 6.5.3 of [HTTP2]).

   Reducing the maximum size of the dynamic table can cause entries to
   be evicted (see Section 4.3).

7.  Security Considerations

   This section describes potential areas of security concern with
   HPACK:

   o  Use of compression as a length-based oracle for verifying guesses
      about secrets that are compressed into a shared compression
      context.

   o  Denial of service resulting from exhausting processing or memory
      capacity at a decoder.

7.1.  Probing Dynamic Table State

   HPACK reduces the length of header field encodings by exploiting the
   redundancy inherent in protocols like HTTP.  The ultimate goal of
   this is to reduce the amount of data that is required to send HTTP
   requests or responses.

   The compression context used to encode header fields can be probed by
   an attacker who can both define header fields to be encoded and
   transmitted and observe the length of those fields once they are
   encoded.  When an attacker can do both, they can adaptively modify
   requests in order to confirm guesses about the dynamic table state.
   If a guess is compressed into a shorter length, the attacker can
   observe the encoded length and infer that the guess was correct.

   This is possible even over the Transport Layer Security (TLS)
   protocol (see [TLS12]), because while TLS provides confidentiality
   protection for content, it only provides a limited amount of
   protection for the length of that content.

      Note: Padding schemes only provide limited protection against an
      attacker with these capabilities, potentially only forcing an
      increased number of guesses to learn the length associated with a
      given guess.  Padding schemes also work directly against
      compression by increasing the number of bits that are transmitted.




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   Attacks like CRIME [CRIME] demonstrated the existence of these
   general attacker capabilities.  The specific attack exploited the
   fact that DEFLATE [DEFLATE] removes redundancy based on prefix
   matching.  This permitted the attacker to confirm guesses a character
   at a time, reducing an exponential-time attack into a linear-time
   attack.

7.1.1.  Applicability to HPACK and HTTP

   HPACK mitigates but does not completely prevent attacks modeled on
   CRIME [CRIME] by forcing a guess to match an entire header field
   value rather than individual characters.  Attackers can only learn
   whether a guess is correct or not, so they are reduced to brute-force
   guesses for the header field values.

   The viability of recovering specific header field values therefore
   depends on the entropy of values.  As a result, values with high
   entropy are unlikely to be recovered successfully.  However, values
   with low entropy remain vulnerable.

   Attacks of this nature are possible any time that two mutually
   distrustful entities control requests or responses that are placed
   onto a single HTTP/2 connection.  If the shared HPACK compressor
   permits one entity to add entries to the dynamic table and the other
   to access those entries, then the state of the table can be learned.

   Having requests or responses from mutually distrustful entities
   occurs when an intermediary either:

   o  sends requests from multiple clients on a single connection toward
      an origin server, or

   o  takes responses from multiple origin servers and places them on a
      shared connection toward a client.

   Web browsers also need to assume that requests made on the same
   connection by different web origins [ORIGIN] are made by mutually
   distrustful entities.

7.1.2.  Mitigation

   Users of HTTP that require confidentiality for header fields can use
   values with entropy sufficient to make guessing infeasible.  However,
   this is impractical as a general solution because it forces all users
   of HTTP to take steps to mitigate attacks.  It would impose new
   constraints on how HTTP is used.





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   Rather than impose constraints on users of HTTP, an implementation of
   HPACK can instead constrain how compression is applied in order to
   limit the potential for dynamic table probing.

   An ideal solution segregates access to the dynamic table based on the
   entity that is constructing header fields.  Header field values that
   are added to the table are attributed to an entity, and only the
   entity that created a particular value can extract that value.

   To improve compression performance of this option, certain entries
   might be tagged as being public.  For example, a web browser might
   make the values of the Accept-Encoding header field available in all
   requests.

   An encoder without good knowledge of the provenance of header fields
   might instead introduce a penalty for a header field with many
   different values, such that a large number of attempts to guess a
   header field value results in the header field no longer being
   compared to the dynamic table entries in future messages, effectively
   preventing further guesses.

      Note: Simply removing entries corresponding to the header field
      from the dynamic table can be ineffectual if the attacker has a
      reliable way of causing values to be reinstalled.  For example, a
      request to load an image in a web browser typically includes the
      Cookie header field (a potentially highly valued target for this
      sort of attack), and web sites can easily force an image to be
      loaded, thereby refreshing the entry in the dynamic table.

   This response might be made inversely proportional to the length of
   the header field value.  Marking a header field as not using the
   dynamic table anymore might occur for shorter values more quickly or
   with higher probability than for longer values.

7.1.3.  Never-Indexed Literals

   Implementations can also choose to protect sensitive header fields by
   not compressing them and instead encoding their value as literals.

   Refusing to generate an indexed representation for a header field is
   only effective if compression is avoided on all hops.  The never-
   indexed literal (see Section 6.2.3) can be used to signal to
   intermediaries that a particular value was intentionally sent as a
   literal.







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   An intermediary MUST NOT re-encode a value that uses the never-
   indexed literal representation with another representation that would
   index it.  If HPACK is used for re-encoding, the never-indexed
   literal representation MUST be used.

   The choice to use a never-indexed literal representation for a header
   field depends on several factors.  Since HPACK doesn't protect
   against guessing an entire header field value, short or low-entropy
   values are more readily recovered by an adversary.  Therefore, an
   encoder might choose not to index values with low entropy.

   An encoder might also choose not to index values for header fields
   that are considered to be highly valuable or sensitive to recovery,
   such as the Cookie or Authorization header fields.

   On the contrary, an encoder might prefer indexing values for header
   fields that have little or no value if they were exposed.  For
   instance, a User-Agent header field does not commonly vary between
   requests and is sent to any server.  In that case, confirmation that
   a particular User-Agent value has been used provides little value.

   Note that these criteria for deciding to use a never-indexed literal
   representation will evolve over time as new attacks are discovered.

7.2.  Static Huffman Encoding

   There is no currently known attack against a static Huffman encoding.
   A study has shown that using a static Huffman encoding table created
   an information leakage; however, this same study concluded that an
   attacker could not take advantage of this information leakage to
   recover any meaningful amount of information (see [PETAL]).

7.3.  Memory Consumption

   An attacker can try to cause an endpoint to exhaust its memory.
   HPACK is designed to limit both the peak and state amounts of memory
   allocated by an endpoint.

   The amount of memory used by the compressor is limited by the
   protocol using HPACK through the definition of the maximum size of
   the dynamic table.  In HTTP/2, this value is controlled by the
   decoder through the setting parameter SETTINGS_HEADER_TABLE_SIZE (see
   Section 6.5.2 of [HTTP2]).  This limit takes into account both the
   size of the data stored in the dynamic table, plus a small allowance
   for overhead.






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   A decoder can limit the amount of state memory used by setting an
   appropriate value for the maximum size of the dynamic table.  In
   HTTP/2, this is realized by setting an appropriate value for the
   SETTINGS_HEADER_TABLE_SIZE parameter.  An encoder can limit the
   amount of state memory it uses by signaling a lower dynamic table
   size than the decoder allows (see Section 6.3).

   The amount of temporary memory consumed by an encoder or decoder can
   be limited by processing header fields sequentially.  An
   implementation does not need to retain a complete list of header
   fields.  Note, however, that it might be necessary for an application
   to retain a complete header list for other reasons; even though HPACK
   does not force this to occur, application constraints might make this
   necessary.

7.4.  Implementation Limits

   An implementation of HPACK needs to ensure that large values for
   integers, long encoding for integers, or long string literals do not
   create security weaknesses.

   An implementation has to set a limit for the values it accepts for
   integers, as well as for the encoded length (see Section 5.1).  In
   the same way, it has to set a limit to the length it accepts for
   string literals (see Section 5.2).

8.  References

8.1.  Normative References

   [HTTP2]     Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
               Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
               DOI 10.17487/RFC7540, May 2015,
               <http://www.rfc-editor.org/info/rfc7540>.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119,
               DOI 10.17487/RFC2119, March 1997,
               <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7230]   Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
               Transfer Protocol (HTTP/1.1): Message Syntax and
               Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014,
               <http://www.rfc-editor.org/info/rfc7230>.







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8.2.  Informative References

   [CANONICAL] Schwartz, E. and B. Kallick, "Generating a canonical
               prefix encoding", Communications of the ACM, Volume 7
               Issue 3, pp. 166-169, March 1964, <https://dl.acm.org/
               citation.cfm?id=363991>.

   [CRIME]     Wikipedia, "CRIME", May 2015, <http://en.wikipedia.org/w/
               index.php?title=CRIME&oldid=660948120>.

   [DEFLATE]   Deutsch, P., "DEFLATE Compressed Data Format
               Specification version 1.3", RFC 1951,
               DOI 10.17487/RFC1951, May 1996,
               <http://www.rfc-editor.org/info/rfc1951>.

   [HUFFMAN]   Huffman, D., "A Method for the Construction of Minimum-
               Redundancy Codes", Proceedings of the Institute of Radio
               Engineers, Volume 40, Number 9, pp. 1098-1101, September
               1952, <http://ieeexplore.ieee.org/xpl/
               articleDetails.jsp?arnumber=4051119>.

   [ORIGIN]    Barth, A., "The Web Origin Concept", RFC 6454,
               DOI 10.17487/RFC6454, December 2011,
               <http://www.rfc-editor.org/info/rfc6454>.

   [PETAL]     Tan, J. and J. Nahata, "PETAL: Preset Encoding
               Table Information Leakage", April 2013,
               <http://www.pdl.cmu.edu/PDL-FTP/associated/
               CMU-PDL-13-106.pdf>.

   [SPDY]      Belshe, M. and R. Peon, "SPDY Protocol", Work in
               Progress, draft-mbelshe-httpbis-spdy-00, February 2012.

   [TLS12]     Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246,
               DOI 10.17487/RFC5246, August 2008,
               <http://www.rfc-editor.org/info/rfc5246>.














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Appendix A.  Static Table Definition

   The static table (see Section 2.3.1) consists in a predefined and
   unchangeable list of header fields.

   The static table was created from the most frequent header fields
   used by popular web sites, with the addition of HTTP/2-specific
   pseudo-header fields (see Section 8.1.2.1 of [HTTP2]).  For header
   fields with a few frequent values, an entry was added for each of
   these frequent values.  For other header fields, an entry was added
   with an empty value.

   Table 1 lists the predefined header fields that make up the static
   table and gives the index of each entry.

          +-------+-----------------------------+---------------+
          | Index | Header Name                 | Header Value  |
          +-------+-----------------------------+---------------+
          | 1     | :authority                  |               |
          | 2     | :method                     | GET           |
          | 3     | :method                     | POST          |
          | 4     | :path                       | /             |
          | 5     | :path                       | /index.html   |
          | 6     | :scheme                     | http          |
          | 7     | :scheme                     | https         |
          | 8     | :status                     | 200           |
          | 9     | :status                     | 204           |
          | 10    | :status                     | 206           |
          | 11    | :status                     | 304           |
          | 12    | :status                     | 400           |
          | 13    | :status                     | 404           |
          | 14    | :status                     | 500           |
          | 15    | accept-charset              |               |
          | 16    | accept-encoding             | gzip, deflate |
          | 17    | accept-language             |               |
          | 18    | accept-ranges               |               |
          | 19    | accept                      |               |
          | 20    | access-control-allow-origin |               |
          | 21    | age                         |               |
          | 22    | allow                       |               |
          | 23    | authorization               |               |
          | 24    | cache-control               |               |
          | 25    | content-disposition         |               |
          | 26    | content-encoding            |               |
          | 27    | content-language            |               |
          | 28    | content-length              |               |
          | 29    | content-location            |               |
          | 30    | content-range               |               |



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          | 31    | content-type                |               |
          | 32    | cookie                      |               |
          | 33    | date                        |               |
          | 34    | etag                        |               |
          | 35    | expect                      |               |
          | 36    | expires                     |               |
          | 37    | from                        |               |
          | 38    | host                        |               |
          | 39    | if-match                    |               |
          | 40    | if-modified-since           |               |
          | 41    | if-none-match               |               |
          | 42    | if-range                    |               |
          | 43    | if-unmodified-since         |               |
          | 44    | last-modified               |               |
          | 45    | link                        |               |
          | 46    | location                    |               |
          | 47    | max-forwards                |               |
          | 48    | proxy-authenticate          |               |
          | 49    | proxy-authorization         |               |
          | 50    | range                       |               |
          | 51    | referer                     |               |
          | 52    | refresh                     |               |
          | 53    | retry-after                 |               |
          | 54    | server                      |               |
          | 55    | set-cookie                  |               |
          | 56    | strict-transport-security   |               |
          | 57    | transfer-encoding           |               |
          | 58    | user-agent                  |               |
          | 59    | vary                        |               |
          | 60    | via                         |               |
          | 61    | www-authenticate            |               |
          +-------+-----------------------------+---------------+

                       Table 1: Static Table Entries

















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Appendix B.  Huffman Code

   The following Huffman code is used when encoding string literals with
   a Huffman coding (see Section 5.2).

   This Huffman code was generated from statistics obtained on a large
   sample of HTTP headers.  It is a canonical Huffman code (see
   [CANONICAL]) with some tweaking to ensure that no symbol has a unique
   code length.

   Each row in the table defines the code used to represent a symbol:

   sym:  The symbol to be represented.  It is the decimal value of an
      octet, possibly prepended with its ASCII representation.  A
      specific symbol, "EOS", is used to indicate the end of a string
      literal.

   code as bits:  The Huffman code for the symbol represented as a
      base-2 integer, aligned on the most significant bit (MSB).

   code as hex:  The Huffman code for the symbol, represented as a
      hexadecimal integer, aligned on the least significant bit (LSB).

   len:  The number of bits for the code representing the symbol.

   As an example, the code for the symbol 47 (corresponding to the ASCII
   character "/") consists in the 6 bits "0", "1", "1", "0", "0", "0".
   This corresponds to the value 0x18 (in hexadecimal) encoded in 6
   bits.

                                                        code
                          code as bits                 as hex   len
        sym              aligned to MSB                aligned   in
                                                       to LSB   bits
       (  0)  |11111111|11000                             1ff8  [13]
       (  1)  |11111111|11111111|1011000                7fffd8  [23]
       (  2)  |11111111|11111111|11111110|0010         fffffe2  [28]
       (  3)  |11111111|11111111|11111110|0011         fffffe3  [28]
       (  4)  |11111111|11111111|11111110|0100         fffffe4  [28]
       (  5)  |11111111|11111111|11111110|0101         fffffe5  [28]
       (  6)  |11111111|11111111|11111110|0110         fffffe6  [28]
       (  7)  |11111111|11111111|11111110|0111         fffffe7  [28]
       (  8)  |11111111|11111111|11111110|1000         fffffe8  [28]
       (  9)  |11111111|11111111|11101010               ffffea  [24]
       ( 10)  |11111111|11111111|11111111|111100      3ffffffc  [30]
       ( 11)  |11111111|11111111|11111110|1001         fffffe9  [28]
       ( 12)  |11111111|11111111|11111110|1010         fffffea  [28]
       ( 13)  |11111111|11111111|11111111|111101      3ffffffd  [30]



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       ( 14)  |11111111|11111111|11111110|1011         fffffeb  [28]
       ( 15)  |11111111|11111111|11111110|1100         fffffec  [28]
       ( 16)  |11111111|11111111|11111110|1101         fffffed  [28]
       ( 17)  |11111111|11111111|11111110|1110         fffffee  [28]
       ( 18)  |11111111|11111111|11111110|1111         fffffef  [28]
       ( 19)  |11111111|11111111|11111111|0000         ffffff0  [28]
       ( 20)  |11111111|11111111|11111111|0001         ffffff1  [28]
       ( 21)  |11111111|11111111|11111111|0010         ffffff2  [28]
       ( 22)  |11111111|11111111|11111111|111110      3ffffffe  [30]
       ( 23)  |11111111|11111111|11111111|0011         ffffff3  [28]
       ( 24)  |11111111|11111111|11111111|0100         ffffff4  [28]
       ( 25)  |11111111|11111111|11111111|0101         ffffff5  [28]
       ( 26)  |11111111|11111111|11111111|0110         ffffff6  [28]
       ( 27)  |11111111|11111111|11111111|0111         ffffff7  [28]
       ( 28)  |11111111|11111111|11111111|1000         ffffff8  [28]
       ( 29)  |11111111|11111111|11111111|1001         ffffff9  [28]
       ( 30)  |11111111|11111111|11111111|1010         ffffffa  [28]
       ( 31)  |11111111|11111111|11111111|1011         ffffffb  [28]
   ' ' ( 32)  |010100                                       14  [ 6]
   '!' ( 33)  |11111110|00                                 3f8  [10]
   '"' ( 34)  |11111110|01                                 3f9  [10]
   '#' ( 35)  |11111111|1010                               ffa  [12]
   '$' ( 36)  |11111111|11001                             1ff9  [13]
   '%' ( 37)  |010101                                       15  [ 6]
   '&' ( 38)  |11111000                                     f8  [ 8]
   ''' ( 39)  |11111111|010                                7fa  [11]
   '(' ( 40)  |11111110|10                                 3fa  [10]
   ')' ( 41)  |11111110|11                                 3fb  [10]
   '*' ( 42)  |11111001                                     f9  [ 8]
   '+' ( 43)  |11111111|011                                7fb  [11]
   ',' ( 44)  |11111010                                     fa  [ 8]
   '-' ( 45)  |010110                                       16  [ 6]
   '.' ( 46)  |010111                                       17  [ 6]
   '/' ( 47)  |011000                                       18  [ 6]
   '0' ( 48)  |00000                                         0  [ 5]
   '1' ( 49)  |00001                                         1  [ 5]
   '2' ( 50)  |00010                                         2  [ 5]
   '3' ( 51)  |011001                                       19  [ 6]
   '4' ( 52)  |011010                                       1a  [ 6]
   '5' ( 53)  |011011                                       1b  [ 6]
   '6' ( 54)  |011100                                       1c  [ 6]
   '7' ( 55)  |011101                                       1d  [ 6]
   '8' ( 56)  |011110                                       1e  [ 6]
   '9' ( 57)  |011111                                       1f  [ 6]
   ':' ( 58)  |1011100                                      5c  [ 7]
   ';' ( 59)  |11111011                                     fb  [ 8]
   '<' ( 60)  |11111111|1111100                           7ffc  [15]
   '=' ( 61)  |100000                                       20  [ 6]



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   '>' ( 62)  |11111111|1011                               ffb  [12]
   '?' ( 63)  |11111111|00                                 3fc  [10]
   '@' ( 64)  |11111111|11010                             1ffa  [13]
   'A' ( 65)  |100001                                       21  [ 6]
   'B' ( 66)  |1011101                                      5d  [ 7]
   'C' ( 67)  |1011110                                      5e  [ 7]
   'D' ( 68)  |1011111                                      5f  [ 7]
   'E' ( 69)  |1100000                                      60  [ 7]
   'F' ( 70)  |1100001                                      61  [ 7]
   'G' ( 71)  |1100010                                      62  [ 7]
   'H' ( 72)  |1100011                                      63  [ 7]
   'I' ( 73)  |1100100                                      64  [ 7]
   'J' ( 74)  |1100101                                      65  [ 7]
   'K' ( 75)  |1100110                                      66  [ 7]
   'L' ( 76)  |1100111                                      67  [ 7]
   'M' ( 77)  |1101000                                      68  [ 7]
   'N' ( 78)  |1101001                                      69  [ 7]
   'O' ( 79)  |1101010                                      6a  [ 7]
   'P' ( 80)  |1101011                                      6b  [ 7]
   'Q' ( 81)  |1101100                                      6c  [ 7]
   'R' ( 82)  |1101101                                      6d  [ 7]
   'S' ( 83)  |1101110                                      6e  [ 7]
   'T' ( 84)  |1101111                                      6f  [ 7]
   'U' ( 85)  |1110000                                      70  [ 7]
   'V' ( 86)  |1110001                                      71  [ 7]
   'W' ( 87)  |1110010                                      72  [ 7]
   'X' ( 88)  |11111100                                     fc  [ 8]
   'Y' ( 89)  |1110011                                      73  [ 7]
   'Z' ( 90)  |11111101                                     fd  [ 8]
   '[' ( 91)  |11111111|11011                             1ffb  [13]
   '\' ( 92)  |11111111|11111110|000                     7fff0  [19]
   ']' ( 93)  |11111111|11100                             1ffc  [13]
   '^' ( 94)  |11111111|111100                            3ffc  [14]
   '_' ( 95)  |100010                                       22  [ 6]
   '`' ( 96)  |11111111|1111101                           7ffd  [15]
   'a' ( 97)  |00011                                         3  [ 5]
   'b' ( 98)  |100011                                       23  [ 6]
   'c' ( 99)  |00100                                         4  [ 5]
   'd' (100)  |100100                                       24  [ 6]
   'e' (101)  |00101                                         5  [ 5]
   'f' (102)  |100101                                       25  [ 6]
   'g' (103)  |100110                                       26  [ 6]
   'h' (104)  |100111                                       27  [ 6]
   'i' (105)  |00110                                         6  [ 5]
   'j' (106)  |1110100                                      74  [ 7]
   'k' (107)  |1110101                                      75  [ 7]
   'l' (108)  |101000                                       28  [ 6]
   'm' (109)  |101001                                       29  [ 6]



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   'n' (110)  |101010                                       2a  [ 6]
   'o' (111)  |00111                                         7  [ 5]
   'p' (112)  |101011                                       2b  [ 6]
   'q' (113)  |1110110                                      76  [ 7]
   'r' (114)  |101100                                       2c  [ 6]
   's' (115)  |01000                                         8  [ 5]
   't' (116)  |01001                                         9  [ 5]
   'u' (117)  |101101                                       2d  [ 6]
   'v' (118)  |1110111                                      77  [ 7]
   'w' (119)  |1111000                                      78  [ 7]
   'x' (120)  |1111001                                      79  [ 7]
   'y' (121)  |1111010                                      7a  [ 7]
   'z' (122)  |1111011                                      7b  [ 7]
   '{' (123)  |11111111|1111110                           7ffe  [15]
   '|' (124)  |11111111|100                                7fc  [11]
   '}' (125)  |11111111|111101                            3ffd  [14]
   '~' (126)  |11111111|11101                             1ffd  [13]
       (127)  |11111111|11111111|11111111|1100         ffffffc  [28]
       (128)  |11111111|11111110|0110                    fffe6  [20]
       (129)  |11111111|11111111|010010                 3fffd2  [22]
       (130)  |11111111|11111110|0111                    fffe7  [20]
       (131)  |11111111|11111110|1000                    fffe8  [20]
       (132)  |11111111|11111111|010011                 3fffd3  [22]
       (133)  |11111111|11111111|010100                 3fffd4  [22]
       (134)  |11111111|11111111|010101                 3fffd5  [22]
       (135)  |11111111|11111111|1011001                7fffd9  [23]
       (136)  |11111111|11111111|010110                 3fffd6  [22]
       (137)  |11111111|11111111|1011010                7fffda  [23]
       (138)  |11111111|11111111|1011011                7fffdb  [23]
       (139)  |11111111|11111111|1011100                7fffdc  [23]
       (140)  |11111111|11111111|1011101                7fffdd  [23]
       (141)  |11111111|11111111|1011110                7fffde  [23]
       (142)  |11111111|11111111|11101011               ffffeb  [24]
       (143)  |11111111|11111111|1011111                7fffdf  [23]
       (144)  |11111111|11111111|11101100               ffffec  [24]
       (145)  |11111111|11111111|11101101               ffffed  [24]
       (146)  |11111111|11111111|010111                 3fffd7  [22]
       (147)  |11111111|11111111|1100000                7fffe0  [23]
       (148)  |11111111|11111111|11101110               ffffee  [24]
       (149)  |11111111|11111111|1100001                7fffe1  [23]
       (150)  |11111111|11111111|1100010                7fffe2  [23]
       (151)  |11111111|11111111|1100011                7fffe3  [23]
       (152)  |11111111|11111111|1100100                7fffe4  [23]
       (153)  |11111111|11111110|11100                  1fffdc  [21]
       (154)  |11111111|11111111|011000                 3fffd8  [22]
       (155)  |11111111|11111111|1100101                7fffe5  [23]
       (156)  |11111111|11111111|011001                 3fffd9  [22]
       (157)  |11111111|11111111|1100110                7fffe6  [23]



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       (158)  |11111111|11111111|1100111                7fffe7  [23]
       (159)  |11111111|11111111|11101111               ffffef  [24]
       (160)  |11111111|11111111|011010                 3fffda  [22]
       (161)  |11111111|11111110|11101                  1fffdd  [21]
       (162)  |11111111|11111110|1001                    fffe9  [20]
       (163)  |11111111|11111111|011011                 3fffdb  [22]
       (164)  |11111111|11111111|011100                 3fffdc  [22]
       (165)  |11111111|11111111|1101000                7fffe8  [23]
       (166)  |11111111|11111111|1101001                7fffe9  [23]
       (167)  |11111111|11111110|11110                  1fffde  [21]
       (168)  |11111111|11111111|1101010                7fffea  [23]
       (169)  |11111111|11111111|011101                 3fffdd  [22]
       (170)  |11111111|11111111|011110                 3fffde  [22]
       (171)  |11111111|11111111|11110000               fffff0  [24]
       (172)  |11111111|11111110|11111                  1fffdf  [21]
       (173)  |11111111|11111111|011111                 3fffdf  [22]
       (174)  |11111111|11111111|1101011                7fffeb  [23]
       (175)  |11111111|11111111|1101100                7fffec  [23]
       (176)  |11111111|11111111|00000                  1fffe0  [21]
       (177)  |11111111|11111111|00001                  1fffe1  [21]
       (178)  |11111111|11111111|100000                 3fffe0  [22]
       (179)  |11111111|11111111|00010                  1fffe2  [21]
       (180)  |11111111|11111111|1101101                7fffed  [23]
       (181)  |11111111|11111111|100001                 3fffe1  [22]
       (182)  |11111111|11111111|1101110                7fffee  [23]
       (183)  |11111111|11111111|1101111                7fffef  [23]
       (184)  |11111111|11111110|1010                    fffea  [20]
       (185)  |11111111|11111111|100010                 3fffe2  [22]
       (186)  |11111111|11111111|100011                 3fffe3  [22]
       (187)  |11111111|11111111|100100                 3fffe4  [22]
       (188)  |11111111|11111111|1110000                7ffff0  [23]
       (189)  |11111111|11111111|100101                 3fffe5  [22]
       (190)  |11111111|11111111|100110                 3fffe6  [22]
       (191)  |11111111|11111111|1110001                7ffff1  [23]
       (192)  |11111111|11111111|11111000|00           3ffffe0  [26]
       (193)  |11111111|11111111|11111000|01           3ffffe1  [26]
       (194)  |11111111|11111110|1011                    fffeb  [20]
       (195)  |11111111|11111110|001                     7fff1  [19]
       (196)  |11111111|11111111|100111                 3fffe7  [22]
       (197)  |11111111|11111111|1110010                7ffff2  [23]
       (198)  |11111111|11111111|101000                 3fffe8  [22]
       (199)  |11111111|11111111|11110110|0            1ffffec  [25]
       (200)  |11111111|11111111|11111000|10           3ffffe2  [26]
       (201)  |11111111|11111111|11111000|11           3ffffe3  [26]
       (202)  |11111111|11111111|11111001|00           3ffffe4  [26]
       (203)  |11111111|11111111|11111011|110          7ffffde  [27]
       (204)  |11111111|11111111|11111011|111          7ffffdf  [27]
       (205)  |11111111|11111111|11111001|01           3ffffe5  [26]



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       (206)  |11111111|11111111|11110001               fffff1  [24]
       (207)  |11111111|11111111|11110110|1            1ffffed  [25]
       (208)  |11111111|11111110|010                     7fff2  [19]
       (209)  |11111111|11111111|00011                  1fffe3  [21]
       (210)  |11111111|11111111|11111001|10           3ffffe6  [26]
       (211)  |11111111|11111111|11111100|000          7ffffe0  [27]
       (212)  |11111111|11111111|11111100|001          7ffffe1  [27]
       (213)  |11111111|11111111|11111001|11           3ffffe7  [26]
       (214)  |11111111|11111111|11111100|010          7ffffe2  [27]
       (215)  |11111111|11111111|11110010               fffff2  [24]
       (216)  |11111111|11111111|00100                  1fffe4  [21]
       (217)  |11111111|11111111|00101                  1fffe5  [21]
       (218)  |11111111|11111111|11111010|00           3ffffe8  [26]
       (219)  |11111111|11111111|11111010|01           3ffffe9  [26]
       (220)  |11111111|11111111|11111111|1101         ffffffd  [28]
       (221)  |11111111|11111111|11111100|011          7ffffe3  [27]
       (222)  |11111111|11111111|11111100|100          7ffffe4  [27]
       (223)  |11111111|11111111|11111100|101          7ffffe5  [27]
       (224)  |11111111|11111110|1100                    fffec  [20]
       (225)  |11111111|11111111|11110011               fffff3  [24]
       (226)  |11111111|11111110|1101                    fffed  [20]
       (227)  |11111111|11111111|00110                  1fffe6  [21]
       (228)  |11111111|11111111|101001                 3fffe9  [22]
       (229)  |11111111|11111111|00111                  1fffe7  [21]
       (230)  |11111111|11111111|01000                  1fffe8  [21]
       (231)  |11111111|11111111|1110011                7ffff3  [23]
       (232)  |11111111|11111111|101010                 3fffea  [22]
       (233)  |11111111|11111111|101011                 3fffeb  [22]
       (234)  |11111111|11111111|11110111|0            1ffffee  [25]
       (235)  |11111111|11111111|11110111|1            1ffffef  [25]
       (236)  |11111111|11111111|11110100               fffff4  [24]
       (237)  |11111111|11111111|11110101               fffff5  [24]
       (238)  |11111111|11111111|11111010|10           3ffffea  [26]
       (239)  |11111111|11111111|1110100                7ffff4  [23]
       (240)  |11111111|11111111|11111010|11           3ffffeb  [26]
       (241)  |11111111|11111111|11111100|110          7ffffe6  [27]
       (242)  |11111111|11111111|11111011|00           3ffffec  [26]
       (243)  |11111111|11111111|11111011|01           3ffffed  [26]
       (244)  |11111111|11111111|11111100|111          7ffffe7  [27]
       (245)  |11111111|11111111|11111101|000          7ffffe8  [27]
       (246)  |11111111|11111111|11111101|001          7ffffe9  [27]
       (247)  |11111111|11111111|11111101|010          7ffffea  [27]
       (248)  |11111111|11111111|11111101|011          7ffffeb  [27]
       (249)  |11111111|11111111|11111111|1110         ffffffe  [28]
       (250)  |11111111|11111111|11111101|100          7ffffec  [27]
       (251)  |11111111|11111111|11111101|101          7ffffed  [27]
       (252)  |11111111|11111111|11111101|110          7ffffee  [27]
       (253)  |11111111|11111111|11111101|111          7ffffef  [27]



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       (254)  |11111111|11111111|11111110|000          7fffff0  [27]
       (255)  |11111111|11111111|11111011|10           3ffffee  [26]
   EOS (256)  |11111111|11111111|11111111|111111      3fffffff  [30]

Appendix C.  Examples

   This appendix contains examples covering integer encoding, header
   field representation, and the encoding of whole lists of header
   fields for both requests and responses, with and without Huffman
   coding.

C.1.  Integer Representation Examples

   This section shows the representation of integer values in detail
   (see Section 5.1).

C.1.1.  Example 1: Encoding 10 Using a 5-Bit Prefix

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

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

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

C.1.2.  Example 2: Encoding 1337 Using a 5-Bit Prefix

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

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

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

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

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

            I % 128 == 26

            26 + 128 == 154

            154 is encoded in 8 bits as: 10011010

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



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

         I, now 10, is encoded in 8 bits as: 00001010.

      The process ends.

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

C.1.3.  Example 3: Encoding 42 Starting at an Octet Boundary

   The value 42 is to be encoded starting at an octet boundary.  This
   implies that a 8-bit prefix is used.

   o  42 is less than 255 (2^8 - 1) and is represented using the 8-bit
      prefix.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 |   42 stored on 8 bits
   +---+---+---+---+---+---+---+---+

C.2.  Header Field Representation Examples

   This section shows several independent representation examples.

C.2.1.  Literal Header Field with Indexing

   The header field representation uses a literal name and a literal
   value.  The header field is added to the dynamic table.

   Header list to encode:

   custom-key: custom-header

   Hex dump of encoded data:

   400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus
   746f 6d2d 6865 6164 6572                | tom-header







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   Decoding process:

   40                                      | == Literal indexed ==
   0a                                      |   Literal name (len = 10)
   6375 7374 6f6d 2d6b 6579                | custom-key
   0d                                      |   Literal value (len = 13)
   6375 7374 6f6d 2d68 6561 6465 72        | custom-header
                                           | -> custom-key:
                                           |   custom-header

   Dynamic Table (after decoding):

   [  1] (s =  55) custom-key: custom-header
         Table size:  55

   Decoded header list:

   custom-key: custom-header

C.2.2.  Literal Header Field without Indexing

   The header field representation uses an indexed name and a literal
   value.  The header field is not added to the dynamic table.

   Header list to encode:

   :path: /sample/path

   Hex dump of encoded data:

   040c 2f73 616d 706c 652f 7061 7468      | ../sample/path

   Decoding process:

   04                                      | == Literal not indexed ==
                                           |   Indexed name (idx = 4)
                                           |     :path
   0c                                      |   Literal value (len = 12)
   2f73 616d 706c 652f 7061 7468           | /sample/path
                                           | -> :path: /sample/path

   Dynamic table (after decoding): empty.

   Decoded header list:

   :path: /sample/path





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C.2.3.  Literal Header Field Never Indexed

   The header field representation uses a literal name and a literal
   value.  The header field is not added to the dynamic table and must
   use the same representation if re-encoded by an intermediary.

   Header list to encode:

   password: secret

   Hex dump of encoded data:

   1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre
   74                                      | t

   Decoding process:

   10                                      | == Literal never indexed ==
   08                                      |   Literal name (len = 8)
   7061 7373 776f 7264                     | password
   06                                      |   Literal value (len = 6)
   7365 6372 6574                          | secret
                                           | -> password: secret

   Dynamic table (after decoding): empty.

   Decoded header list:

   password: secret






















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C.2.4.  Indexed Header Field

   The header field representation uses an indexed header field from the
   static table.

   Header list to encode:

   :method: GET

   Hex dump of encoded data:

   82                                      | .

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET

   Dynamic table (after decoding): empty.

   Decoded header list:

   :method: GET

C.3.  Request Examples without Huffman Coding

   This section shows several consecutive header lists, corresponding to
   HTTP requests, on the same connection.

C.3.1.  First Request

   Header list to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

   Hex dump of encoded data:

   8286 8441 0f77 7777 2e65 7861 6d70 6c65 | ...A.www.example
   2e63 6f6d                               | .com








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   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   86                                      | == Indexed - Add ==
                                           |   idx = 6
                                           | -> :scheme: http
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> :path: /
   41                                      | == Literal indexed ==
                                           |   Indexed name (idx = 1)
                                           |     :authority
   0f                                      |   Literal value (len = 15)
   7777 772e 6578 616d 706c 652e 636f 6d   | www.example.com
                                           | -> :authority:
                                           |   www.example.com

   Dynamic Table (after decoding):

   [  1] (s =  57) :authority: www.example.com
         Table size:  57

   Decoded header list:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

C.3.2.  Second Request

   Header list to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com
   cache-control: no-cache

   Hex dump of encoded data:

   8286 84be 5808 6e6f 2d63 6163 6865      | ....X.no-cache







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   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   86                                      | == Indexed - Add ==
                                           |   idx = 6
                                           | -> :scheme: http
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> :path: /
   be                                      | == Indexed - Add ==
                                           |   idx = 62
                                           | -> :authority:
                                           |   www.example.com
   58                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24)
                                           |     cache-control
   08                                      |   Literal value (len = 8)
   6e6f 2d63 6163 6865                     | no-cache
                                           | -> cache-control: no-cache

   Dynamic Table (after decoding):

   [  1] (s =  53) cache-control: no-cache
   [  2] (s =  57) :authority: www.example.com
         Table size: 110

   Decoded header list:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com
   cache-control: no-cache

C.3.3.  Third Request

   Header list to encode:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value






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   Hex dump of encoded data:

   8287 85bf 400a 6375 7374 6f6d 2d6b 6579 | ....@.custom-key
   0c63 7573 746f 6d2d 7661 6c75 65        | .custom-value

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   87                                      | == Indexed - Add ==
                                           |   idx = 7
                                           | -> :scheme: https
   85                                      | == Indexed - Add ==
                                           |   idx = 5
                                           | -> :path: /index.html
   bf                                      | == Indexed - Add ==
                                           |   idx = 63
                                           | -> :authority:
                                           |   www.example.com
   40                                      | == Literal indexed ==
   0a                                      |   Literal name (len = 10)
   6375 7374 6f6d 2d6b 6579                | custom-key
   0c                                      |   Literal value (len = 12)
   6375 7374 6f6d 2d76 616c 7565           | custom-value
                                           | -> custom-key:
                                           |   custom-value

   Dynamic Table (after decoding):

   [  1] (s =  54) custom-key: custom-value
   [  2] (s =  53) cache-control: no-cache
   [  3] (s =  57) :authority: www.example.com
         Table size: 164

   Decoded header list:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value









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C.4.  Request Examples with Huffman Coding

   This section shows the same examples as the previous section but uses
   Huffman encoding for the literal values.

C.4.1.  First Request

   Header list to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

   Hex dump of encoded data:

   8286 8441 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...A......:k....
   ff                                      | .

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   86                                      | == Indexed - Add ==
                                           |   idx = 6
                                           | -> :scheme: http
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> :path: /
   41                                      | == Literal indexed ==
                                           |   Indexed name (idx = 1)
                                           |     :authority
   8c                                      |   Literal value (len = 12)
                                           |     Huffman encoded:
   f1e3 c2e5 f23a 6ba0 ab90 f4ff           | .....:k.....
                                           |     Decoded:
                                           | www.example.com
                                           | -> :authority:
                                           |   www.example.com

   Dynamic Table (after decoding):

   [  1] (s =  57) :authority: www.example.com
         Table size:  57






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   Decoded header list:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

C.4.2.  Second Request

   Header list to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com
   cache-control: no-cache

   Hex dump of encoded data:

   8286 84be 5886 a8eb 1064 9cbf           | ....X....d..

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   86                                      | == Indexed - Add ==
                                           |   idx = 6
                                           | -> :scheme: http
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> :path: /
   be                                      | == Indexed - Add ==
                                           |   idx = 62
                                           | -> :authority:
                                           |   www.example.com
   58                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24)
                                           |     cache-control
   86                                      |   Literal value (len = 6)
                                           |     Huffman encoded:
   a8eb 1064 9cbf                          | ...d..
                                           |     Decoded:
                                           | no-cache
                                           | -> cache-control: no-cache






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   Dynamic Table (after decoding):

   [  1] (s =  53) cache-control: no-cache
   [  2] (s =  57) :authority: www.example.com
         Table size: 110

   Decoded header list:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com
   cache-control: no-cache

C.4.3.  Third Request

   Header list to encode:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value

   Hex dump of encoded data:

   8287 85bf 4088 25a8 49e9 5ba9 7d7f 8925 | ....@.%.I.[.}..%
   a849 e95b b8e8 b4bf                     | .I.[....























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   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   87                                      | == Indexed - Add ==
                                           |   idx = 7
                                           | -> :scheme: https
   85                                      | == Indexed - Add ==
                                           |   idx = 5
                                           | -> :path: /index.html
   bf                                      | == Indexed - Add ==
                                           |   idx = 63
                                           | -> :authority:
                                           |   www.example.com
   40                                      | == Literal indexed ==
   88                                      |   Literal name (len = 8)
                                           |     Huffman encoded:
   25a8 49e9 5ba9 7d7f                     | %.I.[.}.
                                           |     Decoded:
                                           | custom-key
   89                                      |   Literal value (len = 9)
                                           |     Huffman encoded:
   25a8 49e9 5bb8 e8b4 bf                  | %.I.[....
                                           |     Decoded:
                                           | custom-value
                                           | -> custom-key:
                                           |   custom-value

   Dynamic Table (after decoding):

   [  1] (s =  54) custom-key: custom-value
   [  2] (s =  53) cache-control: no-cache
   [  3] (s =  57) :authority: www.example.com
         Table size: 164

   Decoded header list:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value








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C.5.  Response Examples without Huffman Coding

   This section shows several consecutive header lists, corresponding to
   HTTP responses, on the same connection.  The HTTP/2 setting parameter
   SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing
   some evictions to occur.

C.5.1.  First Response

   Header list to encode:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Hex dump of encoded data:

   4803 3330 3258 0770 7269 7661 7465 611d | H.302X.privatea.
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3120 474d 546e 1768 |  20:13:21 GMTn.h
   7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
   6c65 2e63 6f6d                          | le.com

   Decoding process:

   48                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   03                                      |   Literal value (len = 3)
   3330 32                                 | 302
                                           | -> :status: 302
   58                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24)
                                           |     cache-control
   07                                      |   Literal value (len = 7)
   7072 6976 6174 65                       | private
                                           | -> cache-control: private
   61                                      | == Literal indexed ==
                                           |   Indexed name (idx = 33)
                                           |     date
   1d                                      |   Literal value (len = 29)
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3120 474d 54        |  20:13:21 GMT
                                           | -> date: Mon, 21 Oct 2013
                                           |   20:13:21 GMT
   6e                                      | == Literal indexed ==
                                           |   Indexed name (idx = 46)



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                                           |     location
   17                                      |   Literal value (len = 23)
   6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
   706c 652e 636f 6d                       | ple.com
                                           | -> location:
                                           |   https://www.example.com

   Dynamic Table (after decoding):

   [  1] (s =  63) location: https://www.example.com
   [  2] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  3] (s =  52) cache-control: private
   [  4] (s =  42) :status: 302
         Table size: 222

   Decoded header list:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

C.5.2.  Second Response

   The (":status", "302") header field is evicted from the dynamic table
   to free space to allow adding the (":status", "307") header field.

   Header list to encode:

   :status: 307
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Hex dump of encoded data:

   4803 3330 37c1 c0bf                     | H.307...

   Decoding process:

   48                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   03                                      |   Literal value (len = 3)
   3330 37                                 | 307
                                           | - evict: :status: 302
                                           | -> :status: 307
   c1                                      | == Indexed - Add ==



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                                           |   idx = 65
                                           | -> cache-control: private
   c0                                      | == Indexed - Add ==
                                           |   idx = 64
                                           | -> date: Mon, 21 Oct 2013
                                           |   20:13:21 GMT
   bf                                      | == Indexed - Add ==
                                           |   idx = 63
                                           | -> location:
                                           |   https://www.example.com

   Dynamic Table (after decoding):

   [  1] (s =  42) :status: 307
   [  2] (s =  63) location: https://www.example.com
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  4] (s =  52) cache-control: private
         Table size: 222

   Decoded header list:

   :status: 307
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

C.5.3.  Third Response

   Several header fields are evicted from the dynamic table during the
   processing of this header list.

   Header list to encode:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   location: https://www.example.com
   content-encoding: gzip
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1












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   Hex dump of encoded data:

   88c1 611d 4d6f 6e2c 2032 3120 4f63 7420 | ..a.Mon, 21 Oct
   3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
   54c0 5a04 677a 6970 7738 666f 6f3d 4153 | T.Z.gzipw8foo=AS
   444a 4b48 514b 425a 584f 5157 454f 5049 | DJKHQKBZXOQWEOPI
   5541 5851 5745 4f49 553b 206d 6178 2d61 | UAXQWEOIU; max-a
   6765 3d33 3630 303b 2076 6572 7369 6f6e | ge=3600; version
   3d31                                    | =1

   Decoding process:

   88                                      | == Indexed - Add ==
                                           |   idx = 8
                                           | -> :status: 200
   c1                                      | == Indexed - Add ==
                                           |   idx = 65
                                           | -> cache-control: private
   61                                      | == Literal indexed ==
                                           |   Indexed name (idx = 33)
                                           |     date
   1d                                      |   Literal value (len = 29)
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3220 474d 54        |  20:13:22 GMT
                                           | - evict: cache-control:
                                           |   private
                                           | -> date: Mon, 21 Oct 2013
                                           |   20:13:22 GMT
   c0                                      | == Indexed - Add ==
                                           |   idx = 64
                                           | -> location:
                                           |   https://www.example.com
   5a                                      | == Literal indexed ==
                                           |   Indexed name (idx = 26)
                                           |     content-encoding
   04                                      |   Literal value (len = 4)
   677a 6970                               | gzip
                                           | - evict: date: Mon, 21 Oct
                                           |    2013 20:13:21 GMT
                                           | -> content-encoding: gzip
   77                                      | == Literal indexed ==
                                           |   Indexed name (idx = 55)
                                           |     set-cookie
   38                                      |   Literal value (len = 56)
   666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
   5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
   206d 6178 2d61 6765 3d33 3630 303b 2076 |  max-age=3600; v
   6572 7369 6f6e 3d31                     | ersion=1



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                                           | - evict: location:
                                           |   https://www.example.com
                                           | - evict: :status: 307
                                           | -> set-cookie: foo=ASDJKHQ
                                           |   KBZXOQWEOPIUAXQWEOIU; ma
                                           |   x-age=3600; version=1

   Dynamic Table (after decoding):

   [  1] (s =  98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU;
                    max-age=3600; version=1
   [  2] (s =  52) content-encoding: gzip
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:22 GMT
         Table size: 215

   Decoded header list:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   location: https://www.example.com
   content-encoding: gzip
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1

C.6.  Response Examples with Huffman Coding

   This section shows the same examples as the previous section but uses
   Huffman encoding for the literal values.  The HTTP/2 setting
   parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256
   octets, causing some evictions to occur.  The eviction mechanism uses
   the length of the decoded literal values, so the same evictions occur
   as in the previous section.

C.6.1.  First Response

   Header list to encode:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Hex dump of encoded data:

   4882 6402 5885 aec3 771a 4b61 96d0 7abe | H.d.X...w.Ka..z.
   9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f...
   2d1b ff6e 919d 29ad 1718 63c7 8f0b 97c8 | -..n..)...c.....
   e9ae 82ae 43d3                          | ....C.



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   Decoding process:

   48                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   82                                      |   Literal value (len = 2)
                                           |     Huffman encoded:
   6402                                    | d.
                                           |     Decoded:
                                           | 302
                                           | -> :status: 302
   58                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24)
                                           |     cache-control
   85                                      |   Literal value (len = 5)
                                           |     Huffman encoded:
   aec3 771a 4b                            | ..w.K
                                           |     Decoded:
                                           | private
                                           | -> cache-control: private
   61                                      | == Literal indexed ==
                                           |   Indexed name (idx = 33)
                                           |     date
   96                                      |   Literal value (len = 22)
                                           |     Huffman encoded:
   d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
   e082 a62d 1bff                          | ...-..
                                           |     Decoded:
                                           | Mon, 21 Oct 2013 20:13:21
                                           | GMT
                                           | -> date: Mon, 21 Oct 2013
                                           |   20:13:21 GMT
   6e                                      | == Literal indexed ==
                                           |   Indexed name (idx = 46)
                                           |     location
   91                                      |   Literal value (len = 17)
                                           |     Huffman encoded:
   9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
   d3                                      | .
                                           |     Decoded:
                                           | https://www.example.com
                                           | -> location:
                                           |   https://www.example.com








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   Dynamic Table (after decoding):

   [  1] (s =  63) location: https://www.example.com
   [  2] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  3] (s =  52) cache-control: private
   [  4] (s =  42) :status: 302
         Table size: 222

   Decoded header list:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

C.6.2.  Second Response

   The (":status", "302") header field is evicted from the dynamic table
   to free space to allow adding the (":status", "307") header field.

   Header list to encode:

   :status: 307
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Hex dump of encoded data:

   4883 640e ffc1 c0bf                     | H.d.....

   Decoding process:

   48                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   83                                      |   Literal value (len = 3)
                                           |     Huffman encoded:
   640e ff                                 | d..
                                           |     Decoded:
                                           | 307
                                           | - evict: :status: 302
                                           | -> :status: 307
   c1                                      | == Indexed - Add ==
                                           |   idx = 65
                                           | -> cache-control: private
   c0                                      | == Indexed - Add ==
                                           |   idx = 64



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                                           | -> date: Mon, 21 Oct 2013
                                           |   20:13:21 GMT
   bf                                      | == Indexed - Add ==
                                           |   idx = 63
                                           | -> location:
                                           |   https://www.example.com

   Dynamic Table (after decoding):

   [  1] (s =  42) :status: 307
   [  2] (s =  63) location: https://www.example.com
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  4] (s =  52) cache-control: private
         Table size: 222

   Decoded header list:

   :status: 307
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

C.6.3.  Third Response

   Several header fields are evicted from the dynamic table during the
   processing of this header list.

   Header list to encode:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   location: https://www.example.com
   content-encoding: gzip
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1

   Hex dump of encoded data:

   88c1 6196 d07a be94 1054 d444 a820 0595 | ..a..z...T.D. ..
   040b 8166 e084 a62d 1bff c05a 839b d9ab | ...f...-...Z....
   77ad 94e7 821d d7f2 e6c7 b335 dfdf cd5b | w..........5...[
   3960 d5af 2708 7f36 72c1 ab27 0fb5 291f | 9`..'..6r..'..).
   9587 3160 65c0 03ed 4ee5 b106 3d50 07   | ..1`e...N...=P.








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   Decoding process:

   88                                      | == Indexed - Add ==
                                           |   idx = 8
                                           | -> :status: 200
   c1                                      | == Indexed - Add ==
                                           |   idx = 65
                                           | -> cache-control: private
   61                                      | == Literal indexed ==
                                           |   Indexed name (idx = 33)
                                           |     date
   96                                      |   Literal value (len = 22)
                                           |     Huffman encoded:
   d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
   e084 a62d 1bff                          | ...-..
                                           |     Decoded:
                                           | Mon, 21 Oct 2013 20:13:22
                                           | GMT
                                           | - evict: cache-control:
                                           |   private
                                           | -> date: Mon, 21 Oct 2013
                                           |   20:13:22 GMT
   c0                                      | == Indexed - Add ==
                                           |   idx = 64
                                           | -> location:
                                           |   https://www.example.com
   5a                                      | == Literal indexed ==
                                           |   Indexed name (idx = 26)
                                           |     content-encoding
   83                                      |   Literal value (len = 3)
                                           |     Huffman encoded:
   9bd9 ab                                 | ...
                                           |     Decoded:
                                           | gzip
                                           | - evict: date: Mon, 21 Oct
                                           |    2013 20:13:21 GMT
                                           | -> content-encoding: gzip
   77                                      | == Literal indexed ==
                                           |   Indexed name (idx = 55)
                                           |     set-cookie
   ad                                      |   Literal value (len = 45)
                                           |     Huffman encoded:
   94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
   d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
   3160 65c0 03ed 4ee5 b106 3d50 07        | 1`e...N...=P.
                                           |     Decoded:
                                           | foo=ASDJKHQKBZXOQWEOPIUAXQ
                                           | WEOIU; max-age=3600; versi



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                                           | on=1
                                           | - evict: location:
                                           |   https://www.example.com
                                           | - evict: :status: 307
                                           | -> set-cookie: foo=ASDJKHQ
                                           |   KBZXOQWEOPIUAXQWEOIU; ma
                                           |   x-age=3600; version=1

   Dynamic Table (after decoding):

   [  1] (s =  98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU;
                    max-age=3600; version=1
   [  2] (s =  52) content-encoding: gzip
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:22 GMT
         Table size: 215

   Decoded header list:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   location: https://www.example.com
   content-encoding: gzip
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1



























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RFC 7541                          HPACK                         May 2015


Acknowledgments

   This specification includes substantial input from the following
   individuals:

   o  Mike Bishop, Jeff Pinner, Julian Reschke, and Martin Thomson
      (substantial editorial contributions).

   o  Johnny Graettinger (Huffman code statistics).

Authors' Addresses

   Roberto Peon
   Google, Inc

   EMail: fenix@google.com


   Herve Ruellan
   Canon CRF

   EMail: herve.ruellan@crf.canon.fr





























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