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RFC 7541

 
 
 

HPACK: Header Compression for HTTP/2

Part 2 of 3, p. 11 to 33
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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]

Top      Up      ToC       Page 29 
   '>' ( 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]

Top      Up      ToC       Page 30 
   '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]

Top      Up      ToC       Page 31 
       (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]

Top      Up      ToC       Page 32 
       (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]



(page 33 continued on part 3)

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