Re: Straw Poll: Restore Header Table and Static Table Indices

[Mark Nottingham <[e-mail]>]

Thanks for that, Willy.

At the risk of repeating myself -- making this kind of change (one that does not impact security or interoperability) at this stage in our process needs to have *strong* consensus. 

What do others think?

Regards,


> On 21 Oct 2014, at 3:53 am, Willy Tarreau <[e-mail]> wrote:
> 
> Hi Mark,
> 
> On Fri, Oct 17, 2014 at 10:23:35AM +1100, Mark Nottingham wrote:
>> I think we need to see a proposal here if we?re going to take this seriously
>> ? i.e., either fairly complete text or a pull request.
> 
> Here's the change I'm proposing. It's a diff against the text version because
> it was easier for me to produce this, and it's easily readable by everyone here.
> 
> The diff looks like what's below, I think it addresses what was discussed in
> a way that is pretty straightforward since there's no more the need to explain
> the split between the two tables.
> 
> I have not changed anything else eventhough I'm seeing room for improvement
> (eg: swapping encoding prefix of "literal without indexing" with the one
> for "dynamic table size update" and gain one bit of encoding per header).
> 
> Anyone would please review and/or comment ?
> 
> Thanks,
> Willy
> 
> ------------
> 
> diff --git a/compression.txt b/compression.txt
> index a4c8a40..56b2ea0 100644
> --- a/compression.txt
> +++ b/compression.txt
> @@ -309,9 +309,6 @@ Internet-Draft                    HPACK                     October 2014
>    (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
> @@ -348,37 +345,21 @@ Internet-Draft                    HPACK                     October 2014
> 
> 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).
> -
> +   Both the static and the dynamic tables are indexed starting at 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.
> +   Static table indices strictly greater than the length of the table as
> +   defined in Appendix A MUST be treated as a decoding error. Dynamic table
> +   indices strictly greater than the length of the dynamic table 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
> 
> 2.4.  Header Field Representation
> 
>    An encoded header field can be represented either as a literal or as
> -   an index.
> +   an index in one of the tables.
> 
>    A literal representation defines a header field by specifying its
>    name and value.  The header field name can be represented literally
> @@ -740,14 +721,15 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 1 |        Index (7+)         |
> -   +---+---------------------------+
> +   | 1 | D |     Index (6+)        |
> +   +---+---+---+---+---+---+---+---+
> 
>                       Figure 5: Indexed Header Field
> 
> -   An indexed header field starts with the '1' 1-bit pattern, followed
> -   by the index of the matching pair, represented as an integer with a
> -   7-bit prefix (see Section 5.1).
> +   An indexed header field starts with the '1' 1-bit pattern, followed by
> +   the D bit indicating if the index references the static table (0) or
> +   the dynamic table (1), and the index of the matching pair, represented
> +   as an integer with a 6-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.
> @@ -788,8 +770,8 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 0 | 1 |      Index (6+)       |
> -   +---+---+-----------------------+
> +   | 0 | 1 | D |  Index (5+)       |
> +   +---+---+---+-------------------+
>    | H |     Value Length (7+)     |
>    +---+---------------------------+
>    | Value String (Length octets)  |
> @@ -800,8 +782,8 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 0 | 1 |           0           |
> -   +---+---+-----------------------+
> +   | 0 | 1 | 0 |       0           |
> +   +---+---+---+-------------------+
>    | H |     Name Length (7+)      |
>    +---+---------------------------+
>    |  Name String (Length octets)  |
> @@ -819,11 +801,13 @@ Internet-Draft                    HPACK                     October 2014
>    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.
> +   the index of the entry is represented as the D bit indicating if the
> +   index references the static table (0) or the dynamic table (1),
> +   followed by an integer with a 5-bit prefix (see Section 5.1).  This
> +   value is always non-zero.
> 
>    Otherwise, the header field name is represented as a literal string
> -   (see Section 5.2).  A value 0 is used in place of the 6-bit index,
> +   (see Section 5.2).  A value 0 is used in place of the 5-bit index,
>    followed by the header field name.
> 
>    Either form of header field name representation is followed by the
> @@ -844,8 +828,8 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 0 | 0 | 0 | 0 |  Index (4+)   |
> -   +---+---+-----------------------+
> +   | 0 | 0 | 0 | 0 | D | Index (3+)|
> +   +---+---+---+---+---+-----------+
>    | H |     Value Length (7+)     |
>    +---+---------------------------+
>    | Value String (Length octets)  |
> @@ -855,8 +839,8 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 0 | 0 | 0 | 0 |       0       |
> -   +---+---+-----------------------+
> +   | 0 | 0 | 0 | 0 | 0 |     0     |
> +   +---+---+---+---+---+-----------+
>    | H |     Name Length (7+)      |
>    +---+---------------------------+
>    |  Name String (Length octets)  |
> @@ -874,11 +858,13 @@ Internet-Draft                    HPACK                     October 2014
>    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.
> +   the index of the entry is represented as the D bit indicating if the
> +   index references the static table (0) or the dynamic table (1),
> +   followed by as an integer with a 3-bit prefix (see Section 5.1).
> +   This value is always non-zero.
> 
>    Otherwise, the header field name is represented as a literal string
> -   (see Section 5.2).  A value 0 is used in place of the 4-bit index,
> +   (see Section 5.2).  A value 0 is used in place of the 3-bit index,
>    followed by the header field name.
> 
>    Either form of header field name representation is followed by the
> @@ -900,8 +886,8 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 0 | 0 | 0 | 1 |  Index (4+)   |
> -   +---+---+-----------------------+
> +   | 0 | 0 | 0 | 1 | D | Index (3+)|
> +   +---+---+---+---+---+-----------+
>    | H |     Value Length (7+)     |
>    +---+---------------------------+
>    | Value String (Length octets)  |
> @@ -911,8 +897,8 @@ Internet-Draft                    HPACK                     October 2014
> 
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
> -   | 0 | 0 | 0 | 1 |       0       |
> -   +---+---+-----------------------+
> +   | 0 | 0 | 0 | 1 | 0 |     0     |
> +   +---+---+---+---+---+-----------+
>    | H |     Name Length (7+)      |
>    +---+---------------------------+
>    |  Name String (Length octets)  |
> @@ -957,7 +943,7 @@ Internet-Draft                    HPACK                     October 2014
>      0   1   2   3   4   5   6   7
>    +---+---+---+---+---+---+---+---+
>    | 0 | 0 | 1 |   Max size (5+)   |
> -   +---+---------------------------+
> +   +---+---+---+-------------------+
> 
>                Figure 12: Maximum Dynamic Table Size Change
> 
> ------------

--
Mark Nottingham   https://www.mnot.net/