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Re: Integer Representation in header-compression-draft-03

From: Roberto Peon <grmocg@gmail.com>
Date: Thu, 17 Oct 2013 16:45:08 -0700
Message-ID: <CAP+FsNcApTmgqj05e=eXdWpjJJh7BCxZjf0FE8N8GEP8-YR7OA@mail.gmail.com>
To: Mike Bishop <Michael.Bishop@microsoft.com>
Cc: "Kulkarni, Saurabh" <sakulkar@akamai.com>, HTTP Working Group <ietf-http-wg@w3.org>
I've integrated Fred's suggestion into the github spec version (i.e. N is
always between 1 and 8)

Mike-- any suggestions on further clarification?

(imho, it is suboptimal to assume N=0, as you lose 127 points of codespace
instead of only one.)
-=R


On Thu, Oct 17, 2013 at 4:41 PM, Mike Bishop
<Michael.Bishop@microsoft.com>wrote:

>  Looks like an interpretational difference that needs to be clarified,
> because Firefox looks exactly correct to me.****
>
> ** **
>
> I had interpreted a field being “8+” bits long would be a zero-bit prefix
> integer.  (i.e. N=0, so the partial byte is absent, and you always have at
> least one byte which can represent numbers 0-127)  Certain instances
> explicitly call out zero-bit prefixes on byte boundaries, so I assumed they
> all were.  The spec needs to be consistent about whether integers starting
> on a byte boundary have an eight-bit or a zero-bit prefix, and an example
> would be good for this.****
>
> ** **
>
> With a zero-bit prefix, that’s the correct encoding for 159.  159 is
> 0b10011111.  You only get seven bits of value in the first byte because one
> is reserved for the continuation – which just happens to be the same bit
> that would be set if representing 159 on eight bits.  So the first byte is
> 0b10011111, followed by a second byte with the extra bit, 0b00000001.****
>
> ** **
>
> *From:* Roberto Peon [mailto:grmocg@gmail.com]
> *Sent:* Thursday, October 17, 2013 4:37 PM
> *To:* Kulkarni, Saurabh
> *Cc:* HTTP Working Group
> *Subject:* Re: Integer Representation in header-compression-draft-03****
>
> ** **
>
> Saurabh--****
>
> ** **
>
> Thanks for this.****
>
> It looks like Firefox is getting this wrong, per my interpretation of what
> is supposed to happen here.****
>
> Indeed, though poorly specified, the intent is for the name-length and
> value-list-length fields, N is 8 since there are 8 bits available for
> length up to the next byte boundary, and so any value under 0xFF is (or
> should be) encodable on that byte.****
>
> ** **
>
> -=R****
>
> ** **
>
> On Thu, Oct 17, 2013 at 4:23 PM, Kulkarni, Saurabh <sakulkar@akamai.com>
> wrote:****
>
>  I was debugging my server (Akamai Ghost) with Firefox nightly for
> draft-06 and noticed a discrepancy with the way integer values are being
> represented in header compression. I shot an individual mail to Patrick
> just in case this is a false alarm, or people talked about this offline.**
> **
>
> ** **
>
> So header-compression-draft-03 says:****
>
> "The N-bit prefix allows filling the current byte. If the value is****
>
>  small enough (strictly less than 2^N-1), it is encoded within the****
>
>  N-bit prefix. Otherwise all the bits of the prefix are set to 1 and****
>
>  the value is encoded using an unsigned variable length integer [1]****
>
>  representation."****
>
> ** **
>
> For representing lengths of header values the draft-03 says its 8+ meaning
> N=8. Which corresponds to <255 values can be encoded in 1 byte. But since
> the algorithm uses the MSB for signaling whether to consume the next byte,
> henceforth N needs to be 7. This is potentially confusing. I encountered
> this issue when I received a cookie value of length 159 which can
> potentially be encoded as 1/2 bytes (which is true to all values > 128 and
> < 255). ****
>
> ** **
>
> Firefox encoded this as: 159 = \159\001, but it can also be encoded as
> just \159.****
>
> ** **
>
> Please clarify the text in the draft, because +/- 1 byte can throw-off the
> compressor completely for the subsequent values.****
>
> ** **
>
> Thanks,****
>
> Saurabh****
>
> ** **
>
>  ** **
>
Received on Thursday, 17 October 2013 23:45:36 UTC

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