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Integer Representation in header-compression-draft-03

From: Kulkarni, Saurabh <sakulkar@akamai.com>
Date: Thu, 17 Oct 2013 18:23:14 -0500
To: 'HTTP Working Group' <ietf-http-wg@w3.org>
Message-ID: <CE85BF72.23DE%sakulkar@akamai.com>
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 23:23:43 UTC

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