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[quicwg/base-drafts] Transport: GAP limitation in ACK Frame (#613)

Magnus Westerlund <notifications@github.com> Thu, 08 June 2017 08:46 UTC

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Date: Thu, 08 Jun 2017 01:46:28 -0700
From: Magnus Westerlund <notifications@github.com>
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Subject: [quicwg/base-drafts] Transport: GAP limitation in ACK Frame (#613)
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In draft-ietf-quic-transport-03, Section 8.2.1:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              First ACK Block Length (8/16/32/48)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  [Gap 1 (8)]  |       [ACK Block 1 Length (8/16/32/48)]     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  [Gap 2 (8)]  |       [ACK Block 2 Length (8/16/32/48)]     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  [Gap N (8)]  |       [ACK Block N Length (8/16/32/48)]     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Gap To Next Block (opt, repeated):  An unsigned number specifying the
      number of contiguous missing packets from the end of the previous
      ACK block to the start of the next.  Repeated "Num Blocks" times.

So, to my understanding the gap counter is fixed to 8 bits, and thus have a maximum GAP distance of 255. Considering that packet numbers are not required to be a continuous number, nor have a limit that the largest gap is less than 255, I wonder how one handles situations where the GAP between received packet numbers are larger than 255? Do one use multiple ACK frames, where each acking the sub-set of packet numbers that are separeted by GAPs larger than 255?



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