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2.3.4. Credit Window Status

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Original RFC Text​

2.3.4.  Credit Window Status

   The Credit Window Status Data Item is used by a router to report the
   current credit window size to its peer modem.  One or more Credit
   Window Status Data Items MAY be carried in a Destination Up Response
   Message or a Credit Control Response Message.  As discussed in
   Section 2.3.3, the Destination Up Response Message is used when the
   Data Item is sent in response to a Destination Up Message, and the
   Credit Control Response Message is sent in response to a Credit
   Control Message.  Multiple Credit Window Status Data Items in a
   single message are used to indicate different sizes of different
   credit windows.  Similar to the Credit Window Grant Data Item, credit
   windows are identified using FID values that have been previously
   sent by the modem.

   The format of the Credit Window Status Data Item is as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Data Item Type                | Length (12)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Flow Identifier (FID)         |            Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Current Credit Window Size                   |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:
      33

   Length:
      12

      As specified in [RFC8175], Length is the number of octets in the
      Data Item.  It MUST be equal to twelve (12).  If it is some other
      value, the Data Item is corrupt, so the message in which it
      appears cannot be reliably parsed and is ignored.

   Flow Identifier (FID):
      A 2-octet flow identifier as defined by the Traffic Classification
      Data Item.  The FID also uniquely identifies a credit window.

   Reserved:
      For the Credit Window Status Data Item, this reserved field is
      currently unused.  It MUST be set to all zeros for this version of
      the Data Item and is currently ignored on reception.  This allows
      for future extensions of the Data Item if needed.

   Current Credit Window Size:
      A 64-bit unsigned integer indicating the current number of
      credits, in octets, available for the router to send to the modem.
      This is referred to as the Modem Receive Window in
      [Credit-Window-Extension].

   When receiving this Data Item, a modem MUST identify the credit
   window indicated by the FID.  If the FID is not known to the modem,
   it SHOULD log this information and discard the Data Item.  The method
   of logging is left to the modem implementation.  As with the Credit
   Window Grant Data Item, the FID MAY be unrelated to the destination
   indicated in the message carrying the Data Item.

   Once the credit window is identified, the modem SHOULD check the
   received Current Credit Window Size field value against the
   outstanding credit window's available credits at the time the most
   recent Credit Window Initialization or Credit Window Grant Data Item
   associated with the indicated FID was sent.  If the difference in
   values is greater than what can be accounted for based on observed
   data frames, then the modem SHOULD send a Credit Window
   Initialization Data Item to reset the associated credit window size
   to the modem's current view of the available credits.  As specified
   in Section 2.3.1, Credit Window Initialization Data Items are sent in
   Session Update Messages.  When multiple Data Items need to be sent,
   they SHOULD be combined into a single message when possible.
   Alternatively, and also in cases where there are small differences,
   the modem MAY adjust the values sent in Credit Window Grant Data
   Items to account for the reported credit window.
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