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2.3.3. Credit Window Grant

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Original RFC Text​

2.3.3.  Credit Window Grant

   The Credit Window Grant Data Item is used by a modem to provide
   credits to a router.  One or more Credit Window Grant Data Items MAY
   be carried in the DLEP Destination Up, Destination Announce Response,
   Destination Update, Credit Control, and Credit Control Response
   Messages.  Multiple Credit Window Grant Data Items may be present in
   a single message.  Each item grants credits to a different credit
   window and therefore references a different FID.  In all message
   types, this Data Item provides an additional number of octets to be
   added to the indicated credit window.  Credit windows are identified
   using FID values that have been previously sent by the modem or are
   listed in a Credit Window Initialization Data Item carried in the
   same message as the Data Item.

   The format of the Credit Window Grant 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           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Additional Credits                        |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:
      32

   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 indicates a credit window.

   Reserved:
      For the Credit Window Grant 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.

   Additional Credits:
      A 64-bit unsigned integer representing the credits, in octets, to
      be added to the credit window.  This value includes MAC headers as
      seen on the link between the modem and router.  A value of zero
      indicates that no additional credits are being provided.

   When receiving this Data Item, a router MUST identify the credit
   window indicated by the FID.  If the FID is not known to the router,
   it SHOULD log this information and discard the Data Item.  The method
   of logging is left to the router implementation.  It is important to
   note that while this Data Item can be received in a destination-
   specific message, credit windows are managed independently of the
   destination identified in the message carrying this Data Item, and
   the indicated FID MAY even be disjoint from the identified
   destination.

   Once the credit window is identified, the credit window size MUST be
   increased by the value contained in the Additional Credits field.  If
   the increase results in a window overflow, the credit window must be
   set to its maximum as defined by the Credit Window Max Size carried
   in the Credit Window Initialization Data Item.

   No response is sent by the router to a modem after processing a
   Credit Window Grant Data Item received in a Credit Control Response
   Message.  For each Credit Window Grant Data Item received in other
   message types, the receiving router MUST send a Credit Window Status
   Data Item reflecting the resultant credit window value of the updated
   credit windows.  When the Credit Window Grant Data Item is received
   in a Destination Up Message, the Credit Window Status Data Item(s)
   MUST be sent in the corresponding Destination Up Response Message.
   Otherwise, a Credit Control Message MUST be sent.
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