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    dccp ccid-2: Ack Vector interface clean-up · ff49e270
    Gerrit Renker 提交于
    This patch brings the Ack Vector interface up to date. Its main purpose is
    to lay the basis for the subsequent patches of this set, which will use the
    new data structure fields and routines.
    
    There are no real algorithmic changes, rather an adaptation:
    
     (1) Replaced the static Ack Vector size (2) with a #define so that it can
         be adapted (with low loss / Ack Ratio, a value of 1 works, so 2 seems
         to be sufficient for the moment) and added a solution so that computing
         the ECN nonce will continue to work - even with larger Ack Vectors.
    
     (2) Replaced the #defines for Ack Vector states with a complete enum.
    
     (3) Replaced #defines to compute Ack Vector length and state with general
         purpose routines (inlines), and updated code to use these.
    
     (4) Added a `tail' field (conversion to circular buffer in subsequent patch).
    
     (5) Updated the (outdated) documentation for Ack Vector struct.
    
     (6) All sequence number containers now trimmed to 48 bits.
    
     (7) Removal of unused bits:
         * removed dccpav_ack_nonce from struct dccp_ackvec, since this is already
           redundantly stored in the `dccpavr_ack_nonce' (of Ack Vector record);
         * removed Elapsed Time for Ack Vectors (it was nowhere used);
         * replaced semantics of dccpavr_sent_len with dccpavr_ack_runlen, since
           the code needs to be able to remember the old run length; 
         * reduced the de-/allocation routines (redundant / duplicate tests).
    
    
    Justification for removing Elapsed Time information [can be removed]:
    ---------------------------------------------------------------------
     1. The Elapsed Time information for Ack Vectors was nowhere used in the code.
     2. DCCP does not implement rate-based pacing of acknowledgments. The only
        recommendation for always including Elapsed Time is in section 11.3 of
        RFC 4340: "Receivers that rate-pace acknowledgements SHOULD [...]
        include Elapsed Time options". But such is not the case here.
     3. It does not really improve estimation accuracy. The Elapsed Time field only
        records the time between the arrival of the last acknowledgeable packet and
        the time the Ack Vector is sent out. Since Linux does not (yet) implement
        delayed Acks, the time difference will typically be small, since often the
        arrival of a data packet triggers sending feedback at the HC-receiver.
    
    
    Justification for changes in de-/allocation routines [can be removed]:
    ----------------------------------------------------------------------
      * INIT_LIST_HEAD in dccp_ackvec_record_new was redundant, since the list
        pointers were later overwritten when the node was added via list_add();
      * dccp_ackvec_record_new() was called in a single place only;
      * calls to list_del_init() before calling dccp_ackvec_record_delete() were
        redundant, since subsequently the entire element was k-freed;
      * since all calls to dccp_ackvec_record_delete() were preceded to a call to
        list_del_init(), the WARN_ON test would never evaluate to true;
      * since all calls to dccp_ackvec_record_delete() were made from within
        list_for_each_entry_safe(), the test for avr == NULL was redundant;
      * list_empty() in ackvec_free was redundant, since the same condition is
        embedded in the loop condition of the subsequent list_for_each_entry_safe().
    Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
    ff49e270
ccid2.c 20.0 KB