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    dccp: Refine the wait-for-ccid mechanism · 146993cf
    Gerrit Renker 提交于
    This extends the existing wait-for-ccid routine so that it may be used with
    different types of CCID. It further addresses the problems listed below.
    
    The code looks if the write queue is non-empty and grants the TX CCID up to
    `timeout' jiffies to drain the queue. It will instead purge that queue if
     * the delay suggested by the CCID exceeds the time budget;
     * a socket error occurred while waiting for the CCID;
     * there is a signal pending (eg. annoyed user pressed Control-C);
     * the CCID does not support delays (we don't know how long it will take).
    
    
                     D e t a i l s  [can be removed]
                     -------------------------------
    DCCP's sending mechanism functions a bit like non-blocking I/O: dccp_sendmsg()
    will enqueue up to net.dccp.default.tx_qlen packets (default=5), without waiting
    for them to be released to the network.
    
    Rate-based CCIDs, such as CCID3/4, can impose sending delays of up to maximally
    64 seconds (t_mbi in RFC 3448). Hence the write queue may still contain packets
    when the application closes. Since the write queue is congestion-controlled by
    the CCID, draining the queue is also under control of the CCID.
    
    There are several problems that needed to be addressed:
     1) The queue-drain mechanism only works with rate-based CCIDs. If CCID2 for
        example has a full TX queue and becomes network-limited just as the
        application wants to close, then waiting for CCID2 to become unblocked could
        lead to an indefinite  delay (i.e., application "hangs").
     2) Since each TX CCID in turn uses a feedback mechanism, there may be changes
        in its sending policy while the queue is being drained. This can lead to
        further delays during which the application will not be able to terminate.
     3) The minimum wait time for CCID3/4 can be expected to be the queue length
        times the current inter-packet delay. For example if tx_qlen=100 and a delay
        of 15 ms is used for each packet, then the application would have to wait
        for a minimum of 1.5 seconds before being allowed to exit.
     4) There is no way for the user/application to control this behaviour. It would
        be good to use the timeout argument of dccp_close() as an upper bound. Then
        the maximum time that an application is willing to wait for its CCIDs to can
        be set via the SO_LINGER option.
    
    These problems are addressed by giving the CCID a grace period of up to the
    `timeout' value.
    
    The wait-for-ccid function is, as before, used when the application 
     (a) has read all the data in its receive buffer and
     (b) if SO_LINGER was set with a non-zero linger time, or
     (c) the socket is either in the OPEN (active close) or in the PASSIVE_CLOSEREQ
         state (client application closes after receiving CloseReq).
    
    In addition, there is a catch-all case by calling __skb_queue_purge() after 
    waiting for the CCID. This is necessary since the write queue may still have
    data when
     (a) the host has been passively-closed,
     (b) abnormal termination (unread data, zero linger time),
     (c) wait-for-ccid could not finish within the given time limit.
    Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
    146993cf
timer.c 7.5 KB