1. 07 2月, 2012 1 次提交
    • A
      tipc: Major redesign of broadcast link ACK/NACK algorithms · 7a54d4a9
      Allan Stephens 提交于
      Completely redesigns broadcast link ACK and NACK mechanisms to prevent
      spurious retransmit requests in dual LAN networks, and to prevent the
      broadcast link from stalling due to the failure of a receiving node to
      acknowledge receiving a broadcast message or request its retransmission.
      
      Note: These changes only impact the timing of when ACK and NACK messages
      are sent, and not the basic broadcast link protocol itself, so inter-
      operability with nodes using the "classic" algorithms is maintained.
      
      The revised algorithms are as follows:
      
      1) An explicit ACK message is still sent after receiving 16 in-sequence
      messages, and implicit ACK information continues to be carried in other
      unicast link message headers (including link state messages).  However,
      the timing of explicit ACKs is now based on the receiving node's absolute
      network address rather than its relative network address to ensure that
      the failure of another node does not delay the ACK beyond its 16 message
      target.
      
      2) A NACK message is now typically sent only when a message gap persists
      for two consecutive incoming link state messages; this ensures that a
      suspected gap is not confirmed until both LANs in a dual LAN network have
      had an opportunity to deliver the message, thereby preventing spurious NACKs.
      A NACK message can also be generated by the arrival of a single link state
      message, if the deferred queue is so big that the current message gap
      cannot be the result of "normal" mis-ordering due to the use of dual LANs
      (or one LAN using a bonded interface). Since link state messages typically
      arrive at different nodes at different times the problem of multiple nodes
      issuing identical NACKs simultaneously is inherently avoided.
      
      3) Nodes continue to "peek" at NACK messages sent by other nodes. If
      another node requests retransmission of a message gap suspected (but not
      yet confirmed) by the peeking node, the peeking node forgets about the
      gap and does not generate a duplicate retransmit request. (If the peeking
      node subsequently fails to receive the lost message, later link state
      messages will cause it to rediscover and confirm the gap and send another
      NACK.)
      
      4) Message gap "equality" is now determined by the start of the gap only.
      This is sufficient to deal with the most common cases of message loss,
      and eliminates the need for complex end of gap computations.
      
      5) A peeking node no longer tries to determine whether it should send a
      complementary NACK, since the most common cases of message loss don't
      require it to be sent. Consequently, the node no longer examines the
      "broadcast tag" field of a NACK message when peeking.
      Signed-off-by: NAllan Stephens <allan.stephens@windriver.com>
      Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
      7a54d4a9
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