TCP之ACK/DUPACK

RFC中ACK/DUPACK的说明如下：

   The delayed ACK algorithm specified in [RFC1122] SHOULD be used by a
   TCP receiver.  When using delayed ACKs, a TCP receiver MUST NOT
   excessively delay acknowledgments.  Specifically, an ACK SHOULD be
   generated for at least every second full-sized segment, and MUST be
   generated within 500 ms of the arrival of the first unacknowledged
   packet.

   The requirement that an ACK "SHOULD" be generated for at least every
   second full-sized segment is listed in [RFC1122] in one place as a
   SHOULD and another as a MUST.  Here we unambiguously state it is a
   SHOULD.  We also emphasize that this is a SHOULD, meaning that an