A "keep-alive" mechanism periodically probes the other end of a connection when the connection is otherwise idle, even when there is no data to be sent. The TCP specification does not include a keep-alive mechanism because it could: (1) cause perfectly good connections to break during transient Internet failures; (2) consume unnecessary bandwidth ("if no one is using the connection, who cares if it is still good?"); and (3) cost money for an Internet path that charges for packets.
Some TCP implementations, however, have included a keep-alive mechanism. To confirm that an idle connection is still active, these implementations send a probe segment designed to elicit a response from the peer TCP. Such a segment generally contains SEG.SEQ = SND.NXT-1 and may or may not contain one garbage octet of data. Note that on a quiet connection SND.NXT = RCV.NXT, so that this SEG.SEQ will be outside the window. Therefore, the probe causes the receiver to return an acknowledgment segment, confirming that the connection is still live. If the peer has dropped the connection due to a network partition or a crash, it will respond with a RST instead of an acknowledgment segment.
Unfortunately, some misbehaved TCP implementations fail to respond to a segment with SEG.SEQ = SND.NXT-1 unless the segment contains data. Alternatively, an implementation could determine whether a peer responded correctly to keep-alive packets with no garbage data octet.
A TCP keep-alive mechanism should only be invoked in server applications that might otherwise hang indefinitely and consume resources unnecessarily if a client crashes or aborts a connection during a network failure.
这个资料说，有一种方式是 “ Such a segment generally contains SEG.SEQ = SND.NXT-1 ”，所以我们看到是288=289-1，然后 may or may not contain one garbage octet of data，
文章也说了 ，不幸的是，有些 TCP的实现不支持 SEG.SEQ = SND.NXT-1"some misbehaved TCP implementations fail to respond to a segment with SEG.SEQ = SND.NXT-1" ,但是又来了一个unless ，“ unless the segment contains data”，
我这里有一个包，你看看 |
发表于 2008-7-9 18:05:13
发表于 2008-7-9 21:36:12