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原帖由 chenzhanyiczy 于 2008-11-11 14:03 发表 ![]()
就按你说的，正常的关闭流程，我们讨论的是为什么不按4次握手流程去走
为什么非要按4次握手流程呢？rfc1122说的很清楚了，我把这篇贴全吧
4.2.2.13 Closing a Connection: RFC-793 Section 3.5
A TCP connection may terminate in two ways: (1) the normal
TCP close sequence using a FIN handshake, and (2) an "abort"
in which one or more RST segments are sent and the
connection state is immediately discarded. If a TCP
connection is closed by the remote site, the local
application MUST be informed whether it closed normally or
was aborted.
The normal TCP close sequence delivers buffered data
reliably in both directions. Since the two directions of a
TCP connection are closed independently, it is possible for
a connection to be "half closed," i.e., closed in only one
direction, and a host is permitted to continue sending data
in the open direction on a half-closed connection.
A host MAY implement a "half-duplex" TCP close sequence, so
that an application that has called CLOSE cannot continue to
read data from the connection. If such a host issues a
CLOSE call while received data is still pending in TCP, or
if new data is received after CLOSE is called, its TCP
SHOULD send a RST to show that data was lost.
When a connection is closed actively, it MUST linger in
TIME-WAIT state for a time 2xMSL (Maximum Segment Lifetime).
However, it MAY accept a new SYN from the remote TCP to
reopen the connection directly from TIME-WAIT state, if it:
(1) assigns its initial sequence number for the new
connection to be larger than the largest sequence
number it used on the previous connection incarnation,
and
(2) returns to TIME-WAIT state if the SYN turns out to be
an old duplicate.
DISCUSSION:
TCP's full-duplex data-preserving close is a feature
that is not included in the analogous ISO transport
protocol TP4.
Some systems have not implemented half-closed
connections, presumably because they do not fit into
the I/O model of their particular operating system. On
these systems, once an application has called CLOSE, it
can no longer read input data from the connection; this
is referred to as a "half-duplex" TCP close sequence.
The graceful close algorithm of TCP requires that the
connection state remain defined on (at least) one end
of the connection, for a timeout period of 2xMSL, i.e.,
4 minutes. During this period, the (remote socket,
local socket) pair that defines the connection is busy
and cannot be reused. To shorten the time that a given
port pair is tied up, some TCPs allow a new SYN to be
accepted in TIME-WAIT state.
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楼主: wishel
发表于 2008-11-11 11:58
发表于 2008-11-11 13:33
