TCP half-open: Difference between revisions

Revision as of 15:30, 30 August 2010

Revision as of 11:49, 11 November 2011

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The term '''half-open''', when referring to [[Transmission Control Protocol|TCP]] connections, haswhich state is out of synchronization between the two distinctcommunicating hosts, possibly due to crash of one side. A connection which is in the process of being established, also known as '''embryonic connection''', lack of synchronization could be due to [[SYN flood|malicious meaningsintent]].

== RFC 793 ==

According to RFC 793, a TCP connection is referred to as ''half-open'' when the host at one end of that TCP connection has crashed, or has otherwise removed the socket without notifying the other end. If the remaining end is idle, the connection may remain in the half-open state for unbounded periods of time.

== Embryonic connection ==

Nowadays, however, the term ''half-open connection'' is most often used to describe [[an '''embryonic connection''', i.e. a [[Transmission Control Protocol|TCP]]s connection which is in the process of being established.

The TCP protocol has a three state system for opening a connection. First, the originating endpoint (A) sends a [[SYN (TCP)|SYN packet]] to the destination (B). A is now in an embryonic state (specifically, SYN_SENT), and awaiting a response. B now updates its kernel information to indicate the incoming connection from A, and sends out a request to open a channel back (the SYN/ACK packet).

At this point, B is also in an embryonic state (specifically, SYN_RCVD). Note that B was put into this state by another machine, outside of B's control.

Under normal circumstances (see [[denial-of-service attack]] for deliberate failure cases), A will receive the SYN/ACK from B, update its tables (which now have enough information for A to both send and receive), and send a final ACK back to B.

Once B receives this final ACK, it also has sufficient information for two-way communication, and the connection is fully open. Both endpoints are now in an established state.

== See also ==

* [[Transmission Control Protocol]]

* [[EmbryonicSYN connectionflood]]

* [[SYN cookies]]

== External links ==

* [http://www.ietf.org/rfc/rfc0793.txt Transmission Control Protocol DARPA Internet Program Protocol Specification]

[[Category:TCP/IP|Half-Open]]

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