Difference between revisions of "TCP connection"

Revision as of 16:18, 25 July 2012

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-This page describes how we handle established TCP connections
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-== TCP repair mode in kernel ==
-The sockoption called TCP_REPAIR was recently added to the kernel and helps with doing C/R for TCP sockets.
-When set this option turn the socket into a special state in which any action performed on it doesn't
-result in any defined by protocol actions, but instead directly puts the socket into a state, which
-should be at the end of the successfully finished operation.
-E.g. calling connect() on a repaired socket just switches one to the ESTABLISHED state with the peer set as requested.
-The bind() call forcibly binds the socket to a given address (ignoring any potential conflicts). Close()-ing the
-socket under repair happens without any transient FIN_WAIT/TIME_WAIT/etc states. Socket is silently killed.
-=== Sequences ===
-In order to restore the connection properly only binding and connecting it is not enough. One also needs to restore the
-TCP sequence numbers. To do so the TCP_REPAIR_QUEUE and TCP_QUEUE_SEQ options were introduced.
-The former one selects which queue (input or output) will be repaired and the latter gets/sets the sequence. Note, that
-setting the sequence is only possible on CLOSE-d socket.
-=== Packets in queue ===
-When set the queue to repair as described above, one can call recv or send syscalls on a repaired socket. Both calls
-result on peeking or poking data from/to the respective queue. This sounds funny, but yes, for repaired socket one
-can receve the outgoing and send the incoming queues. Using the MSG_PEEK flag for recv is required.
-=== Options ===
-There are 4 options that are negotiated by the socket at the connecting stage. These are
-* mss_clamp -- the maximum size of the segment peer is ready to accept
-* snd _scale -- the scale factor for a window
-* sack -- whether selective acks are permitted or not
-* tstamp -- whether timestamps on packets are supported
-All four can be read with getsockopt calls to a socket and in order to restore them the TCP_REPAIR_OPTIONS sockoption
-is introduced.
-== Checkpoint and restore TCP connection ==
-With the above sockoptions dumping and restoring TCP connection becomes possible. The crtools just reads the socket
-state and restores it back letting the protocol resurrect the data sequence.
-One thing to note here -- while the socket is closed between dump and restore the connection should be "locked", i.e.
-no packets from peer should enter the stack, otherwise the RST will be sent by a kernel. In order to do so a simple
-netfilter rule is configured that drops all the packets from peer to a socket we're dealing with. This rule sits
-in the host netfilter tables after the crtools dump command finishes and it should be there when you issue the
-crtools restore one.
-That said, the command line option --tcp-established should be used when calling crtools to explicitly state, that the
-caller is aware of this "transitional" state of the netfilter.

Difference between revisions of "TCP connection"

Revision as of 16:18, 25 July 2012

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