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In TCP 3-way handshake, 3 segments will be sent (SYN, SYN/ACK, ACK). What happens if the third segment(ACK) is lost?

asked 2019-08-30 06:19:26 +0000

net user gravatar image

In TCP 3-way handshake, 3 segments will be sent (SYN, SYN/ACK, ACK). What happens to the destination host if the source host fails to acknowledge the SYN/ACK segment? (In other words third segment ACK is lost)

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answered 2019-09-01 06:54:03 +0000

hackslash gravatar image

In general , if client send syn, the server will return an ack, and server will set a expired time named "RTO", which means "retranmission time out";if client ack didnt arrive in time , it will trigger "tcp retranmission " ; if client ack didnt arrive in time again, send "tcp retranmission " again and set "RTO" to double or incrase in power; In the end , the server will think client has lost , so server will send "Reset " to client and tell client to release this connection; Most devices use this way to connect and release, but if there is a firewall , then the case is different

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answered 2019-08-30 11:04:14 +0000

SYN-bit gravatar image

That depends on the protocol. If it is a protocol where the client sends data first, then after the ACK, there will be a data packet which contains the same ACK value, so all is fine.

If it is a protocol where the server sends data first, it will retransmit the SYN/ACK until it does receive an ACK and then all is fine.

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Thank you for the response.

net user gravatar imagenet user ( 2019-08-30 21:35:47 +0000 )edit

Sake is correct, of course. I just need to add (my OCD kicks in here) that if there is never an ACK (which can happen in some situations) the server will at some point give up and most likely send an RST packet to reset the connection.

Jasper gravatar imageJasper ( 2019-09-01 21:27:44 +0000 )edit

Thanks for the addition @Jasper.

SYN-bit gravatar imageSYN-bit ( 2019-09-02 11:33:51 +0000 )edit

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Asked: 2019-08-30 06:19:26 +0000

Seen: 106 times

Last updated: Sep 01