CODEWITHSUNDEEP
tcp
ZHOU
ZHOU

Reputation: 1181

What if a TCP handshake segment is lost?

In TCP 3-way handshake, 3 segments will be sent (SYN, SYN ACK, ACK). What if the third segment(ACK) is lost? Is the sender going to resend the segment or give up establishing the connection? And how do the two hosts know the segment is lost?

Upvotes: 29

Views: 31546

Answers (2)

Lennart Regebro
Lennart Regebro

Reputation: 172179

TCP has a sequence number in all packets. Hence it's easy to know if a packet was lost or not. If a host doesn't get an ACK on a packet he just resends it.

In most cases though, even if that ACK was lost, there will be no resending for a very simple reason. Directly after the ACK, the host that opened the TCP protocol is likely to start sending data. That data will, as all TCP packets, have an ACK number, so the recipient would get an ACK that way. Hence, the sender of the SYN-ACK should reasonably not care that it didn't get the ACK, because it gets an "implicit" ACK in the following package.

The re-send of the SYN-ACK is only necessary of there no data is received at all.

Update: I found the place in the RFC that specified exactly this:

If our SYN has been acknowledged (perhaps in this incoming segment) the precedence level of the incoming segment must match the local precedence level exactly, if it does not a reset must be sent.

In other words, if the ACK is dropped but the next packet is not dropped, then everything is fine. Otherwise, the connection must be reset. Which makes perfect sense.

Upvotes: 36

Remy Lebeau
Remy Lebeau

Reputation: 595319

I am not an expert on this particular situation, but I suspect what will happen is the client will think it is connected but the server will not. If the client tries to send data to the server, the server will reject it and send a RST packet to the client so it can reset its "connection".

Upvotes: 2

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