when TCP RST packet generated? what are the possible cause of generation on TCP RST packet?
The RST flag is used to signal any kind of error and terminate the connection.
If you send a packet with a wrong ACK, you will get a RST
If you don't acknowledge data in a timely manner, you get a RST
Basically, if you do anything wrong, a TCP will send a RST and terminate the connection.
It is also possible that Linux Kernel sent these RSTs.
An example:
http://blog.facilelogin.com/2010/12/hand-crafting-tcp-handshake-with-scapy.html
Related
I had an opportunity to work with wireshark.
I installed wireshark on my VMware and made some tests on it, I found an interesting thing.
I sent the HTTP request to non-existing computer and captured all the packet flow with wireshark.
The first packet was SYN packet, the second one was re-transmission packet.(the system hadn't received any response from the targeted computer).
But surprisingly, RST packet from the target computer reached.
I couldn't understand where this packet came from, so I tested for 2 target computers, and in both cases, I had the RST packet.
So are there anyone who can tell me about those suspicious RST packets?
Thanks for reading.
Regards.
https://ask.wireshark.org/upfiles/img2_fpJSr9b.png
https://ask.wireshark.org/upfiles/img3_Rxmvejp.png
I am facing a problem related to the TCP retransmissions.
My Sender starts sending some data to receiver (which is not in the network after opening the connection), after sending 3 packets, it retransmits first packet 3 times (as per the retransmission timeouts)and start sending next packets.
Then it retransmits first packet again. I am not able to understand this behavior and want to know if there is some way I can disable this and force TCP to retransmit first packet and then close the connection if no ack is received.
Thanks.
No there isn't. It's a streaming protocol, not a datagram protocol.
if there is a tcp connection between A and B,
A send some packets and then a TCP RST(or TCP FIN/ACK) to close the connection,
let me say?
PKT1, PKT2, PKT3, TCP_RST
or
PKT1, PKT2, PKT3, TCP_FIN/ACK
but the packet arrival is out of order
PKT1, TCP_RST(or TCP_FIN/ACK), PKT2, PKT3
then how will B react?
according to the sequence number of TCP_RST and TCP_FIN/ACK,
B knows there are some packets missing(PKT2 and PKT3),
will B wait for PKT2 and PKT3 before it close the connection,
or B immediately close the connection when it receives TCP_RST(or TCP_FIN/ACK)?
thanks
The TCP protocol will reorder the packets before sending them further up the stack. This means it will wait for out of order packets according to the sequence number, ask for retransmission if needed, etc. and wait for the last ack before closing the connection.
You can find the TCP state diagram here:
http://www.ssfnet.org/Exchange/tcp/tcpTutorialNotes.html#ST
TCP guarantees sequence. That includes the sequence of the EOS. It must be delivered after all the data.
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?
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.
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".
The way I understand this, there are 2 ways to close TCP connection:
send FIN flag
send RST flag
RST causes immediate connection termination, while in FIN you get a confirmation.
Do I understand this right, and are there any other distinctions between the two? Can those 2 flags be used together?
FIN says: "I finished talking to you, but I'll still listen to everything you have to say until you say that you're done."
RST says: "There is no conversation. I won't say anything and I won't listen to anything you say."
RST is useful if you have long lasting TCP connection with little traffic. If one of the computers is restarted, it forgets about the connection, and the other computer gets RST, as soon as it sends another packet.
FIN or RST would be sent in the following case
your process close the socket
OS is doing the resource cleanup when your process exit without closing socket.
If your process call close(), FIN would be sent from the closing side by default (note: you can set socket option SO_LINGER to make it send RST instead of FIN)
If your process exit without closing the socket, kernel would close the tcp connection and do the clean up for your process. FIN or RST can be sent. If there is data in your receive queue, RST would be sent. Otherwise, FIN would be sent.
You can loop through tcp_close() in tcp.c for more details.(I am using kernel-2.6.32-573.7.1 from redhat branch)
From RFC 1122, which everybody keeps citing, but not actually quoting, against me:
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.
It is not possible to use both at the same time. The concept doesn't even begin to make sense.
It is possible by means of trickery which I will not describe here to close a TCP connection with an RST instead of a FIN, but it's a stupid idea, which is why I am not documenting it. For one thing, all pending data in flight is lost.