As per my understanding, if a TCP RST is sent, the sender(A) closes the socket and the connection is closed at its end. Now, since the RST is lost, the intended receiver(B) is not yet aware of the RST sent. If B now tries to send more data to A, how will A react and what response is got at B?
As per my understanding, if a TCP RST is sent, the sender(A) closes the socket
No. The sender had already closed the socket, or maybe the connection never existed in the first place. Then something arrived for the connection, then the RST was issued. [It is possible for close to issue an RST but it is both infrequent and undesirable.]
and the connection is closed at its end.
Already, if it ever existed.
Now, since the RST is lost, the intended receiver(B) is not yet aware of the RST sent. If B now tries to send more data to A, how will A react and what response is got at B?
This is no different from the original situation. There is no connection for the data to be delivered to, so an RST is issued.
Related
I am taking a packet trace and the server on my system receives a RST flag,, it then replies with a packet were the ACK,RST and PSH bits are all set. This does not seem normal to me? I m not sure why I would see the PSH bit set in a RST scenario? does anyone know why this might be?
Once the connection is established, all packets need to have ACK set and match the sequence number of the received packets for reliable transport/security. RST without ACK will not be accepted. When one side sends RST, the socket is closed immediately and the receiving side also closes the socket immediately after receiving valid RST. It does not need to be and can't be acknowledged.
after TCP handshake
A --->B Syn=x, Ack=y, len=z, ACK Flag
B --->A Syn=y, Ack=x+z, len=o, ACK Flag
A --->B Syn=x+z, Ack=y+o, len=p, ACK Flag
B --->A Syn=y+o, ACK=x+z+p,len=q, RST, ACK Flag
B closes the socket after it sends the last packet and A closes the socket after it receives it.
(not considering TCP window here, or there might be more packets from one end before the acknoledgement)
ACK Flag, acknowledgement number and the procedure of acknowledgement are related but not the same thing.
Per RFC793
RFC793
Acknowledgment Number: 32 bits
If the ACK control bit is set this field contains the value of the
next sequence number the sender of the segment is expecting to
receive. Once a connection is established this is always sent.
Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN.
The receiver of a RST first validates it, then changes state. If the receiver was in the LISTEN state, it ignores it. If the receiver was in SYN-RECEIVED state and had previously been in the LISTEN state, then the receiver returns to the LISTEN state, otherwise the receiver aborts the connection and goes to the CLOSED state. If the receiver was in any other state, it aborts the connection and advises the user and goes to the CLOSED state.
I am very new to networking, so this might sound simple. Though I have tried to look here and here and here and have got few basics of TCP, there are few questions whose answers I am not certain about.
Is a request and response part of 2 different TCP establishments. To explain that :
Is a connection established, kept alive until all packets are delivered, request sent and connection closed for each request and same happens for its response.
or
A connection is opened, request sent, connection kept alive, response arrives and connection closed.
Is the ACK number always 1 + sequence number of sent segment.
Is a request and response part of 2 different TCP establishments
You need just 3 packets to handshake and establish a bidirectional TCP connection. So no, you do not establish TCP connection for receiving and sending parts.
On the other hand, there is a sutdown() system call which allows to shutdown a part of the bidirectional connection. See man shutdown(2). So there is a possibility to establish a unidirectional connection by opening a bidirectional and then shutdown one of the sides.
Is the ACK number always 1 + sequence number of sent segment.
We usually do not send ACK for every received packet. There are also selective ACKs, retransmissions etc. So in general, the answer is no, the ACK number is not always seq + 1.
On the other hand, if you are sending a small amount of data and waiting for the confirmation, no errors or packet loss occurred, most probably there will be just one packet with that data and one ACK with seq + 1.
Hope that helps.
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.
We are facing random RST packet problem in our environments, which causes some unexpected behaviors, following image is snapshot of the tcp data generated by wireshark, which shows the problem:
Client (117.136.2.181) successfully sets up the connection with the server (192.168.40.16)
Client sends some data to the server, as well the KEEP_ALIVE signal.
Server receives the data, process it and sends the result back to client.
Server close the socket.
Server does not receive the ACK signal from client, so it re-transmits the result data as well as the FIN signal, this is automatically done by TCP protocol. However, server still does not receive the ACK signal from client.
Server sends a RST signal to client so connection is closed.
After some analysis, we think some network problem happens after step 3, so all the result data and FIN signal sent from server are not ack'd by client, but we are very confused about the RST signal sent from the server. Based on our understanding, a RST signal is sent if a half-closed socket receives some data, or if there is data in the receive queue when closes a socket. But both these seem not be the root cause of our case.
Can some one help to elaborate why this is happening?
RST usually happens when close is called on the socket without shutdown, or after a shutdown while the other party is still trying to send data (still has not replied with an FIN).
Some programming languages have a socket.close(timeout) for example .NET, that calls shutdown then close after timeout has passed.
So the client have up to timeout to finish sending and closing the connection with FIN, if it fails to do so, the connection will be forcibly closed by RST.
See https://stackoverflow.com/a/23483487/1438522 for a more thorough explanation about difference between close and shutdown.
I'm not sure how to tell in TCP when the sender finished sending me the information.
For example if A needs to send 200 bytes to B, how will B know that A finished sending, and that the information is ready to be transferred to the application layer?
There's the FIN flag, but as far as I know it's only there to symbolizes that your going to close the connection.
Thanks.
TCP has no obligation to tell the receiver when the sender has finished sending data on the connection. It can't because it has no understanding of the higher level protocol data it's transporting.
However it will tell you if the connection closes (then you'll see a FIN, FIN-ACK) sequence in a network trace. From a programming perspective, assuming that you're using C the function recv will return 0 when the peer closes the connection.
You define a protocol such as first sending a 4 byte int in a specified byte order which indicates how many bytes will follow in the message body.
If you're on unix/linux, select and poll can signal you that the other end finished transfer (did a half/full close). read will also return with an error if you've read all the data and want to read from a closed connection.
If you do multiple transfers on one connection and want to signal the end of a "package" you have to build that into your protocol.