If a TCP connection is established between two hosts (A & B), and lets say host A has sent 5 octets to host B, and then the host B crashes (due to unknown reason).
The host A will wait for acknowledgments, but on not getting them, will resend octets and also reduce the sender window size.
This will repeat couple times till the window size shrinks to zero because of packet loss. My question is, what will happen next?
In this case, TCP eventually times out waiting for the ack's and return an error to the application. The application have to read/recv from the TCP socket to learn about that error, a subsequent write/send call will fail as well. Up till the point that TCP determined that the connection is gone, write/send calls will not fail, they'll succeed as seen from the application or block if the socket buffer is full.
In the case your host B vanishes after it has sent its ACKs, host A will not learn about that until it sends something to B, which will eventually also time out, or result in an ICMP error. (Typically the first write/send call will not fail as TCP will not fail the connection immediately, and keep in mind that write/send calls does not wait for ACKs until they complete).
Note also that retransmission does not reduce the window size.
Please follow this link
now a very simple answer to your question in my view is, The connection will be timed out and will be closed. another possibility that exists is that some ICMP error might be generated due to due un-responsive machine.
Also, if the crashed machine is online again, then the procedure described in the link i just pasted above will be observed.
Depends on the OS implementation. In short it will wait for ACK and resend packets until it times out. Then your connection will be torn down. To see exactly what happens in Linux look here other OSes follow similar algorithm.
in your case, A FIN will be generated (by the surviving node) and connection will eventually migrate to CLOSED state. If you keep grep-ing for netstat output on the destination ip address, you will watch the migration from ESTABLISHED state to TIMED_WAIT and then finally disappear.
In your case, this will happen since TCP keeps a timer to get the ACK for the packet it has sent. This timer is not long enough so detection will happen pretty quickly.
However, if the machine B dies after A gets ACK and after that A doesn't send anything, then the above timer can't detect the same event, however another timer (calls idle timeout) will detect that condition and connection will close then. This timeout period is high by default. But normally this is not the case, machine A will try to send stuff in between and will detect the error condition in send path.
In short, TCP is smart enough to close the connection by itself (and let application know about it) except for one case (Idle timeout: which by default is very high).
cforfun
In normal cases, each side terminating its end of the connectivity by sending a special message with a FIN(finish) bit set.
The device receiving this FIN responds with an acknowledgement to the FIN to indicate that it has been received.
The connection as a whole is not considered terminated until both the devices complete the shut down procedure by sending an FIN and receiving an acknowledgement.
Related
I have been testing a program which has simple communication between two machines over a 1Gbps line. While running TCP communications over the line I occasionally receive write errors on the client side (due to a timeout) when the network is totally flooded (running at or close to 100% usage). This generally happens when I am running multiple instances of the same program going to different ports.
My question is, is it possible to get a write error but still receive the message on the server side. It appears that is what is happening, and I am not quite sure why. Could it be that the ACK coming back to the client is what is timing out?
Yes, that is possible. TCP does not guarantee you that data you sent successfully is received and that data that is sent unsuccessfully is not received. This problem is unsolvable. It is called the Generals Problem. There is always a way to loose messages/packets such that the sender comes to the wrong conclusion. TCP guarantees that the receiver receives the same stream of bytes that the sender sent, but possibly cut off at an arbitrary point.
This unreliability has performance reasons, too. TCP data is buffered on both hosts as well as on the network. Acknowledgement is delayed.
You have to live with this. If you make your scenario more concrete I can suggest some strategies of dealing with this.
send puts data into the TCP send buffer.
If the send buffer has no enough space, send will block util the data is completely or partly copied into the send buffer, or the designed timeout arrives.
Read timeout and write timeout is OK. You should check and process them. The way is restarting read/write operation after timeout. You also pay attention to other read/write error except timeout.
I'm trying to determine if a client has closed a socket connection from netty. Is there a way to do this?
On a usual case where a client closes the socket via close() and the TCP closing handshake has been finished successfully, a channelInactive() (or channelClosed() in 3) event will be triggered.
However, on an unusual case such as where a client machine goes offline due to power outage or unplugged LAN cable, it can take a lot of time until you discover the connection was actually down. To detect this situation, you have to send some message to the client periodically and expect to receive its response within a certain amount of time. It's like a ping - you should define a periodic ping and pong message in your protocol which practically does nothing but checking the health of the connection.
Alternatively, you can enable SO_KEEPALIVE, but the keepalive interval of this option is usually OS-dependent and I would not recommend using it.
To help a user implement this sort of behavior relatively easily, Netty provides ReadTimeoutHandler. Configure your pipeline so that ReadTimeoutHandler raises an exception when there's no inbound traffic for a certain amount of time, and close the connection on the exception in your exceptionCaught() handler method. If you are the party who is supposed to send a periodic ping message, use a timer (or IdleStateHandler) to send it.
If you are writing a server, and netty is your client, then your server can detect a disconnect by calling select() or equivalent to detect when the socket is readable and then call recv(). If recv() returns 0 then the socket was closed gracefully by the client. If recv() returns -1 then check errno or equivalent for the actual error (with few exceptions, most errors should be treated as an ungraceful disconnect). The thing about unexpected disconnects is that they can take a long time for the OS to detect, so you would have to either enable TCP keep-alives, or require the client to send data to the server on a regular basis. If nothing is received from the client for a period of time then just assume the client is gone and close your end of the connection. If the client wants to, it can then reconnect.
If you read from a connection that has been closed by the peer you will get an end-of-stream indication of some kind, depending on the API. If you write to such a connection you will get an IOException: 'connection reset'. TCP doesn't provide any other way of detecting a closed connection.
TCP keep-alive (a) is off by default and (b) only operates every two hours by default when enabled. This probably isn't what you want. If you use it and you read or write after it has detected that the connection is broken, you will get the reset error above,
It depends on your protocol that you use ontop of netty. If you design it to support ping-like messages, you can simply send those messages. Besides that, netty is only a pretty thin wrapper around TCP.
Also see this SO post which describes isOpen() and related. This however does not solve the keep-alive problem.
I have an embedded device running an http client. Until now we have always used linux servers but recently have needed to use windows servers and am now having issues.
My device opens a connection sends some data and then closes it ( not neccessarily gracefully as a reset or power failure can occur anytime!).
What im finding is that after a few days at some locations only the server stops accepting new connections from the device.
Originally I thought this might be due to trying to open a connection on already estabilished port as here: What will happen if I send a SYN packet to the server when there has already been a TCP connection established?
but now that I randomize the initial port value after boot up the issue still occurs (I would also expect the old connection to timeout).
I have noticed several things that look strange:
The SYN packet has a zero window size and a zero length, is this correct?
It also has a zero sequence number, I think this is right?
I apologize for lack of detail, I may be able to put up a link to a Wireshark capture at some point. The windows servers we are dealing with are also in 'the cloud' so I have very little in the way of access to server logs (which I find a bit ridiculous).
From what I know, a blocking receive on a TCP socket does not always detect a connection error (due either to a network failure or to a remote-endpoint failure) by returning a -1 value or raising an IO exception: sometimes it could just hang indefinitely.
One way to manage this problem is to set a timeout for the blocking receive. In case an upper bound for the reception time is known, this bound could be set as timeout and the connection could be considered lost simply when the timeout expires; when such an upper bound is not known a priori, for example in a pub-sub system where a connection stays open to receive publications, the timeout to be set would be somewhat arbitrary but its expiration could trigger a ping/pong request to verify that the connection (and the endpoint too) is still up.
I wonder whether the use of asynchronous receive also manages the problem of detecting a connection failure. In boost::asio I would call socket::asynch_read_some() registering an handler to be asynchronously called, while in java.nio I would configure the channel as non-blocking and register it to a selector with an OP_READ interest flag. I imagine that a correct connection-failure detection would mean that, in the first case the handler would be called with a non-0 error_code, while in the second case the selector would select the faulty channel but a subsequent read() on the channel would either return -1 or throw an IOException.
Is this behaviour guaranteed with asynchronous receive, or could there be scenarios where after a connection failure, for example, in boost::asio the handler will never be called or in java.nio the selector will never select the channel?
Thank you very much.
I believe you're referring to the TCP half-open connection problem (the RFC 793 meaning of the term). Under this scenario, the receiving OS will never receive indication of the lost connection, so it will never notify the app. Whether the app is readding synchronously or asynchronously doesn't enter into it.
The problem occurs when the transmitting side of the connection somehow is no longer aware of the network connection. This can happen, for example, when
the transmitting OS abruptly terminates/restarts (power outage, OS failure/BSOD, etc.).
the transmitting side closes its side while there is a network disruption between the two sides and cleans up its side: e.g transmitting OS reboots cleanly during disruption, transmitting Windows OS is unplugged from the network
When this happens, the receiving side may be waiting for data or a FIN that will never come. Unless the receiving side sends a message, there's no way for it to realize the transmitting side is no longer aware of the receiving side.
Your solution (a timeout) is one way to address the issue, but it should include sending a message to the transmitting side. Again, it doesn't matter the read is synchronous or asynchronous, just that it doesn't read and wait indefinitely for data or a FIN. Another solution is using a TCP KEEPALIVE feature that is supported by some TCP stacks. But the hard part of any generalized solution is usually determining a proper timeout, since the timeout is highly dependent on characteristics of the specific application.
Because of how TCP works, you will typically have to send data in order to notice a hard connection failure, to find out that no ACK packet will ever be returned. Some protocols attempt to identify conditions like this by periodically using a keep-alive or ping packet: if one side does not receive such a packet in X time (and perhaps after trying and failing one itself), it can consider the connection dead.
To answer your question, blocking and non-blocking receive should perform identically except for the act of blocking itself, so both will suffer from this same issue. In order to make sure that you can detect a silent failure from the remote host, you'll have to use a form of keep-alive like I described.
There are 3 different accept versions in winsock. Aside from the basic accept which is there for standard compliance, there's also AcceptEx which seems the most advanced version (due to it's overlapped io capabilities), and WSAAccept. The latter supports a condition callback, which as far as I understand, allows the rejection of connection requests before they're accepted (when the SO_CONDITIONAL_ACCEPT option is enabled). None of the other versions supports this functionality.
Since I prefer to use AcceptEx with overlapped io, I wonder how come this functionality is only available in the simpler version?
I don't know enough about the inner workings of TCP to tell wehter there's actually any difference between rejecting a connection before it has been accepted, and disconnecting a socket right after a connection has been established? And if there is, is there any way to mimic the WSAAccept functionality with AcceptEx?
Can someone shed some light over this issue?
When a connection is established, the remote end sends a packet with the SYN flag set. The server answers with a SYN,ACK packet, and after that the remote end sends an ACK packet, which may already contain data.
There are two ways to break a TCP connection from forming. The first is resetting the connection - this is the same as the common "connection refused" message seen when connecting to a port nobody is listening to. In this case, the original SYN packet is answered with a RST packet, which terminates the connection immediately and is stateless. If the SYN is resent, RST will be generated from every received SYN packet.
The second is closing the connection as soon as it has been formed. On the TCP level, there is no way to close the connection both ways immediately - the only thing you can say is that "I am not going to send any more data". This happens so that when the initial SYN, SYN,ACK, ACK exchange has finished, the server sends a FIN packet to the remote end. In most cases, telling the other end with FIN that "I am not going to send any more data" makes the other end close the connection as well, and send it's own FIN packet. A connection terminated this way isn't any different from a normal connection where no data was sent for some reason. This means that the normal state tracking for TCP connections and the lingering close states will persist, just like for normal connections.
Now, on the C API side, this looks a bit different. When calling listen() on a port, the OS starts accepting connections on that port. This means that is starts replying SYN,ACK packets to connections, regardless if the C code has called accept() yet. So, on the TCP side, it makes no difference whether the connection is somehow closed before or after accept. The only additional concern is that a listening socket has a backlog, which means the number of non-accepted connections it can have waiting, before it starts saying RST to the remote end.
However, on windows, the SO_CONDITIONAL_ACCEPT call allows the application to take control of the backlog queue. This means that the server will not answer anything to a SYN packet until the application does something with the connection. This means, that rejecting connections at this level can actually send RST packets to the network without creating state.
So, if you cannot get the SO_CONDITIONAL_ACCEPT functionality enabled somehow on the socket you are using AcceptEx on, it will show up differently to the network. However, not many places actually use the immediate RST functionality, so I would think the requirement for that must mean a very specialized system indeed. For most common use cases, accepting a socket and then closing it is the normal way to behave.
I can't comment on the Windows side of things but as far as TCP is concerned, rejecting a connection is a bit different than disconnecting from it.
For one, disconnecting from a connection means there were more resources already "consumed" (e.g. ports state maintained in Firewalls & end-points, forwarding capacity used in switches/routers etc.) in both the network and the hosts. Rejecting a connection is less resource intensive.