Re: No error while sending via TCP Socket
On 2006-07-03, Ben Sizer <[EMAIL PROTECTED]> wrote: > Grant Edwards wrote: >> You're talking about the case where there's un-ACKed data. >> Breaking a link when there's no un-ACKed data (which is what >> the OP did) will require an hour or two to timeout _iff_ >> keepalive is enabled. If keepalive has not been enabled, a >> broken connection with no un-ACKed data will never timeout. > > My mistake, Grant. I was under the impression that a blocking > send() would wait for an ACK before proceeding, but upon > checking the man pages I see that this is not the case. It would be awfully nice if there was such an option. I've needed to do exactly that once or twice, and had to add a kernel-mode ioctl() call to the Linux kernel so I could wait until the data sent on a socket had been ACKed. The reason I needed to do that was because the application layer protocol was broken with no way to change it. > It does seem to make the oft-cited TCP guarantee of data > arrival a little weaker however, if you can't necessarily know > how many of your sends have succeeded or not. Doubtless I am > missing something here, too. Time to dig out my old networking > notes, perhaps. If you really do need to know if the data got there, I guess you're supposed to use an application layer ACK. Even if you could wait for an ACK, that only means that the TCP/IP stack on the other end got the packet. It doesn't mean that the application has read the data (the application could well be locked up somehow with the data that has been ACKed just sitting in a buffer). -- Grant Edwards grante Yow! Now that we're at in LOVE, you can BUY visi.comthis GOLDFISH for a 48% DISCOUNT. -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Grant Edwards wrote: > You're talking about the case where there's un-ACKed data. > Breaking a link when there's no un-ACKed data (which is what > the OP did) will require an hour or two to timeout _iff_ > keepalive is enabled. If keepalive has not been enabled, a > broken connection with no un-ACKed data will never timeout. My mistake, Grant. I was under the impression that a blocking send() would wait for an ACK before proceeding, but upon checking the man pages I see that this is not the case. It does seem to make the oft-cited TCP guarantee of data arrival a little weaker however, if you can't necessarily know how many of your sends have succeeded or not. Doubtless I am missing something here, too. Time to dig out my old networking notes, perhaps. -- Ben Sizer -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
On 2006-07-03, Ben Sizer <[EMAIL PROTECTED]> wrote: >>> and give me a hint how to get an exception >> >> You can't -- unless you've enabled the keepalive option on the >> TCP connection and you've waited the requisite time after the >> cable is cut before sending your data (IIRC it takes a couple >> hours for an idle TCP connection to time out because of a link >> being down). >> >> TCP/IP is designed to be fault-tolerant. Temporary breaks in >> cables aren't supposed to cause failures. > > But surely a permanent break - which is what I believe was > implied - means the data never arrives, which in turn means > you never get an ack, and eventually the TCP connection should > drop, Correct. But in the case described by the OP, he disconnects the cable when there is no traffic. Unless the keepalive feature has been enabled, TCP won't detect the cable is broken until some time _after_ the OP has written data to it [it won't timeout until after the stack send and resends that data repeatedly and doesn't get an ACK]. There's no way the TCP stack can raise an exception when the data is written, because the TCP stack hasn't yet discovered that the link is dead. > and Python should raise an exception. Right? No. The TCP connection timeout only changes the state of the socket. There's no way for the TCP stack to cause an exception in a Python program until the Python program accesses the socket again after the timeout has occurred. You'll get an error when you try to write to the closed connection or an EOF when you try to read from the closed connection. > I'm very used to connections dropping after much less than a > minute because the host became unreachable or took too long to > send a response. You're talking about the case where there's un-ACKed data. Breaking a link when there's no un-ACKed data (which is what the OP did) will require an hour or two to timeout _iff_ keepalive is enabled. If keepalive has not been enabled, a broken connection with no un-ACKed data will never timeout. -- Grant Edwards grante Yow! .. I must be a at VETERINARIAN... visi.com -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Q: I have been looking through Volume 1 & 2 on the topics of TCP timeouts. I have been looking in the section on "Timeout And Retransmission" where you talk about round trip times. My question to you would be what would make a tcp connection timeout? Is there a certain number of retries that need to happen before a timeout? A: TCP does not define a maximum number of retransmissions before giving up. However, it does define a maximum segment lifetime, and assumes that no response within two maximum segment lifetimes indicates a hopeless situation. Thus, if one attempts to communicte with a machine that is down, TCP will give up after retransmitting for about 5 minutes Copied from a FAQ. TCP will not let you know that the socket is down until you try to send or recieve on the socket. At that time a socket error will occur. And unless he has odd gear, the nagle algorithm will not hold a packet back if there's no packets still being transmitted (ack pairs haven't been received). I've had one radio which repackaged things funny, but the data still got through, it just took some time. -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Grant Edwards wrote: > On 2006-06-30, Martin Bürkle <[EMAIL PROTECTED]> wrote: > > > I have writen a programm using TCP sockets. After i get the > > connection to another socket I cut the Ethernet cable. Then I > > send a message. The program doesnt raise any exception. Can > > somebody tell me why > > Because send() has successfully passed the packet to the > network stack. If you want to know whether the application on > the other end received it, you need an application-layer > protocol that acknowleges the data. > > > and give me a hint how to get an exception > > You can't -- unless you've enabled the keepalive option on the > TCP connection and you've waited the requisite time after the > cable is cut before sending your data (IIRC it takes a couple > hours for an idle TCP connection to time out because of a link > being down). > > TCP/IP is designed to be fault-tolerant. Temporary breaks in > cables aren't supposed to cause failures. But surely a permanent break - which is what I believe was implied - means the data never arrives, which in turn means you never get an ack, and eventually the TCP connection should drop, and Python should raise an exception. Right? I'm very used to connections dropping after much less than a minute because the host became unreachable or took too long to send a response. (Sorry for the large quote but all the context seemed relevant.) -- Ben Sizer -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
On 2006-06-30, Martin Bürkle <[EMAIL PROTECTED]> wrote: > I have writen a programm using TCP sockets. After i get the > connection to another socket I cut the Ethernet cable. Then I > send a message. The program doesnt raise any exception. Can > somebody tell me why Because send() has successfully passed the packet to the network stack. If you want to know whether the application on the other end received it, you need an application-layer protocol that acknowleges the data. > and give me a hint how to get an exception You can't -- unless you've enabled the keepalive option on the TCP connection and you've waited the requisite time after the cable is cut before sending your data (IIRC it takes a couple hours for an idle TCP connection to time out because of a link being down). TCP/IP is designed to be fault-tolerant. Temporary breaks in cables aren't supposed to cause failures. -- Grant Edwards grante Yow! I feel like I'm at in a Toilet Bowl with a visi.comthumbtack in my forehead!! -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Laszlo Nagy írta: > Ben Sizer írta: > >> Martin Bürkle wrote: >> >> >>> I have writen a programm using TCP sockets. After i get the connection >>> to another socket I cut the Ethernet cable. Then I send a message. >>> The program doesnt raise any exception. Can somebody tell me why and >>> give me a hint how to get an exception >>> >>> >> Have you tried waiting 30 seconds or so? The connection may just take a >> while to time out. >> >> > I'm not sure if this will help him. He told that the program doesn't > raise an exception. I suppose this also means that socket.send() was > successful. If his program does not want to send or receive anything > else, then he will not get an exception. Otherwise if socket.send() > blocks his program, then you are right Another idea: probably his message is too small to fit one TCP packet. socket.send() succeeds, but the TCP packet is not sent out. By the time the packet driver tries to send out the (short) TCP packet, the socket.send() call is not active, so it cannot raise an exception. Laszlo -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Ben Sizer írta: > Martin Bürkle wrote: > >> I have writen a programm using TCP sockets. After i get the connection >> to another socket I cut the Ethernet cable. Then I send a message. >> The program doesnt raise any exception. Can somebody tell me why and >> give me a hint how to get an exception >> > > Have you tried waiting 30 seconds or so? The connection may just take a > while to time out. > I'm not sure if this will help him. He told that the program doesn't raise an exception. I suppose this also means that socket.send() was successful. If his program does not want to send or receive anything else, then he will not get an exception. Otherwise if socket.send() blocks his program, then you are right - he needs to wait. But I think he would have seen that his program is still running, and he would not have written what he wrote. If his program is still running, than he shouldn't have told "it did not raise an exception". :-) Laszlo -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Martin Bürkle írta: > Hi NG, > > I have writen a programm using TCP sockets. After i get the connection > to another socket I cut the Ethernet cable. Then I send a message. > The program doesnt raise any exception. Can somebody tell me why and > give me a hint how to get an exception > Okay, let's suppose you do not cut the ethernet cable and your message arrives on the other side, but the receiver program has a problem (not enough memory, bug in the handling code etc.) so your message is not processed on the other side. Most of the time, you do not only want to check if your message was transferred - you also need to check if that is actually processed. The TCP protocol cannot do this for you.Your clients needs to send back another message that notifies the sender about successful processing. Your sender program can wait for this notification, and then raise an exception after some timeout. In other words, you need to develop your own communication protocol over TCP/IP. You should try to minimize the number of send-receive-notification cycles, because the answer time can be high, compared to the transfer speed of the communication channel. And also because notifications are usually small, and they a buffered for a while... I have seen a paper about designing protocols over TCP/IP, but I do not remember now. Others will help you for sure. Please also consider UDP (if possible), which can be much more efficient. Best, Laszlo -- http://mail.python.org/mailman/listinfo/python-list
Re: No error while sending via TCP Socket
Martin Bürkle wrote: > I have writen a programm using TCP sockets. After i get the connection > to another socket I cut the Ethernet cable. Then I send a message. > The program doesnt raise any exception. Can somebody tell me why and > give me a hint how to get an exception Have you tried waiting 30 seconds or so? The connection may just take a while to time out. -- Ben Sizer -- http://mail.python.org/mailman/listinfo/python-list
No error while sending via TCP Socket
Hi NG, I have writen a programm using TCP sockets. After i get the connection to another socket I cut the Ethernet cable. Then I send a message. The program doesnt raise any exception. Can somebody tell me why and give me a hint how to get an exception Thanks for your help. Martin -- http://mail.python.org/mailman/listinfo/python-list
