Hello Samarth... > I was struggling for around 2 weeks without any success. You seem to > have given me the hint. Thanks a lot for replying to the mail You're welcome. Actually, we share common interest (Linux network stack), but I am still a beginner too. So, this is somekind of "quiz" for me :) BTW, you shouldn't do top posting. The reason is simple IMHO. If you are replying by writing below the related paragraph/words, the e-mail receiver will easily follow the flow of your logic without forced to look up and down between yours and the replied sentences. > 1) I want to send some additional data everytime an ack is sent > 2) When the sender recieves the ack packet it would extract this > additional data and use it for some computation Understood. So, by looking at your proposed step #2, logically, you need to modify something at the receiver side, right? If not, the receiver will think the real ack + your additional data = invalid ack, thus asking the ack sender to resend the packet or think that it is a valid data and thus sending another ACK. CMIIW And maybe, do we need to consider Maximum Transfer Unit or Fragmentation issue here? I mean, maybe your ack+modified data is splitted into two fragment and somehow the receiver sees it in reverse order or incorrectly re-assemble the packet? Personally, I am not so sure, i am just throwing ideas to watch for. Another thing, do you append your data such as "ack+modified data" (ack is on lower address, but sorry, so far I never studied the way NIC stores data to its own buffer) or "modified data+ack" (ack is on higher address)? > I plan to put the additional data that I intent to send in the extra > space allocated in the tcp header.(alloc_skb(2*MAX_TCP_HEADER)) . > I have attached the tcp_output.c file.To get the initial version > working I put only a string "Test\n" in the data section.Later on i > will put the acutal data that i intend to send Next time, please just attach your patch (not the entire C file), or if you really need to send the whole lines, you can compress it (gz, zip, bz2, anything you prefer). > 1) A sends data to B > 2) B sends modified acknowledgement header to A > 3) A recieves the acknowldegement from B and also recieves the data > that was added to the end of the header > 4) A sends a modified_acknowledgement packet to the acknowledge the > packet it recieves from B... IMHO, these are the things that really happens: #1 - #2 correct in #3, ack+data is received by A. A tries to parse the received ACK, but failed due to the additional data. Chances are: a) Received data (ack+your data) is simply typecasted by A. A then tried to find the string/number to identify if it is really an ACK or not. But it failed and think that B is actually sending data to A and A quickly sends ACK to acknowledge this data. Got my idea here? b) A correctly understood the the packet is indeed an ACK packet. But then it thinks "hm, what does this additional mean? A flag? Well, maybe host B did something wrong here" Both scenario (maybe there are others) create a never ending ping pong between A and B (as you already guessed). But somehow I feel, case (a) is the one you faced. Try to implement my ideas above about modifying the receiver side. And let's see these codes: --------------------------------------------------------------------------------- skb_put(buff,MAX_TCP_HEADER); unsigned char *temp = (unsigned char *)(buff+MAX_TCP_BUFFER); strncpy(temp,"Test\n",6); ------------------------------------------------------------------------------------ a) 6? are you sure? Not 5? And, you sure you don't need extra "\0" so it will be null terminated? b) +MAX_TCP_BUFFER? Not MAX_TCP_HEADER? NB: Boiled down by having to look at 2 computer at once or need to peek directly on how network stack code works? Several times I suggested folks who did network stack hacking to use User Mode Linux, maybe you can adopt it too. regards Mulyadi -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
