When I was doing my network lab, I catched these tcp packets. I use gns3 to simulate the network, use the iperf3 to generate tcp packets.
iperf3 -c 10.0.3.33 -t 30
I do not know why there are so many ack packets, as well as high ack payload.
Piggybacking of acknowledments:The ACK for the last received packet need not be sent as a new packet, but gets a free ride on the next outgoing data frame(using the ACK field in the frame header). The technique is temporarily delaying outgoing ACKs so that they can be hooked on the next outgoing data frame is known as piggybacking. But ACK can't be delayed for a long time if receiver(of the packet to be acknowledged) does not have any data to send.
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I have an application and I am seeing packets being re-transmitted multiple times. connection is reset after multiple re-transmits.
In Wireshark, I can see the packet reaching the server, but I do not see the packet at the application level. I want to know how I can check if the packet is dropped at the TCP layer?
When does TCP send DATA packet ACK? After delivering the packet to an application or after receiving the DATA packet on the socket?
When does TCP send DATA packet ACK? After delivering the packet to an application or after receiving the DATA packet on the socket?
The ACK is sent by the OS after the data are successfully put into the sockets read buffer. No application logic is involved here yet.
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.
I'm filtering packets with libpcap with a filter like "tcp src localhost". It filters all the packets whose source is localhost (my host).
When localhost doesn't receive a TCP confirmation of an already sendt packet, localhost will forward the packet.
Not all the packets filtered by libpcap will arrive to its destination, and I need to identify when a packet is a "forwarded packet". Is there any way with libpcap to identify a forwarded packet?
By my understanding, you're looking for TCP retransmissions. These can be found by display fitters in wireshark after capturing. These two should help you:
Retransmitted packets can be found through the display filter tcp.analysis.retransmission (more such filters).
When the receiver gets an out-of-order packet (usually indicates lost packet), it sends a ACK for the missing seq number. This is a duplicate ACK and these can be found by using tcp.analysis.duplicate_ack (details).
I use raw socket to create TCP packets, with focus on the sequence number and TCP flags(SYN, ACK)
I used one machine S to send a tcp ACK packet (flag ACK is set to 1) and another machine R to receive it these two machines are in different subnets, all in my school
meanwhile, I used tcpdump to capture the packets.
Strange things happens! On machine S, the captured packet is as expected, it is an ACK packet however, on the receiving machine R, the packet becomes a SYN packet, and the sequence number is changed, the seq no is 1 smaller the expected and the ack_seq become 0!
what are potential problems?
my guess is that the router/firewall modified the ACK packet to a SYN packet because it never sees a SYN SYN/ACK exchange ahead of the ACK?
is it possible or not?
the two captured packets are:
https://docs.google.com/file/d/0B09y_TWqTtwlVnpuUlNwUmM1YUE/edit?usp=sharing
https://docs.google.com/file/d/0B09y_TWqTtwlTXhjUms4ZnlkMVE/edit?usp=sharing
The biggest problem you will encounter will be that the receiving TCP stack in each case will receive the packet and possibly reply to it. What you are attempting is really not possible.
I have a question on how TCP_ACK works when the original packet are fragmented.
For example, original packet size is 1,500*N bytes and MTU is 1,500. Then, the packet will be frgmented into (approximately) N packets.
In this case, how does the receiver sends TCP_ACK to the sender?
I checked with wireshark, it seems that the receiver sends TCP_ACK for every two fragmented packet. Is it right?
Could you give me some refereces for this or explanation?
Thanks.
IP layer on the receiver stack reassembles all the IP fragments into a single TCP segment before handing the packet over to TCP. Under normal conditions, TCP should send only one ACK for the entire TCP segment. The ACK # would be the next expected SEQ # as usual.