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.
Related
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 using raw sockets to communicate with a TCP server. For the purposes of my project, I need to emulate a TCP timeout.
Whenever a timeout occurs, server re-transmits the first lost packet. On receiving ACK for this packet, the sever re-transmits the second packet and also sends a packet that was previously unseen (due to F-RTO algorithm). In order to stop F-RTO, I need to send duplicate ACK for the later packet.
Lets says the congestion window is 20 at the time of time out. Server will send packet 1 and I will ACK packet 1. Server will then send packet 2 and packet 21. I will ACK packet 2 and send duplicate ACK for packet 21 to stop F-RTO. The problem that I am having is that although client is sending 2 ACKs, for some unknown reasons server is only getting one ACK. As a results it gets stuck in F-RTO.
Wireshark shows client sends multiple duplicate ACKs but from server side I can only see a single ACK. Since the second ACK is duplicate to first one, their fields and checksums are same. Can some one please help me out?
I have a quick question. If a TCP connection between two host is established and negotiated for ECN, then the sender will be notified of any network congestion by the receiver.
In case if ECN is not negotiated and a packet is dropped by a router,does it send ICMP notification to sender about the dropping packet?
i am just wondering, Why cant cant an intermediate router send icmp message before dropping packets? because it has a way to communicate to sender right. why should sender wait till it receives acknowledgement from receiver?
No. The ECN bit is set and transmitted along with the packet as a mechanism to inform the sender that it should reduce its transmission rate. This should eliminate the dropping of packets.
However, the TCP protocol stack is resonsible for guaranteeing delivery, not the intervening routers. If a packet is dropped it is the responsibility of the receiver to detect this and request the packet again.
if there is a tcp connection between A and B,
A send some packets and then a TCP RST(or TCP FIN/ACK) to close the connection,
let me say?
PKT1, PKT2, PKT3, TCP_RST
or
PKT1, PKT2, PKT3, TCP_FIN/ACK
but the packet arrival is out of order
PKT1, TCP_RST(or TCP_FIN/ACK), PKT2, PKT3
then how will B react?
according to the sequence number of TCP_RST and TCP_FIN/ACK,
B knows there are some packets missing(PKT2 and PKT3),
will B wait for PKT2 and PKT3 before it close the connection,
or B immediately close the connection when it receives TCP_RST(or TCP_FIN/ACK)?
thanks
The TCP protocol will reorder the packets before sending them further up the stack. This means it will wait for out of order packets according to the sequence number, ask for retransmission if needed, etc. and wait for the last ack before closing the connection.
You can find the TCP state diagram here:
http://www.ssfnet.org/Exchange/tcp/tcpTutorialNotes.html#ST
TCP guarantees sequence. That includes the sequence of the EOS. It must be delivered after all the data.
In TCP 3-way handshake, 3 segments will be sent (SYN, SYN ACK, ACK). What if the third segment(ACK) is lost? Is the sender going to resend the segment or give up establishing the connection? And how do the two hosts know the segment is lost?
TCP has a sequence number in all packets. Hence it's easy to know if a packet was lost or not. If a host doesn't get an ACK on a packet he just resends it.
In most cases though, even if that ACK was lost, there will be no resending for a very simple reason. Directly after the ACK, the host that opened the TCP protocol is likely to start sending data. That data will, as all TCP packets, have an ACK number, so the recipient would get an ACK that way. Hence, the sender of the SYN-ACK should reasonably not care that it didn't get the ACK, because it gets an "implicit" ACK in the following package.
The re-send of the SYN-ACK is only necessary of there no data is received at all.
Update: I found the place in the RFC that specified exactly this:
If our SYN has been acknowledged (perhaps in this
incoming segment) the precedence level of the incoming segment must
match the local precedence level exactly, if it does not a reset
must be sent.
In other words, if the ACK is dropped but the next packet is not dropped, then everything is fine. Otherwise, the connection must be reset. Which makes perfect sense.
I am not an expert on this particular situation, but I suspect what will happen is the client will think it is connected but the server will not. If the client tries to send data to the server, the server will reject it and send a RST packet to the client so it can reset its "connection".