I am taking a packet trace and the server on my system receives a RST flag,, it then replies with a packet were the ACK,RST and PSH bits are all set. This does not seem normal to me? I m not sure why I would see the PSH bit set in a RST scenario? does anyone know why this might be?
Once the connection is established, all packets need to have ACK set and match the sequence number of the received packets for reliable transport/security. RST without ACK will not be accepted. When one side sends RST, the socket is closed immediately and the receiving side also closes the socket immediately after receiving valid RST. It does not need to be and can't be acknowledged.
after TCP handshake
A --->B Syn=x, Ack=y, len=z, ACK Flag
B --->A Syn=y, Ack=x+z, len=o, ACK Flag
A --->B Syn=x+z, Ack=y+o, len=p, ACK Flag
B --->A Syn=y+o, ACK=x+z+p,len=q, RST, ACK Flag
B closes the socket after it sends the last packet and A closes the socket after it receives it.
(not considering TCP window here, or there might be more packets from one end before the acknoledgement)
ACK Flag, acknowledgement number and the procedure of acknowledgement are related but not the same thing.
Per RFC793
RFC793
Acknowledgment Number: 32 bits
If the ACK control bit is set this field contains the value of the
next sequence number the sender of the segment is expecting to
receive. Once a connection is established this is always sent.
Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN.
The receiver of a RST first validates it, then changes state. If the receiver was in the LISTEN state, it ignores it. If the receiver was in SYN-RECEIVED state and had previously been in the LISTEN state, then the receiver returns to the LISTEN state, otherwise the receiver aborts the connection and goes to the CLOSED state. If the receiver was in any other state, it aborts the connection and advises the user and goes to the CLOSED state.
Related
Say we have sender A sending a message to receiver B using TCP. Say the message to be sent from A to B is split into three packets of length 500 bytes, 500 bytes and 50 bytes, to be sent in that order. How does A indicate to B that the packet of length 50 bytes is the last part of the message? I can understand that an ACK from B to A, sent every other packet received by B, indicates using the sequence number how much data has been received by B since the last ACK was sent by B. I read that FIN is used to terminate the connection between the sender and receiver. However, I can't find a description of how the the last packet, of a message split into several packets, is indicated. I'm thinking the packets have to be reassembled, in order, before the message is sent to the receiving application. I think that as one of TCPs actions is to split the message into packets, there must be some way of the sender flagging the last packet of a message has been sent.
I think that as one of TCPs actions is to split the message into
packets
No, TCP takes a stream of data and segments it into PDUs called segments. It is IP that uses the TCP segments as the payload of IP packets, which are in turn the payload of the data-link protocol, e.g. ethernet, frames.
However, I can't find a description of how the the last packet, of a
message split into several packets, is indicated.
Something like that is up to a higher protocol, e.g. HTTP. I think you are looking at TCP the wrong way. A TCP connection is like a bidirectional pipe; whatever you put in one end comes out the other end. TCP has no idea of the data structure, it just sends whatever it gets from the application or application-layer protocol. When an application or application-layer protocol is through using the connection, it tells TCP to tear it down.
The receiving TCP simply receives data and reorders it, asking for lost or missing segments. It passes properly ordered data up to the application or application-layer protocol, having no idea of the data structure because it is just a data stream to TCP.
Also, remember that both ends of a TCP connection are peers that can send and receive, and either end can send a segment with FIN that tells the other end that it is done sending, but the end sending the FIN is obligated to continue to receive until the other end also sends a FIN to say it is done sending. Either side could also kill the connection with a RST segment.
there must be some way of the sender flagging the last packet of a
message has been sent.
Probably, but that is not the job of TCP, that is up to the application or application-layer protocol. When the application-layer is done, it tells TCP to close, and that starts the FIN process. TCP has no idea what is the last part of a message is because it knows nothing about the data. It keeps the pipe open until it is told to close it.
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'm working on implementing TCP myself for a class project, and there's one detail I can't seem to understand. What is a FIN+ACK message? In the diagram I included here, receipt of a FIN+ACK will take a host from the FIN_WAIT_1 state to the TIME_WAIT state. Yet, NO state transition in the entire diagram sends a FIN+ACK. So how could a FIN+ACK ever be received if nothing is ever sending it?
When an application calls close it moves to FIN_WAIT_1
From FIN_WAIT_1 multiple things can happen:
Application receives ACK:
This means that the peer as acknowledged the last sent data packet. Local application moves to FIN_WAIT_2
Application receives FIN:
This indicates that peer has called close. And local application should acknowledge that. Hence ACK goes out to peer. Local application moves to CLOSING
Application receives FIN + ACK:
What FIN+ACK as you put it means is that the peer has called close as well as in the same TCP segment is acknowledging the data received last. Local application will acknowledge the FIN and this takes the state to TIME_WAIT.
TCP is defined by more than just that state diagram, the basic specification can be found in RFC 793. One particular statement is as follows (page 15, description of ACK field):
Once a connection is established this is always sent.
So basically this says an ACK must always be present after the initial three-way handshake, including during the four-way disconnect phase. There are subsequently only 2 messages that do not include an ACK:
The very first SYN as there is nothing to ACK
A RST as this usually means the connection state is non-existent or so messed up that an ACK does not make sense.
So to answer your question: in that diagram, whenever a FIN is sent, the ACK flag will also be set and an ACK nr will be present, even though it is not explicitly stated.
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".