I'm building my own webserver based on a tutorial.
I have found a simple way to initiate a TCP connection and send one segment of http data (the webserver will run on a microcontroller, so it will be very small)
Anyway, the following is the sequence I need to go through:
receive SYN
send SYN,ACK
receive ACK (the connection is now established)
receive ACK with HTTP GET command
send ACK
send FIN,ACK with HTTP data (e.g 200 OK)
receive FIN,ACK <- I don't recieve this packet!
send ACK
Everything works fine until I send my acknowledgement and HTTP 200 OK message.
The client won't send an acknowledgement to those two packages and thus
no webpage is being displayed.
I've added a pcap file of the sequence how I recorded it with wireshark.
Pcap file: http://cl.ly/5f5/httpdump2.pcap
All sequence and acknowledgement numbers are correct, checksum are ok. Flags are also right.
I have no idea what is going wrong.
I think that step 6. should be just FIN, without ACK. What packet from the client are you ACKing at that place? Also I don't see why 4. should be an ACK instead of just a normal data packet - the client ACKed the connection at 3.
This diagram on TCP states might help.
WireShark says (of the FIN packet):
Broken TCP: The acknowledge field is
nonzero while the ACK flag is not set
I don't know for sure that's what's causing your problem, but if WireShark doesn't like that packet, maybe the client doesn't either. So, it should be FIN+ACK, or you should set the acknowledge field to 0.
If that doesn't solve it, you might also try sending the data first, then a separate FIN packet. It's valid to include data with the FIN, but it's more common to send the FIN by itself (as seen in the other pcap trace you posted earlier).
Also, you should probably be setting the PUSH flag in the packet with the 200 OK
Finally, I don't see any retransmission attempts for the FIN packet - is that because you stopped the capture right away?
The IP length field was consequently counting 8 bits too much. I made a mistake in my calculations. Everythings works like a charm now!
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.
This question was posted on StackExchange - NetworkEngineering. People suggested me to post it here. Here is my original post.
I am trying to write a client that initiates a TCP connection and sends some data. On the server side, I am using nc that listens to a certain port. Now I am able to complete the 3-way handshaking. netstats shows that the connection is established. However, after my client starts sending data, it never gets an ACK.
The client is implemented on top of DPDK, and thus bypasses the kernel stack. The server binds to a different NIC. The two NICs are directly connected. The TCP part is handled by my own code. Due to the lack of knowledge, the implementation is greatly simplified in the sense that I set a lot of fields to some fixed numbers, such as the window size.
As a newbie in networking, I have no clue what could go wrong, and thus not sure what information I should provide to help you identify the problem. Here is a screenshot from wireshark. My client is at 192.168.0.10:12345 and my server listens at 192.168.0.42:3456. No ACK is sent from the server side after packet 6.
Also, the reason for the incorrect FCS is that I had to pad zeros to the SYN and the first ACK packets, so that they are at least 64 bytes, which is a requirement from the client NIC.
I did a comparison between packets from my client and packets from nc client. It seems that for the first data packet, the only real difference is that mine does not have any TCP options, while the nc one carries a timestamp. Could this be the problem?
Please let me know if you spot anything that may cause this no-ACK issue.
We are doing some load testing on our servers and I'm using tshark to capture some data to a pcap file then using the wireshark GUI to see what errors or warnings are showing up by going to Analyze -> expert Info with my pcap loaded in..
I'm seeing various things that I'm not sure or do not completely understand yet..
Under Warnings I have:
779 Warnings for TCP: ACKed segment that wasn't captured (common at capture start)
446 TCP: Previous segment not captured (common at capture start)
An example is :
40292 0.000 xxx xxx TCP 90 [TCP ACKed unseen segment] [TCP Previous segment not captured] 11210 > 37586 [PSH, ACK] Seq=3812 Ack=28611 Win=768 Len=24 TSval=199317872 TSecr=4506547
We also ran the pcap file though a nice command that creates a command line column of data
command
tshark -i 1 -w file.pcap -c 500000
basically just saw a few things in the tcp.analysis.lost_segment column but not many..\
Anyone enlighten what might be going on? tshark not able to keep up with writing data, some other issue? False positive?
That very well may be a false positive. Like the warning message says, it is common for a capture to start in the middle of a tcp session. In those cases it does not have that information. If you are really missing acks then it is time to start looking upstream from your host for where they are disappearing. It is possible that tshark can not keep up with the data and so it is dropping some metrics. At the end of your capture it will tell you if the "kernel dropped packet" and how many. By default tshark disables dns lookup, tcpdump does not. If you use tcpdump you need to pass in the "-n" switch. If you are having a disk IO issue then you can do something like write to memory /dev/shm. BUT be careful because if your captures get very large then you can cause your machine to start swapping.
My bet is that you have some very long running tcp sessions and when you start your capture you are simply missing some parts of the tcp session due to that. Having said that, here are some of the things that I have seen cause duplicate/missing acks.
Switches - (very unlikely but sometimes they get in a sick state)
Routers - more likely than switches, but not much
Firewall - More likely than routers. Things to look for here are resource exhaustion (license, cpu, etc)
Client side filtering software - antivirus, malware detection etc.
Another cause of "TCP ACKed Unseen" is the number of packets that may get dropped in a capture. If I run an unfiltered capture for all traffic on a busy interface, I will sometimes see a large number of 'dropped' packets after stopping tshark.
On the last capture I did when I saw this, I had 2893204 packets captured, but once I hit Ctrl-C, I got a 87581 packets dropped message. Thats a 3% loss, so when wireshark opens the capture, its likely to be missing packets and report "unseen" packets.
As I mentioned, I captured a really busy interface with no capture filter, so tshark had to sort all packets, when I use a capture filter to remove some of the noise, I no longer get the error.
Acked Unseen sampleHi guys! Just some observations from what I just found in my capture:
On many occasions, the packet capture reports “ACKed segment that wasn't captured” on the client side, which alerts of the condition that the client PC has sent a data packet, the server acknowledges receipt of that packet, but the packet capture made on the client does not include the packet sent by the client
Initially, I thought it indicates a failure of the PC to record into the capture a packet it sends because “e.g., machine which is running Wireshark is slow” (https://osqa-ask.wireshark.org/questions/25593/tcp-previous-segment-not-captured-is-that-a-connectivity-issue)
However, then I noticed every time I see this “ACKed segment that wasn't captured” alert I can see a record of an “invalid” packet sent by the client PC
In the capture example above, frame 67795 sends an ACK for 10384
Even though wireshark reports Bogus IP length (0), frame 67795 is
reported to have length 13194
Frame 67800 sends an ACK for 23524
10384+13194 = 23578
23578 – 23524 = 54
54 is in fact length of the
Ethernet / IP / TCP headers (14 for Ethernt, 20 for IP, 20 for TCP)
So in fact, the frame 67796 does represent a large TCP packets (13194
bytes) which operating system tried to put on the wore
NIC driver will fragment it into smaller 1500 bytes pieces in order to transmit over the network
But Wireshark running on my PC fails to understand it is a valid packet and parse it. I believe Wireshark running on 2012 Windows
server reads these captures correctly
So after all, these “Bogus IP
length” and “ACKed segment that wasn't captured” alerts were in fact
false positives in my case
I just came across this and would like to share my observation of TCP ACKed unseen segment. In my case the client was trying to initiate connection on same source port and destination port it used previously and thus the server was confused and replied with old TCP SEQ number saying I havent seen this new packet
PSH is a way to send data via TCP. Besides that, I can find very little info on how to implement it properly.
Here is what interests me:
Let's say, server window is 8000 bytes, and I send 2 requests with 150 and 600 bytes. Do I get some sort of confirmation that the data has been received? Can I somehow trigger a confirmacion?
I've seen some ACK packets, which does not contain PSH but do contain some sort of payload data (Wireshark marks it as "TCP segment data"). Is this data passed on to user, and if it is, why do we need PSH flag?
TCP PSH generally doesn't 'work' at all. Berkely-derived TCP implementations completely ignore it.
Source: W.R. Stevens, TCP/IP Illustrated, vol I: 20.5 PUSH Flag.
#Arsen: Answering to the second part of your question "why do we need PSH flag?"
The PSH flag in the TCP header informs the receiving host that the data should be pushed up to the receiving application immediately.
We are using PSH flag to exchange Time Stamp value between two servers.
i am assuming , if we set push flag , packet wont wait in receive buffer , it will directly send to receiver.
The data doesn't sit waiting in the receive buffer anyhow.
TCP apps must go out of their way to have the TCP layer bulk up a few packets and deliver full data buffers.
In fact its somewhat frustrating to see applications allocate 64KB buffers to receive data and see them getting a gazillion 1480/1472 byte messages.
I am sending HTTP requests from IP_ADDR1 to IP_ADDR2. I observed that HTTP requests are not reaching to application level. When I take wireshark logs I noticed some issue at TCP level. What are these issue? when this occurs ? How to get rid of this? Attaching the Wireshark snapshot here.
'TCP port number reused' means that it saw a successful connection handshake, then the client sent another SYN packet with the same port numbers. If the client hadn't already acknowledged the SYN-ACK, this would have been reported as a retransmission. But since it did acknowlege the SYN-ACK, it shouldn't need to retransmit the SYN. This could mean that something on your network is duplicating packets.
'TCP out-of-order' means that the packets aren't being received in the order that their sequence numbers indicate. It might be a side effect of the duplicate packet that's causing the reused port number error -- that may be resetting the sequence numbers back to the beginning of the connection. Because otherwise it looks like the packet is in order; an HTTP command should be the next thing after the connection handshake.