Questions tagged [tcp]

For questions about Transmission Control Protocol (TCP), a transport protocol used to encapsulate data across a network for reliable communication.

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What are the TCP effects of a CPU migration in Linux (with modern ixgbe NIC driver) [on hold]

I see a long tail TCP latency of up to 1 second on a Linux box with an ixgbe NIC using a specific kernel version. I see occasional hard retransmits (less than 3 dupe acks), and the cwnd go from 10 to ...
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TCP cwnd is increasing when congestion occur

I am a newbie network engineer. I did a small experiment with iperf3. But I run into a small problem. (1) My question. I think TCP flow do not act properly despite of its congestion control. The ...
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1answer
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How TCP find the correct application?

I am learning how TCP and application communicate with each other. I am not sure if my understanding is correct, I will use below example to illustrate my understanding and want to ask some questions. ...
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30 views

Rebuild video stream of TCP packages

I would like to rebuild a live stream that I captured via WireShark. I receive several decrypted TCP packages but am not able to export them as mp4 file or something similar. Here is an illustration ...
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1answer
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What technique is my ISP using to censor web traffic? [closed]

BBC news is censored in the country where I live. Out of interest I decided to briefly look at how this is done. First of all they apparently MITM DNS - if I look up BBC domains on Google's 8.8.8.8 I ...
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2answers
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Why do transport layer do data chunking. If there is fragmentation in Network Layer

I just curiously wanted to know about the question regarding L4, L3 Data Chunks, IP Fragmentation : Data chunking happened at Layer 4(Transport, TCP), and Fragmentation at Layer 3(Network, ipv4) ...
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1answer
98 views

How can I accomplish VPN across Dual Wan connections?

How can I accomplish VPN across Dual Wan connections in a optimal way? Idea is to make: Site to site VPN. Make single connection inside the VPN to use full bandwidth of both wan connections using (...
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1answer
279 views

TCP connections hang during handshake [closed]

For the last few days, connections originating behind my LAN's NAT have been having issues completing the TCP handshake (i.e. web pages only load after hitting F5 a few times, network applications ...
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3answers
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Sending less data than the TCP buffer could take

The accepted answer of this question explains: When you send data, your TCP buffers it. So if you send a character it won't send it immediately but wait to see if you've got more. But what if I ...
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Juniper EX Switch sending TCP reset on SSH connection randomly

So as stated in Title I'm observing that one of our switch randomly send RESET flag in TCP and ends the ssh connection in the middle and sends and Window size of 0 as well. Randomly it refuses to ...
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1answer
40 views

Does HTTP proxy sends its own TCP packets or bypasses clients packets?

Need help to understand what's happening on TCP level when http proxy is used. When client wants to open a website through a proxy, first connection is established between client and proxy. Then ...
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1answer
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Sending TCP packet to the other device without connection

I think the sending packet to other device works like the following. 1. The server device listens with port. for example 80 2. The client connects to the server devices. for example, the port of ...
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Why isn't UDP with reliability (implemented at Application layer) a substitute of TCP?

TCP provides reliability at transport layer while UDP does not. So, UDP is fast. But, a protocol at application layer can implement reliable mechanism while using UDP. In this sense, why isn't UDP ...
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1answer
53 views

TCP sequence numbers in Actual Transmission vs. in Wireshark Capture

Whenever, we do a wireshark capture and Follow--> TCP Stream, we see the sequence numbers as fixed values. In reality, if this were the case, the intruder can easily predict the transmission patterns....
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1answer
57 views

Response time of TCP congestion control

I would like to know about the overall latency of the TCP congestion control for adapting to varying network conditions. Specifically, my questions are: How fast can the system react after a change ...
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2answers
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How to measure max bandwidth with Wireshark?

I'm trying to measure the max bandwidth usage of my LAN with Wireshark. I´ve done several packet captures but I don't know how to obtain the max bandwidth usage. Any Idea? Thank you in advance
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1answer
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Why the Ack number is bigger than the last Seq number?

Why the Ack number is bigger than the last Seq number? you see the Ack response for each two TCP packages. but there I have a question, you see the Ack response for Seq=19997, Seq=22893 and Seq=...
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1answer
81 views

Dropping packets between router and firewall without firewall rule

I have a user that is trying to access his company network resources on his home network. He is able to access SMB shares and other resources, but he is unable to access their AS400 on any computer in ...
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1answer
47 views

Wireshark captures TCP packet with no flag [closed]

In our network, I captured a TCP packet by Wireshark that has no flag set. What's that mean? Is it normal?
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Behaviour of TCP SACK

I will ask the question based on the following scenario. Assume I have to send 10 packets using TCP using a window size of 5. I send 1, 2 and receive ACK for them, I send 3 and the ACK is lost, I send ...
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1answer
54 views

Why server cannot send data in TCP three-way handshaking?

In the three-way handshaking, only from the ACK packet we can send data. Why cannot the server send data with SYN-ACK packet? I understated why a client cannot send data to a server (preventing, for ...
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2answers
99 views

What's the use of an acknowledgement number without the ACK flag set?

Isn't it redundant if the ACK flag in the header isn't also on? Do you ever mix an acknowledgement with a normal message - send data and acknowledge a message in the same TCP segment? I feel like ...
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does ipsec phase1 renewal keep connections?

I have an IPSec tunnel UP and I open a TCP connection when the phase1 expire and is renewed, Is the TCP connection supposed to stay alive or drop ? both in the theory and practical (cisco, ...
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Why do I observe a 4-way TCP handshake instead of an expected 3-way when establishing a connection? [duplicate]

I'm doing a simple HTTP GET query on a localhost from curl to a local webserver. After I did a tcpdump I noticed these packets to be sent each time a request is done: 11:11:27.537300 IP 127.0.0.1....
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1answer
298 views

TCP Three Way Handshake?

I have the following query about the tcp handshake. To establish a connection the server must send a packet with SYN flag to 1 Then the server replies with a SYN,ACK flag to 1 , and afterwards the ...
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1answer
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Why does TCP need segments if the network layer automatically breaks things up into fragments?

I'm trying to better understand the difference between IP fragments and TCP segments (https://www.shubhsblog.com/tech/difference-ip-fragmentation-tcp-segmentation.html) and there's one thing I'm not ...
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2answers
129 views

If an old FIN is delivered, will TCP terminate the new connection?

Two programs use a TCP connection (3-way handshake) to open, communicate, terminate the connection and open a new connection. If a FIN message sent to shut down the first connection is duplicated and ...
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0answers
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Why does TCP exit out of slow-start faster than expected?

For context, I am testing a client-server TCP flow, where the client makes a request for a file, and the server sends the file using regular TCP streams. Both servers are using the "Cubic" ...
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2answers
558 views

Transferring data speed of Fast Ethernet

I am wondering that between 2 hosts in LAN, when i send one file which weights 1Mb versus 1 file weights 0.3Mb from the first host to the second host using TCP/IP protocol, would the latter come first?...
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Why do I not see slow start in this TCP packet capture?

I am looking at the packet flow between a server and a client (which is downloading a file from the server). As you can see in the packet capture below (with IP addresses omitted), after the initial ...
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1answer
66 views

How to send data from one PC to another PC over internet? [closed]

I'm basically a developer, and this is the first time i'm trying to develop a chat application over internet without any servers in between. It's easy to send data from PC to server and then to ...
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3answers
127 views

Is IP an abstraction

If packets "hop" from network device to network device across networks using MAC addresses, resolved by ARP, does this suggest that IP is essentially an abstraction? By abstraction, I mean, the ...
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2answers
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Can anyone Explain me difference Between PSH and URG flag in TCP segment

According to me when PSH flag is set , it indicates that data will have to be sent immediately to the wire and on receiving side the data has to be pushed to the application immediately . When URG ...
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What do the multiple occurencies mean in a packet trace file?

I have a trace file and I used tshark to find out the number of duplicate ACKs, with the following command: tshark -r file.pcap -T fields -e frame.number -e tcp.analysis.duplicate_ack After some ...
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2answers
228 views

Is a TCP segment with FIN but not ACK ever encountered today?

A fin-scan involves sending a TCP segment with FIN and no other flags other than (possibly) PSH and/or URG. Are such packets ever legitimately encountered in 2019? Note that in Is a FIN only segment ...
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1answer
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why can't we use the sequence number to distinguish them since every packet has a different sequence number? [closed]

It is a question from Networking stanford course. A server typically listens for TCP connections on a particular port (e.g. 80 for HTTP or 22 for SSH). Which fields can the server use to ...
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Which fields can the server use to distinguish packets for a particular client connection? [duplicate]

A server typically listens for TCP connections on a particular port (e.g. 80 for HTTP or 22 for SSH). Which fields can the server use to distinguish packets for a particular client connection?It is a ...
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How CUBIC TCP behaves when the sender doesn't use all the available bandwidth?

While I was reading about CUBIC TCP (https://en.wikipedia.org/wiki/CUBIC_TCP) I asked myself one question, what happens to the congestion window if the sender doesn't send much data (enough to trigger ...
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1answer
61 views

Is an RST only segment ever legitimate?

Is a TCP segment with RST but no ACK ever legitimately encountered today? While some kernels emit such an RST when a SYN segment is sent "illegitimately" from themselves (e.g., nmap half-syn, scapy, ...
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1answer
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How is SampleRTT measured once every RTT, without measuring every segment?

According to the RFC, SampleRTT is measured once every RTT. My text: Computer Networking: A top-down approach (6ed By: Kurose & Ross) also states, that: Instead of measuring a SampleRTT for ...
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2answers
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Is it correct to assume that ordering of frames at switch level is not important?

My understanding is that if the payload of the frame contains any data that needs to be reassembled, at least for TCP payload, it can be done at the OS level, once all the packets are received and ...
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1answer
32 views

Is WRED configured for TCP traffic only?

I am looking out for a practical usage of WRED. After reading/searching I found that it is more suitable for TCP traffic. When the queue is full, all the TCP session at that time will start with slow ...
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1answer
76 views

TCP - correlation between ACKs and receive window

EDIT: I think i get it now. So a simple sliding window mechanism basically assumes the correctly received data is processed immediately. Therefore an acknowledgement implies the data was received AND ...
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1answer
100 views

TCP or UDP for Stop & Wait / Go Back N

I'm creating server/client classes for "Stop & Wait" and "Go Back N" (variant of sliding window) protocols, though I'm unsure on whether I should pursue TCP or UDP. From my current understanding I ...
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4answers
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Can throughput exceed the bandwidth of a network

I took an introductory course to networking this semester and I was wondering: looking at things at the layer 4 level using TCP can the throughput on the network exceed its bandwidth? According to the ...
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1answer
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sequence number layer 4 vs sequence number LLC layer 2

I had read about TCP in layer 4 where sequence number and acknowledgment numbers were assigned to each segment so that each side of the connection could keep track of the process of sending/receiving ...
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1answer
54 views

Message formatting and encapsulation

I've gotten confused when reading about encapsulation and the OSI and TCP/IP models. I first read about message formatting and encapsulation and the diagram I read showed: Destination | Source and ...
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1answer
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OSI Model vs Network Model: Does It Matter Which One You Use?

The OSI (Open Systems Interconnection) model is comprised of the following layers: 7. Application Layer 6. Presentation Layer 5. Session Layer 4. Transport Layer 3. Network Layer 2. Data Link ...
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2answers
3k views

Why maximum length of IP, TCP, UDP packet is not suit?

From many tutorials, I get follow knowledge(maybe I'm misunderstood) : maximum of an Ethernet packet is about 1500 bytes. maximum of an IP packet is about 65535 bytes. maximum of a UDP packet is ...
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Why maximum length of IP、TCP、UDP packet is not suit? [duplicate]

From many tutorials, I get follow knowledge(maybe I'm misunderstood) : maximum of an Ethernet packet is about 1500bytes. maximum of an IP packet is about 65535bytes. maximum of a UDP packet is 65515 ...