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66 votes
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What happens to TCP connections when I remove the ethernet cable?

By definition on a layered model as OSI or TCP/IP each layer works independent and not-aware of the lower layers. When you remove the cable, it's a physical disruption (layer 1), so almost ...
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44 votes
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How are source ports determined and how can I force it to use a specific port

How are local ports determined The port number is chosen by the TCP implementation software from a range of port numbers called Ephemeral Ports. The exact mechanism for choosing the port number and ...
  • 149k
34 votes

Why don't we have a usable IP address in 127.0.0.0?

In IPv4, the first address of a subnet is reserved for...unclear reasons, really, but most likely because very long ago it used to be the "broadcast" address. (Later, the last address became ...
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29 votes

Is it possible to force TCP to find another path to the host?

Is it possible that the route is somehow cached? If so, is it possible to reset this cache? No, only the first hop (i.e. the route to your local gateway) is under control of the OS, but you have no ...
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28 votes
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how to access windows localhost from wsl2?

localhost doesn't work because WSL2 is running with a virtual network (vNIC) created by the Windows Virtual Machine Platform (a subset of Hyper-V). Inside WSL2, localhost is the address of the vNIC. ...
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26 votes
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Must TCP use IP?

TCP and IP (v4 & v6) are definitely separable, and one does not imply the other, as proved by the example of TCP over IPX (RFC 1791). However, TCP cannot be built over just any network protocol. ...
25 votes

Is an ACK necessary when using reliable protocols like TCP? How much overhead / throughput penalty does it create?

Is an ACK necessary when using reliable protocols like TCP? Yes. the ACKs are the mechanism that makes TCP a reliable protocol. If ACKs were removed from the protocol, then TCP would no longer know if ...
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21 votes
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Why ICMP is different that TCP and UDP?

ICMP is a control protocol, meaning that it designed to not carry application data, but rather information about the status of the network itself. The best known example of ICMP in practice is the ...
20 votes
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What ports are used by an application

You can use netstat for this. See the example (I grepped for ssh): netstat -putan | grep ssh tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 1725/sshd tcp 0 ...
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19 votes
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Does the sequence number of TCP packet headers wrap around?

It wraps around going to 0. According to RFC 793: It is essential to remember that the actual sequence number space is finite, though very large. This space ranges from 0 to 2**32 - 1. ...
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18 votes

Does the sequence number of TCP packet headers wrap around?

Does the sequence number wrap around and become 0? Yes. All the details can be found in the TCP Specification RFC 793 - Transmission Control Protocol. Sequence Numbers It is essential to remember ...
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16 votes
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ICMP packet with TCP?

According to the ICMP message type (3 "Destination Unreachable"), these are not monitoring but error packets (which is the most common use of ICMP). They do not use TCP, but they were caused ...
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15 votes
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OpenVPN working with TCP not with UDP

These particular symptoms (with a VPN) are fairly common symptoms indicating an issue with your path MTU. MTU basics The path MTU (Maximum transmission unit) is the largest packet that can traverse ...
  • 6,553
14 votes

what is TCP Half Open Connection and TCP half closed connection

The other guys did a fairly decent job of describing what half-open and half-closed connections actually are, but the idea of half-open connections is also often searched for in the context of a them ...
12 votes
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Why is the TCP MSS (Maximum Segment Size) 1460 bytes?

1460 is not the maximum MSS. The MSS is calculated as the MTU minus the IP and TCP header sizes. On networks with higher path MTUs than 1500 (example: data center networks that use nonstandard 6k or ...
  • 99.3k
11 votes

Open a test TCP connection to a specified IP/Port

pkgmgr is now replaced by dism. Install telnet from command line (run it as administrator): dism /online /Enable-Feature /FeatureName:TelnetClient Then you can test TCP connection by: telnet ...
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10 votes
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If HTTP runs on a port, does that mean TCP can also run on that port?

TCP and HTTP are different things. TCP is the transport layer. By definition, it's responsible for carrying application layer protocols (HTTP in your case) over it. TCP does not run over a port. ...
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10 votes
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Practical (vs theoretical) max limit of TCP packet size

Ethernet has always been influential on TCP/IP packet sizes. Ethernet has a standard MTU of 1500 bytes, which, after typical IPv4 header overhead of 20 bytes and typical modern TCP header overhead of ...
  • 99.3k
9 votes

How are source ports determined and how can I force it to use a specific port

David Postill's answer is perfectly right. I would like just to add to it, by stressing that changing the ephemeral port range in Linux is so simple, that the OP has an affirmative answer. You ...
9 votes

How to send binary data in netcat to an already established connection?

Note: while following this solution, type commands in a single shell session (or use a single script) unless said otherwise. This is because it uses variables and file descriptors, they are not ...
8 votes

How can I see 127.0.0.1 traffic on Windows using Wireshark?

To keep this current, as hsluoyz said, install npcap http://wiki.wireshark.org/CaptureSetup/Loopback When installed on Windows Vista or later (including Win7, Win8 and Win10) with option "...
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8 votes

Get the current/default value of TCP initcwnd on Linux

Well, I can't say I'm 100 % sure this should be the answer, buuut, as it often comes, ss is the good choice to get some info revealed, for e. g.: ss -nli|fgrep cwnd westwood rto:1000 mss:536 ...
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8 votes

Redirect serial com to tcp port

I think socat has the functionality you are looking for as well. $ socat TCP-LISTEN:4161,fork,reuseaddr FILE:/dev/ttyUSB0,b57600,raw
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8 votes

Why do computers have 65,535 TCP ports?

"Why do computers have 65535 TCP ports?" Computers do not have ports. The concept of "ports" to which you're referring is strictly related to the TCP and UDP protocols of the TCP/IP networking stack. ...
7 votes
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Localhost tcp throughput performance differences

I believe that the reason that the PowerPC on Windows 7 is much faster on localhost loopback throughput, is because it can use NetDMA. The Microsoft article NetDMA (Windows Drivers) defines NetDMA as ...
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7 votes

What ports are used by an application

Another tool that can do this is lsof: # lsof -i -a -p 1981 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME sshd 1981 root 3u IPv4 917 0t0 TCP host.example.com:ssh (LISTEN) # ...
  • 1,697
7 votes

Does the sequence number of TCP packet headers wrap around?

Yes, it does wrap around. You can read it on Wikipedia or on RFC1323, which shows how to protect against wrapped sequence numbers. Let me quote: TCP timestamps are used in an algorithm known as ...
7 votes
Accepted

How to check the TCP congestion control algorithm flavour in Ubuntu

There aren't TCP variants; there are TCP congestion control algorithms: sysctl net.ipv4.tcp_congestion_control cat /proc/sys/net/ipv4/tcp_congestion_control The default is usually cubic or reno, ...
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6 votes

Why is my UDP so slow?

As Nick answered, iPerf uses a default of 1Mbit/sec for UDP. In order to use the same bandwidth as TCP while performing a UDP test, simply provide -b flag with the value of 0. iperf3 --udp -b 0 -f ...
6 votes

If HTTP runs on a port, does that mean TCP can also run on that port?

If I understand your question correctly, I might rephrase it as, "If the network infrastructure allows HTTP traffic to pass on a certain port, will it also allow pure TCP (without full HTTP-compliant ...
  • 99.3k

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