TCP stands for Transmission Control Protocol and is one of the core protocols of the Internet Protocol Suite. TCP complements the Internet Protocol (IP), and therefore the entire suite is commonly referred to as TCP/IP.

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Gaining insight on TCP traffic

I have an iMac on which I turned off almost all applications. Browser, Skype, Spotify all turned off. Despite that, when I do a tcpdump I record still a lot of TCP traffic. How can I get insight into ...
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209 views

How does TCP get the MSS values for a certain path?

when a TCP client initiates a TCP connection with a remote TCP server, it will announce its MSS in the TCP option. I'm at a loss how TCP knows the path MSS value. This value is specific to the path ...
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202 views

Where can I find a table of TCP header options, which lists the kind number and corresponding option [closed]

Several days ago I came across a table of common TCP header options (tens of them), which lists the kind number and corresponding option, now I can't find it through google. like kind ...
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2answers
177 views

If there are 2 NICs on the same machine, can a TCP client on one NIC connect to a TCP server on the other NIC?

I use TUN/TAP to create a tun NIC tun0 with IP 10.10.10.2, and I have a real NIC eth0 with IP 202.112.34.49 Then I run a TCP client which binds its socket to 202.112.34.49 and a TCP server binding ...
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33 views

Can anyone explain how the loopback interface works? [duplicate]

I got to understand how TUN/TAP works from the following illustration: TUN/TAP illustration I'm at a loss how the loopback interface works. Can anyone explain a bit about how the loopback interface ...
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55 views

is it possible to update tcp window scaling factor after 3-way handshake by sending a syn/ack?

tcp window scaling factor is notified during 3-way handshake period is it possible to update tcp window scaling factor after 3-way handshake for example, by sending a SYN/ACK with tcp header option ...
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57 views

TCP vs UDP Error-Checking

I've bounced upon two articles that describe in-depth on how TCP and UDP connections work. However there is some kind of inconsistency. Where one article tells me that UDP does no Error-Checking at ...
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1answer
4k views

The best way to do TCP/IP over USB on Linux

I want to establish a TCP/IP link between two Linux machines that have several USB 2.0 ports as the only connection options. Since both host and slave ports are available, I can connect them via a ...
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2answers
102 views

Port forwarding beginner

I have been trying to set up port forwarding for the past 48 hours and I can't quite get it. I have used online port scanners to see what ports are open and they are saying none, not even the port I ...
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85 views

Sending TCP command from external network to specific device in home network

I'm working on a mobile phone application which has to send a TCP command to from external network (like a coffee shop or it's connecting to 3G) to a specific device in home network. Let's say A ...
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361 views

DNS: can't dig 8.8.8.8 directly

me@ubuntu:# dig @8.8.8.8 google.com ; <<>> DiG 9.9.2-P2 <<>> @8.8.8.8 google.com ; (1 server found) ;; global options: +cmd ;; connection timed out; no servers could be ...
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644 views

Scapy: TCP Retransmission error

I've been using Scapy to send SCCP or Skinny packets to my Cisco Desk Phone. However I keep getting these TCP Retransmission errors, and I believe this is causing the Cisco Phone to not respond. I ...
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1k views

MTU, RWIN, and MSS

I have been doing some reading into TCP Receive Window (RWIN), Maximum Segment Size (MSS) and Maximum Transmission Unit (MTU). Right off the bat I have some questions and I need a power user's help! ...
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455 views

How to use NetLimiter to block inbound TCP traffic?

I'm using NetLimiter to block TCP traffic. (Rule type: Firewall, Direction: In, Action: Deny) I can block outbound, but not inbound. You can try blocking inbound TCP on Chrome to see it is not ...
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3answers
3k views

How to pipe data over tcp from the command line?

I would like to pipe data from one machine in the command line to another machine over tcp. I guess I could write a socket server but this must already be implemented. For example I could use it to xz ...
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239 views

how tcp stack deal with the ack_seq number and seq number of tcp FIN/ACK packet?

from my knowledge, seq and ack_seq are processed by tcp stack independently. when a packet arrives with a inconsistent ack_seq number if: 1) the ack_seq number is obsolete(smaller than expected), ...
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1answer
61 views

Automating an IP Phone

I've been very interested in Automating an IP phone, currently I am automating phone calls by using PuTTY to a Dial-Up modem through simple AT commands. However, this is a poor method because it does ...
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1answer
1k views

Can't connect to SSH over VPN when using 3G only

I'm having a bit of a strange problem that I'm not sure how to debug/fix. I have a Raspberry Pi connected to a Windows VPN via either a wired interface or 3G dongle. With the wired interface I can ...
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299 views

TCP Throughput versus Packet Loss

How does the packet loss rate affect the TCP throughput rate: throughput rate reduce exponentially with respect to loss increase or throughput rate reduces by the same amount as loss increase?
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241 views

find the process which is listening on a specific port

I want to find some processes which are running on a specified port for example 80 I ran fuser 80/tcp and got nothing and then netstat -tulpn|grep "80\|PID" and got Proto Recv-Q Send-Q Local ...
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3answers
10k views

TCP Retransmission vs. TCP Fast Retransmission

In Wireshark I can see things that say "TCP Retransmission" and "TCP Fast Retransmission." What's the difference? How is Wireshark able to tell the difference?
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368 views

Testing network equipment, TCP window size matters

We are currently testing some network equipment (consumer routers with 1 Gbit/s ports) with Jpef 2.0. All kinds of connection parameters are tested: wired and wireless, different wireless standards, ...
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1k views

using iptables to prevent RST related to a specific port

I have a program which uses libpcap to capture incoming TCP SYN packets, these SYN packets are destined for a specific port. But I have no tcp listening socket for that port, so in practice, the OS ...
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3answers
393 views

How can I open a TCP/UDP connection on Linux?

I am looking for a simple command line tool to open TCP/UDP server/client on a desired port on a Linux machine. Are there any applications or commands that I can use to achieve this?
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2k views

Connecting Android via wifi to Wifly RN-XV RN-171 using Access Point (soft AP) mode not working

I hope somebody has encountered this problem and knows how to fix it. I purchased a Roving Networks RN-XV module to communicate directly with an Android device. I thought it would be perfect because ...
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181 views

Why does Knoppix mess up my internet connection?

I have this problem that when I boot Knoppix on my Windows box, it screws up the internet connection somehow, so Windows cannot get an IP when I reboot back to XP. I use Verizon FIOS. Normally I have ...
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1answer
119 views

How to set TCP SEQ number generation

There is a Windows 2000 system, that when I scanned it, it said, that the TCP SEQ number prediction is really low (nmap – trivial joke). While I'm still trying to verify that, I wondered, how could ...
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6k views

What do these remote addresses, local addresses, and states in TCPview mean?

I have been using TCPview lately to see what connections are made by different processes on my PC. Would somebody please explain what the following situations mean? Thanks. TCP Local Address: ...
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801 views

Is it possible for a PC to use two net connections simultaneously?

We have two net connections in our office - a 10mb Wired DSL provider and a 3mb Wireless backup provider. The wireless is a backup, but it has the advantage of 3mb upload speeds whereas the wired is ...
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188 views

How do NAT or proxy react to incoming TCP SYN packet?

In some messaging systems, two messaging clients send/receive packets directly from each other in chatting or voice call. I think the basic mechanism is (TCP for example): these client programs open a ...
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1answer
434 views

Virtual Windows Server 2008 R2 system unexpectedly responds on TCP echo port 7

We have a virtual system running Windows Server 2008 R2, and it responds on TCP port 7 (echo). (I.e, in response to a SYN it sends ACK,RST, which is enough to make InetAddress#isReachable() return ...
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442 views

intercept TCP/UDP Packets on a host before they are sent to or recevied from the network [closed]

for example, 1 when I'm using some programs, like visiting websites using firefox if the webserver send my program some data, my program TCP stack will issue an ACK packet. Is it possible to ...
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1answer
959 views

is tcp header timestamp mandatory or optional? and how to generate tcp timestamp

I noticed that in all tcp communications, in the tcp header option parts, tcp timestamp is always there is it mandatory or not? so if there is no tcp timestamp in a tcp packet, the tcp client or ...
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1answer
3k views

How is TCP/UDP checksum working?

I used raw socket to create some UDP packets and then send them to a UDP server I notice that when I put a wrong UDP checksum number the UDP server doesn't accept it, but if I put all 0 on the ...
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1answer
90 views

sent packet is different from received packet

I use raw socket to create TCP packets, with focus on the sequence number and TCP flags(SYN, ACK) I used one machine S to send a tcp ACK packet (flag ACK is set to 1) and another machine R to receive ...
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1k views

tcp port numbers reused on wireshark

I used raw socket to build a tcp client program on machine A to reproduce tcp 3-way handshake with a simple regular tcp server on machine B I captured packets on the client host and server host, and ...
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1answer
434 views

stopped a service but outgoing port still exists on netstat -aon

bit more detail.... I have a port, specifically port 2869 for Windows Media Player Network Sharing Service that uses UPnP port 2869, however even when turning off all the sharing possibilities I can ...
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1answer
268 views

TCP number of connections differ from client to server

How can this be ? netstat shows x 'established' connections from the client to y 'established' connections on the server. Situation stays like this for 10 minutes, then x and y change, then another ...
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1answer
247 views

How does the browser know which packet is the response to a specific request?

I am sniffing the in/out packets over TCP. I have some GET output, but I want to know which of the packets is the input? How does the browser determine this? Simpler explaination: I'm sniffing and I ...
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1answer
5k views

Remotely push DNS server to client via OpenVPN

When I try to push a DNS server via the OpenVPN server-config I don't get that server to be the first DNS server on the connected client system. It ends up being specified as an alternative DNS ...
4
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1answer
3k views

Is there a way to kill an open connection without killing the process?

I've search around and all the suggestions just say to kill the process. But I'm surprised I can't find any simple way to force a "connection" to close, without killing a pid associated with it. ...
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1answer
405 views

What does this tcpdump printout mean?

I'm not very experience with networking, so on my free time, I usually try and learn some more things about the internet and how it works. While at school, I came across tcpdump and wanted to give it ...
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1answer
2k views

tcpview cannot kill connection/process

I have a program that sometime misses to close a tcp connection. After the program has ended I can see that the port is taken. TCPViewer shows "non-existent" in the "Process" column. If i try to ...
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213 views

Artificial network delay tcp/udp (bigger ping) for defined application? (windows)

I have a game where people can easy detect who i am looking at my ping, i want to make my ping highter. How can i set network delay for all network connections for defined application?
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2answers
3k views

Get the current/default value of TCP initcwnd on Linux

I can manipulate such value with: ip route change ... initcwnd 10 and then have a feedback with: ip route show But what about the default value before any modification? Is there a way to query ...
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91 views

What happens if i send html over tcp without using the http protocol? [closed]

Will the html be delivered properly to the browser, or will it lack formatting? Will it get delivered at all?
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374 views

Is the following analogy to describe the relationship between TCP and HTTP correct?

TCP as a hose-pipe and HTTP as the water flowing through it I've just been reading some, and trying to conceptualize the difference between TCP and HTTP. I stole the analogy from here ...
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233 views

Windows 7 cannot connect to network shares, Exchange server, Webmail… anything requiring authentication

I have been handed this Asus Ultrabook that had Outlook 2010 that was not connecting to our Exchange server. It used to when in a different office. On our office network, it can't. I have 36 other ...
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2answers
230 views

Why does the IP address matter to IE9?

Start up Wireshark. Point IE9 to a local, nonroutable IP address. You will see a single TCP SYN packet try to connect to the host. Now point IE9 to a remote, routable IP address. You will see 2 ...
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2answers
999 views

Why can't a PC with 2 network cards be accessed by hostname?

I set up PC with 2 network cards, connected to the same LAN. I can connect to this PC (e.g. by remote desktop) only via IP addresses. Accessing by hostname does not work. Why is this the case? ...