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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TCP SYN does not reach TCP server

I have a TCP server listening on 10.10.10.101, port 2000, running on a Windows 7 machine. Below is the corresponding output line for the command netstat -aon: TCP 10.10.10.101:2000 0.0.0.0:0 ...
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193 views

Find processes listening on the wildcard address in `lsof`

Using lsof, I can find all of the processes with a TCP socket listening for connections: lsof -Pni -sTCP:LISTEN e.g.: COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME cupsd 662 ...
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675 views

Why does TCP sometimes not acknowledge the packets it receives?

I use firefox to visit a web server running on a computer on my LAN. I notice that sometimes TCP doesn't acknowledge the packets it receives. For example, in the following captured packets: ...
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8k views

How exactly does a proxy work?

If I want to connect to say, IP 100.100.100.100, Port 80, my computer will send a tcp packet with this adress into the wire. Now If I use a proxy server say, 200.200.200.200 Port 8080 (such kind of ...
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2answers
5k views

TCP Slow start and congestion control, and how to modify them

I'm trying to change the behavior of the tcp slow start protocol on a CentOS5 box (uname -r => 2.6.18-238.12.1.el5). I have read that the following command will change my icwnd to 8: ip route change ...
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3k views

Detect what outgoing ports are bypassed by firewall

My school has a firewall which limits most outgoing ports. There're only TCP/80, TCP/443, TCP/21 allowed. Is there's a way to find out all the outgoing port allowed by the firewall? My current idea: ...
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1k views

Open TCP port in Windows 7 and read incoming data, what program to use?

I have an external device which can connect over GPRS to the Internet using a SIM card and I want to see what data it sends. I can program it to use my computer as a server and then use a program to ...
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502 views

about combinations of TCP flags

TCP FIN packet, ACK flag is always set? In TCP tear-down phase, there are usually 4 packets: FIN/ACK, ACK, FIN/ACK, ACK. Is it possible some of these packets may not be sent by a TCP peer? When there ...
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348 views

TCP: when in TIME_WAIT state, will TCP starts a new 2MSL time if it receives a retransmitted FIN with 2MSL

we know that when a TCP program is in TIME_WAIT state, it waits for a FIN for 2MSL. if it receives a retransmitted FIN with 2MSL and retransmits an ACK, will it starts another 2MSL timeout or not?
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302 views

How to intentionally delay TCP packets in one direction

I've looked at this already: How to deliberately introduce a delay for incoming UDP packets I am testing a server-client system, and need to delay messages from the server to the client by some ...
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1answer
214 views

Disable tcp slow start

I am looking for disable tcp slow start mechanism for high-speed network. Currently clients send 100MB flows to random servers with non-blocking 1Gbps link. In my math, the completion time of the ...
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838 views

TCP header option: SACK-permitted (Selective Acknowledgments) negotiation

I'm doing a research project, which needs to split tcp connection. so I have some pecular questions, which may happen in my development. The problem is understanding of TCP SACK-permitted ...
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605 views

if a TCP packet got partially acknowledged, how will the retransmission mechanism react?

if a tcp client send a packet, with sequence number from 10000 to 20000, to a tcp server. the tcp will respond with an ACK with seq_ack 20001. if I intercept the TCP packet from the client, and ...
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1answer
636 views

Ports 1935 and 843 are getting blocked for no reason

I have Windows 7, and my computer has been blocking ports 1935 and 843 for no reason for a while now. If for example, I try to watch twitch, nothing will work because those two are getting blocked. ...
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130 views

Does pipelineing HTTP requests really result in faster page loads? Why?

A frequently cited argument is basically the one contained in this picture However, this does not reflect the reality of an underlying TCP connection which has to slowstart, and where the client ...
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1answer
94 views

in a browsing session, is it possible that web server initiates the TCP tear-down process?

when a user use browser to visit a website, it is the browser who initiates a TCP tear-down , namely it is the browser who sends the FIN/ACK first. is it possible that web server sends the FIN/ACK ...
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1answer
117 views

Hexadecimal Values Are Seemingly Backwards When Analyzing Packets Using Wireshark

Using Wireshark I am trying to debug an error that I am experiencing with one of my applications. I am having difficulty understanding how the packet contents are being formatted and displayed in ...
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225 views

how to maintain a inactive TCP connection?

if a TCP server establishes a TCP connection with a TCP client. Then the TCP client never sends any packet to the server. I roughly know firewalls near the TCP server may send a RST to it. or there ...
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912 views

should every TCP data packet be acknowledge or not? [closed]

If a tcp program A send two data packets to another TCP program B, packet 1: sequence number from 1000 to 2000; packet 2: seq number from 2000 to 3000. Generally B should reply with 2 ACKs, one for ...
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1k views

can TCP client use the same port to connec to different remote TCP servers?

I'm wondering whether TCP client can use the same port to connec to different remote TCP servers or not? In network-programming, there are two functions: sendto and send. When WE use send we don't ...
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1answer
292 views

Is it possible that a web server initiates a tcp connection to a browsing client? [closed]

when users use browser to visit a web site, it is of course the client send TCP SYN to initiate a tcp connection with the web server. I'm wondering are there any cases that a web server initiates a ...
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1answer
225 views

Router/NAT and IP/TCP header checksum

We know that IP/TCP has checksum mechanism to guarantee communication reliability. However, when a IP/TCP packet is relayed by router or NAT, the IP header may be modified, if if the router/NAT ...
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1answer
146 views

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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181 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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153 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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164 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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1answer
49 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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1answer
52 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
3k 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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96 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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84 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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1answer
297 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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1answer
567 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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1answer
917 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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328 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
2k 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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195 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
56 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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234 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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177 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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1answer
347 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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1answer
824 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
376 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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1answer
172 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
117 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: ...