Skip to main content

networking - What happens to TCP connections when I remove the ethernet cable?


For a TCP connection, when I remove the cable and reconnect after 30-40 seconds there is no packet loss issues. But when the reconnect time is more than a few minutes all the packets are lost. I know it works with the retransmission timer running out but I want to know what exactly goes about when a network cable is unplugged.



Answer



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 inmediately ethernet (layer 2) detects a loss of signal (if you're on Windows you will see the very dreaded pop-up informing network disconnected)


IP (layer 3) and TCP (layer 4) won't notice it, so they will try to keep on working.


TCP won't break a established TCP connection during a period of time because when TCP sends data, it expects an ACK in reply and if it doesn't arrive within a period of time, it re-transmits the data.


TCP will re-transmit the data, passing it to IP, who will pass it to Ethernet, who is unable to send it and simply discard it.


TCP will be waiting again and repeating this process until a timeout happens that let it declares that the connection is over. TCP resets the segment sequence number, discard the information that was trying to send and let free the buffer and memory resources that were allocated for that connection.


Plug the cable in before it happens and everything will keep going. This is what makes TCP reliable and at the same time vulnerable to DDos attacks.


If the OS has more than one interface (for example, ethernet and wi-fi), it is possible that when the ethernet goes down, it will try through wifi. It depends how the routing is configured, but in general terms "TCP won't be aware of that".


The basic structure of DDoS attacks is: thousands of clients opening each one a TCP connection every few seconds to a server and then abandoning the connection. Each TCP connection keeps open on the server during a long time (wasting valuable assets as TCP ports, allocated memory, bandwith, etc.) clogging the server resources to attend legitimate users.


Comments

Post a Comment

Popular Posts

linux - Extract/save a mail attachment using bash

Using normal bash tools (ie, built-ins or commonly-available command-line tools), is it possible, and how to extract/save attachments on emails? For example, say I have a nightly report which arrives via email but is a zip archive of several log files. I want to save all those zips into a backup directory. How would I accomplish that? Answer If you're aiming for portability, beware that there are several different versions of mail(1) and mailx(1) . There's a POSIX mailx command, but with very few requirements. And none of the implementations I have seem to parse attachments anyway. You might have the mpack package . Its munpack command saves all parts of a MIME message into separate files, then all you have to do is save the interesting parts and clean up the rest. There's also metamail . An equivalent of munpack is metamail -wy .
Post a Comment
Read more

How do I transmit a single hexadecimal value serial data in PuTTY using an Alt code?

I am trying to sent a specific hexadecimal value across a serial COM port using PuTTY. Specifically, I want to send the hex codes 9C, B6, FC, and 8B. I have looked up the Alt codes for these and they are 156, 182, 252, and 139 respectively. However, whenever I input the Alt codes, a preceding hex value of C2 is sent before 9C, B6, and 8B so the values that are sent are C2 9C, C2 B6, and C2 8B. The value for FC is changed to C3 FC. Why are these values being placed before the hex value and why is FC being changed altogether? To me, it seems like there is a problem internally converting the Alt code to hex. Is there a way to directly input hex values without using Alt codes in PuTTY? Answer What you're seeing is just ordinary text character set conversion. As far as PuTTY is concerned, you are typing (and reading) text , not raw binary data, therefore it has to convert the text to bytes in whatever configured character set before sending it over the wire. In other words, when y...
Post a Comment
Read more

keyboard - Is there any utility/method to change Windows key bindings to type rare chars to currently empty bindings?

I'm currently typing this post with my windows XP machine and (Spanish) keyboard, and I'd like to add some extra symbols to my text. I could open the "char map" windows utility, look for the desired symbols, and paste them. But I'd like something quickier. For example, when I'm using my OSX Mac at work, I can easily add a ©, ™, ® or similar symbols, just pressing some weird ALT-GR + G / H / J, key combinations. In my (Spanish) keyboard mapping, these combinations are empty, as they don't produce any char at all, which, on the other hand, is perfectly normal and desirable. So, I thought: Why couldn't I add some extra key mappings on top of my currently empty ALT-GR + G/J/H Keys in my Spanish keyboard, and thus, being able to quickly type these special symbols? So that's my question: Is there any utility/method to achieve that effect under windows? (My version is XP). I've even googled this for some time but no luck. I've been a long term Hot...
Post a Comment
Read more

Desktop reboots itself on sleep or hibernate

I have been using an ASUS M2NPV-VM motherboard for main home desktop workstation, operating Windows Vista x64. This computer has right from day one not been able to enter hibernate or standby; after Windows performs its final actions and brings the machine down, it would automatically revive itself for a reboot. Updating to the second latest BIOS (1201)has not helped (the latest BIOS revision would induce video refresh problems rendering it unusable). I have been reading related discussions on incidents similar to mine to no avail of a true workable solution. They appear to be more speculative guesses rather than actual knowledge on the inner workings of motherboard hardware. Does anybody have any electronic engineering experience on PC energy-saving standards to provide a more informed opinion how to go about getting this to work? More stories: this motherboard could not even reboot properly the first thing i used it. It was due to refresh rate of the onboard GPU, which had no influe...
Post a Comment
Read more