I once tried a denial of service attack on my friend. I knew he has a 32 bit computer, so I tried to send him a Facebook message 2^32 characters long thinking that it would crash his computer (and possibly lead to a blue screen of death). My computer crashed before I could send the message.
Somebody should make a video. 3 guys are sitting in a room. Guy 1 puts on a mask that looks like guy 3 Guy 1 whispers "Monlist" to guy 2 Guy 2 throws a huge stack of paper at guy 3
A funny story from fall semester during the 2020 pandemic: because my university's classes were all online, one of my professors set up a website on a server he physically owned in his basement that he would upload assignments on. Turns out that was a bad idea because after every class when he would give us this week's assignment, he would get kicked offline from every student downloading from his server at the same time.
This reminds me of a project we had to do for University. We had to build a database driven web application. However, in order to fill our database, we were required to get our data from websites. In the end, it turned out that at least 5 groups were mass scraping the same website during a few weekends. Even though we never meant it to be a DOS attack, the poor server was in trouble.
"What time is it?" Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?* Server: *You want to know the time?*
That "monlist" command sounds like something that exists for debugging, and should require special permission to use. Even without the DoS attack part, it seems like a massive privacy/security violation to just let anyone anywhere ask for a list of everyone who has accessed something. My guess is that if it wasn't just an accident that it was left in, they left it in because removing it would break something. "We can't fix it because something else needs it to stay exactly the same" is a thing in programming.
Two things I'd like to point out: 1. You can spoof the IPv4/IPv6 address for any IP-based protocol (TCP, ICMP, IGMP, etc.), not just UDP. 2. It's the ISP's job to drop packets that have a spoofed source address as they know exactly where the packet came from (based on MAC address), and even the big routing services have some level of detecting whether or not a source address is spoofed.
I've been a programmer of web apps for the last 15 years, and I like to consider myself more security-aware than the common developer, but this was actually new information for me. Thank you for sharing the knowledge.
Thanks for explaining the difference between TCP and UDT. I didn't know it was that simple. Now I know why the setting on my surveillance system is the way it is
Really good video. I love Tom, he's so enthusiastic! :) Also, he talks about topics that I find very interesting (not that I'd actually follow any of his "tips", I just find it interesting). :)
Very very good explanation! I work for a company with a reasonable sized network (couple of gbits) and own datacenter. For a while now we get multiple abuse reports a day for exactly this type of attack. Sending in these abuse reports is actually useful, as we will block customers after receiving too many reports and not seeing any action on their part.
It sounds like you work for a server/VPS provider. If that's the case, you should look into some automated network monitoring software. Depending on your infrastructure there's most likely free ways of doing so, and if you're company's a decent size, it could afford the cost of some of the better solutions. In the long run, you'd save money from having to manually deal with those situations. And who knows... Maybe management would be appreciative of your recommendation. Might get a nice bonus/raise? :)
An an employee of a local ISP, we've been seeing these NTP attacks on a high-bandwidth customer of ours for some time now. We definitely notice. I believe we've simply blocked NTP from outside sources and are only allowing a select few popular ones, but that's slightly above me.
I have to say I lean more to the chemistry side of the Brady universe, but I have to say I am completely riveted to watching this guy, he is so enthusiastic about his subject and so interesting to listen too, more please!
He is an excellent orator and teacher!There is a lot I don't understand until explained the right way... I'm sure a lot of people feel that way. Thank You!
ive watched so much computer file, tom scott, matt and tom, and ashens, that youtube seems to genuinely think im british, and is now showing me ads for companies that dont exist in america
Actually you can do a couple of things: 1) Replication 2) Distributed Caching 3) Content Delivery Networks (a.g. Akamai) Theese are all expensive things, but if you are under a DDoS attack, you probably have enough money to pay for these services :)
I love this video because the only way I know of to truly raise security in this very attacker centric world is to raise the base knowledge of the average user.
@DerpTrolling on twitter currently holds the world record for the biggest DoS attack ever, it maxed out at over 420Gb/s and was against a company who wanted to test their infrastructure...They also delayed the customization packs for CoD by knocking the DNS servers that send the files, as wekk as every single LoL server, which is why they have had so much down time recently...
I actually remember that back during my school days there was this script you could run called a flooder that did pretty much this, and if you picked the right port to send traffic to on those old windows computers they not only lost all internet connections but also instantly crashed to bluescreen. I remember having a lot of fun with THAT on my school's intranet... =p
so many different attack vectors exist in the tcp/ip stack. we are sitting here talking about host to host but the real fun begins when you start attacking the routers and routing protocols.. not that I'm in to that but the potential is huge.
When you started talking about TCP vs UDP, I was thinking of a new name for the reflected DoS via SYN flood. That NTP thing is pure evil! Sounds like a command that was put in to an early ntpd for remote testing/diagnostic/logging and ended up staying in
Where the pressure really need to get put is on Internet Service Providers to do egress filtering on the outer edges of their networks, and ingress filtering on anyone they sell or rent a block of addresses to. That would actually reduce spoofing instead of just chasing it from protocol to protocol.
The person who inserted "MONLIST" command in the protocol, is THE Culprit/Hacker! While misguiding you as "This is one useless command", he got way of hacking forever! Otherwise "last 600 users? Really? 600?", come on, you don't need Sherlock Holmes!
"How can you protect your servers? The easiest way to update to NTP version 4.2.7, which removes the monlist command entirely. If upgrading is not an option, you can start the NTP daemon with noquery enabled in the NTP conf file. This will disable access to mode 6 and 7 query packetts (which includes monlist). By disabling monlist, or upgrading so the the command is no longer there, not only are you protecting your network from unwanted reconnaissance, but you are also protecting your network from inadvertently being used in a DDoS attack."
Wow! I don't really know what it is, but it's been a while since I watched this channel because of college stuff, and I feel like something changed on the transictions or the camera motion, that really make me get more concentrated on the video. I just loved his explanation, thanks so much for this video.
Truly, I think the only way to defend against DoS attacks is to improve the protocol itself. For instance UDP could handshake every once in a while, with the receiver sending a single "flag" saying that you can continue with the streaming. This will not only stop DoS, but could also save server bandwidth since the server will know that it doesn't need to keep sending packets to a recipient that doesn't want to listen (this would greatly improve the stream quality of sites like Justin, Twitch, Hulu, and Netflix). And the recipient can effortlessly let only one UDP transfer through, or none at all. another protocol idea is that when a server receives a request for a large amount of data (TCP), or just always (UDP), it 1st sends a "handshake" confirmation to the destination to make sure they really want it. This solves 2 things as one the server can ignore duplicated requests from the same "source", and again save even more bandwidth if the source turned down the handshake. There may even be a "timeout" flag included so that even the confirmation handshakes themselves aren't spammed. The only way a criminal could bypass this is if they hack the actual servers themselves.
So basically, it's the bad guys which are creating the need to defend against attacks of any kind that help improve security on the long run. I find this an interesting thought...
Aww, I expected you to mention amplification attacks in the IRC protocol itself and was disappointed. (CTCP VERSION or INFO for instance) These were all the rage when I just got on to IRCNet.
The problem with UDP is the fact that there is no absolutely no protection against source IP spoofing; Any ISP that the UDP packets are traveling through cannot verify the legitimacy of those packets. Therefore, any public UDP service which can send more data than it receives is vulnerable to (D)DoS amplification, and in fact it is extremely trivial to accomplish this. With my knowledge and $1000 worth of VPS (like Amazon Web Services), I could bring down a big site for a couple of hours and make headlines. I would never risk it though because that would trace straight back to me. The people who do these attacks in the real world use BitCoins and Tor or VPNs only so that they're much harder to find. I don't have any BitCoins, no use for VPNs, and I personally find it immoral and even childish to perform DoS attacks. I find the technicalities slightly interesting but, compared to real vulnerabilities, to me it's just child's play.
MONLIST is used to know who have already synced their time, so that you don't send them a sync command again. it's not useless dude. it's very a useful command. It's funny how it's being exploited though.
Sorry if this won't make sense, but can't that monlist type of communication/request be blocked by firewall? Doesn't it have some specific attribute that could be set to firewall rules and firewall just wouldn't let it through?
Here's an interesting question that I just thought of. What are the chances that the average person will be the victim of a direct, intentional attack on their computer over the internet. I don't mean viruses or malwares that are designed to spread to as many networks as possible. I mean a direct attack from a hacker/generally unfriendly person onto their personal computer?
+William Sculley Rather low, if your say somebody's grandfather that uses the computer only to read news articles - virtually never. if your someone that either has a geeky, tech enthusiast as a close friend or actually engages in arguments, etc online with other individuals then your chances are significantly higher but still rather low. generally speaking though you need to be in some sort of position that holds power, this could be: big youtuber, popular game dev, facebook famous, major corporation employee, etc.. just generally anybody with a large following or connected to an enterprise with a large influence. just think of it this way - the larger your following the more likely. NOTE: just because your chances are very low DOES NOT mean it can't and won't happen to you, it very well could.
If you're a gamer or someone who engages in online communities. You would be very familiar with ddos attacks. Every 12 year old and has mates have access to ddos tools. You can buy 20-30min duration attack tools, for a few dollars.
+William Sculley You can be sure that (Like +Vrox said) if you are a gamer, a normal DDoS will be something "normal" ... but a BIG DDoS or a REAL attack from a REAL (bad) hacker? No!, I'm not saying that you are no-one, but a REAL (bad) hacker has better stuff to do than scaring normal users... they would be working to find new way to fix this (probably because big companys pay for this kinda fixes, like Google) or going for the big stuff, or better known as "the big jackpot" :P. (I said "Real (bad) hackers" because HACKERS aren't bad, they work on finding exploits/bugs/security issues, but the bad people that get called "HACKERS" just for getting your Facebook conversations or e-mail password should be called Cyber Criminals). My point: :) Important people = High chances of being in Cyber Criminal's eye, anything else it's almost impossible
This is why the world's Internet providers must band together and implement egress filtering. I say "band together" because unless a sufficiently large proportion of carriers do it, it will be ineffectual. Malicious people will just hop to an ISP which doesn't do it, so there needs to be a united front.
I once tried a denial of service attack on my friend. I knew he has a 32 bit computer, so I tried to send him a Facebook message 2^32 characters long thinking that it would crash his computer (and possibly lead to a blue screen of death). My computer crashed before I could send the message.
Somebody should make a video. 3 guys are sitting in a room.
Guy 1 puts on a mask that looks like guy 3
Guy 1 whispers "Monlist" to guy 2
Guy 2 throws a huge stack of paper at guy 3
I was gonna tell you guys a joke about UDP, but you might not get it.
A funny story from fall semester during the 2020 pandemic: because my university's classes were all online, one of my professors set up a website on a server he physically owned in his basement that he would upload assignments on. Turns out that was a bad idea because after every class when he would give us this week's assignment, he would get kicked offline from every student downloading from his server at the same time.
This reminds me of a project we had to do for University.
We had to build a database driven web application. However, in order to fill our database, we were required to get our data from websites. In the end, it turned out that at least 5 groups were mass scraping the same website during a few weekends. Even though we never meant it to be a DOS attack, the poor server was in trouble.
And I though I was scary pulling a flash drive out without safely ejecting it.........
Tom Scott is to computerphile what James Grime is to numberphile
"Monlist!". The force of his disgust towards this command cracked me up no end! You just got yourself a new subscriber. Well done!
Tom Scott is by far my favorite person they interview on Computerphile
Yeah. It's especially funny how worked up and passionate he gets about some things.
"What time is it?"
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
Server: *You want to know the time?*
haha i love this guy. he is always so energetic.
Koseiku he has his own youtube channel - Tom Scott
He's brilliant :D
Tom Lloyd Ha. I thought he looked familiar.
i love how into it tom gets with the drawings
Why don't we just remove Monlist?
That "monlist" command sounds like something that exists for debugging, and should require special permission to use. Even without the DoS attack part, it seems like a massive privacy/security violation to just let anyone anywhere ask for a list of everyone who has accessed something. My guess is that if it wasn't just an accident that it was left in, they left it in because removing it would break something. "We can't fix it because something else needs it to stay exactly the same" is a thing in programming.
I love how angry Tom gets over this subject. The _passion!_
All of Computerphile's videos are cool and all, but....
Tom is just amazing!
I love this guy. He always sounds so excited when he's talking.
His passion is addicting.
n6i9k4a He has the rare talent that he's passionate about a subject but also has the ability to communicate that passion to the layman.
More of tom please, his enthusiasm keeps you glued to the screen.
I like to imagine someone setting this up but forgetting to spoof the return and destroying themselves
7:06 what do the other people in the building you're in think when they see you randomly scribbling and shouting 206 times? 😂
The fax paper to computerphile is like the brown paper to numberphile. Same for Tom Scott and James Grime
"I approve this stream being sent to me"
I swear this guy
I could just listen to him all day
This guy makes me feel bad for a computer...
Two things I'd like to point out:
1. You can spoof the IPv4/IPv6 address for any IP-based protocol (TCP, ICMP, IGMP, etc.), not just UDP.
2. It's the ISP's job to drop packets that have a spoofed source address as they know exactly where the packet came from (based on MAC address), and even the big routing services have some level of detecting whether or not a source address is spoofed.
I just found a new favorite channel. This guy loves what he does.
I've been a programmer of web apps for the last 15 years, and I like to consider myself more security-aware than the common developer, but this was actually new information for me. Thank you for sharing the knowledge.
More videos with this guy please. He explains everything so well
Thanks for explaining the difference between TCP and UDT. I didn't know it was that simple. Now I know why the setting on my surveillance system is the way it is
amplified distributed DoS? Yep, I get it. When you try to kick the internet's butt, be sure to wear ADIDoS!
Tom Scott is back on Computerphile! YAY! :)
Really good video. I love Tom, he's so enthusiastic! :) Also, he talks about topics that I find very interesting (not that I'd actually follow any of his "tips", I just find it interesting). :)
Very very good explanation!
I work for a company with a reasonable sized network (couple of gbits) and own datacenter. For a while now we get multiple abuse reports a day for exactly this type of attack. Sending in these abuse reports is actually useful, as we will block customers after receiving too many reports and not seeing any action on their part.
It sounds like you work for a server/VPS provider. If that's the case, you should look into some automated network monitoring software. Depending on your infrastructure there's most likely free ways of doing so, and if you're company's a decent size, it could afford the cost of some of the better solutions. In the long run, you'd save money from having to manually deal with those situations. And who knows... Maybe management would be appreciative of your recommendation. Might get a nice bonus/raise? :)
Chris Miller thanks for the suggestion. I might just look into that.
An an employee of a local ISP, we've been seeing these NTP attacks on a high-bandwidth customer of ours for some time now. We definitely notice. I believe we've simply blocked NTP from outside sources and are only allowing a select few popular ones, but that's slightly above me.
I have to say I lean more to the chemistry side of the Brady universe, but I have to say I am completely riveted to watching this guy, he is so enthusiastic about his subject and so interesting to listen too, more please!
He is an excellent orator and teacher!There is a lot I don't understand until explained the right way... I'm sure a lot of people feel that way. Thank You!
ive watched so much computer file, tom scott, matt and tom, and ashens, that youtube seems to genuinely think im british, and is now showing me ads for companies that dont exist in america
Meanwhile in captions: [SCRIBBLES FURIOUSLY]
love when this guy explains!
All the experts are so well spoken.
Actually you can do a couple of things:
1) Replication
2) Distributed Caching
3) Content Delivery Networks (a.g. Akamai)
Theese are all expensive things, but if you are under a DDoS attack, you probably have enough money to pay for these services :)
And that is something you might not have known!
Great Video! Thank you very much!
This guy really has a natural talent for teaching. I had never been interested in computer science until I started watching his videos.
I love this video because the only way I know of to truly raise security in this very attacker centric world is to raise the base knowledge of the average user.
@DerpTrolling on twitter currently holds the world record for the biggest DoS attack ever, it maxed out at over 420Gb/s and was against a company who wanted to test their infrastructure...They also delayed the customization packs for CoD by knocking the DNS servers that send the files, as wekk as every single LoL server, which is why they have had so much down time recently...
I actually remember that back during my school days there was this script you could run called a flooder that did pretty much this, and if you picked the right port to send traffic to on those old windows computers they not only lost all internet connections but also instantly crashed to bluescreen. I remember having a lot of fun with THAT on my school's intranet... =p
Thanks a lot for this, Haley Joel Osment! Seriously, this is highly informative and worth sharing.
This man, should have his own T.V show!
Charb thabowz He does, it's called Gadget Geeks.
so many different attack vectors exist in the tcp/ip stack. we are sitting here talking about host to host but the real fun begins when you start attacking the routers and routing protocols.. not that I'm in to that but the potential is huge.
Thanks Tom! Another great video with a really interesting and also terrifying topic.
ok wtf is there to dislike about this video??
knowledge is priceless
When you started talking about TCP vs UDP, I was thinking of a new name for the reflected DoS via SYN flood.
That NTP thing is pure evil! Sounds like a command that was put in to an early ntpd for remote testing/diagnostic/logging and ended up staying in
My IP is 192.168.1.1. a lot of people say they have the same IP but i thought IP's were only for one router/household. help?
computerphile needs to have tom Scott on more, his videos are great!
I think that just happended...
My internet died when you talked about you're Internet getting denied :/
Where the pressure really need to get put is on Internet Service Providers to do egress filtering on the outer edges of their networks, and ingress filtering on anyone they sell or rent a block of addresses to. That would actually reduce spoofing instead of just chasing it from protocol to protocol.
i really love this channel and the explanation of Tom Scott is really great. No matter what topic he is telling.
Turned Closed Captions on. Saw [furiously scribbling] at 7:03. Nice
The person who inserted "MONLIST" command in the protocol, is THE Culprit/Hacker! While misguiding you as "This is one useless command", he got way of hacking forever! Otherwise "last 600 users? Really? 600?", come on, you don't need Sherlock Holmes!
Note that the title says "disrupt" not "take down"
"How can you protect your servers? The easiest way to update to NTP version 4.2.7, which removes the monlist command entirely. If upgrading is not an option, you can start the NTP daemon with noquery enabled in the NTP conf file. This will disable access to mode 6 and 7 query packetts (which includes monlist).
By disabling monlist, or upgrading so the the command is no longer there, not only are you protecting your network from unwanted reconnaissance, but you are also protecting your network from inadvertently being used in a DDoS attack."
This is really interesting because I've noticed that some sites have been acting strangely last few days
Wow! I don't really know what it is, but it's been a while since I watched this channel because of college stuff, and I feel like something changed on the transictions or the camera motion, that really make me get more concentrated on the video. I just loved his explanation, thanks so much for this video.
This is a great video, He explains the concepts very well.
It's scary to think that something so intangible and seemingly untouchable as the internet...could soon actually be very vulnerable.
There is such amazing content on this channel.
Took 'em two years to get to terabit level attacks.
I absolutely love these videos. Tom is the man!
How did I not know about this beautiful channel?!?!
Truly, I think the only way to defend against DoS attacks is to improve the protocol itself.
For instance UDP could handshake every once in a while, with the receiver sending a single "flag" saying that you can continue with the streaming. This will not only stop DoS, but could also save server bandwidth since the server will know that it doesn't need to keep sending packets to a recipient that doesn't want to listen (this would greatly improve the stream quality of sites like Justin, Twitch, Hulu, and Netflix). And the recipient can effortlessly let only one UDP transfer through, or none at all.
another protocol idea is that when a server receives a request for a large amount of data (TCP), or just always (UDP), it 1st sends a "handshake" confirmation to the destination to make sure they really want it. This solves 2 things as one the server can ignore duplicated requests from the same "source", and again save even more bandwidth if the source turned down the handshake. There may even be a "timeout" flag included so that even the confirmation handshakes themselves aren't spammed. The only way a criminal could bypass this is if they hack the actual servers themselves.
Explaining DoS like a boss.
Flash forward three years and we've seen 620Gbps attacks with zero amplification...
So basically, it's the bad guys which are creating the need to defend against attacks of any kind that help improve security on the long run. I find this an interesting thought...
I love this guy..... so simply explained, well done...
I would love to see a feature-length Documentary (60-120mins) made by the computerphile guys! I imagine that it would be amazing.
Please do a video explaining the heartbleed bug.
6:00 If I were in charge of the Internet time protocol I would get rid of MONLIST immediately!
Loving this channel! Thanks for the great vids! :)
Tom, insightful as always!
Well that's a thought to keep me awake tonight…
2:10 Soviet Union battling against German soilders, circa 1945, colourized
I am forever going to remember these metaphors. UDP, _the firehose of data._
Nice video but fyi TCP doesn't require packets to be sent in order. It just makes sure that all packets are received at some point in time.
Aww, I expected you to mention amplification attacks in the IRC protocol itself and was disappointed. (CTCP VERSION or INFO for instance) These were all the rage when I just got on to IRCNet.
The problem with UDP is the fact that there is no absolutely no protection against source IP spoofing; Any ISP that the UDP packets are traveling through cannot verify the legitimacy of those packets. Therefore, any public UDP service which can send more data than it receives is vulnerable to (D)DoS amplification, and in fact it is extremely trivial to accomplish this. With my knowledge and $1000 worth of VPS (like Amazon Web Services), I could bring down a big site for a couple of hours and make headlines. I would never risk it though because that would trace straight back to me. The people who do these attacks in the real world use BitCoins and Tor or VPNs only so that they're much harder to find. I don't have any BitCoins, no use for VPNs, and I personally find it immoral and even childish to perform DoS attacks. I find the technicalities slightly interesting but, compared to real vulnerabilities, to me it's just child's play.
Wow I just found this channel and I love it.
MONLIST is used to know who have already synced their time, so that you don't send them a sync command again.
it's not useless dude. it's very a useful command.
It's funny how it's being exploited though.
Sorry if this won't make sense, but can't that monlist type of communication/request be blocked by firewall? Doesn't it have some specific attribute that could be set to firewall rules and firewall just wouldn't let it through?
spooky! "all you can do is hope that you won't get targeted"
Tom Scott is awesome. Thanks!
In the 8 years since this video's release, has this issue been resolved at all? Even partially?
How did Tom get "206" times the data....Also "206 x ?" What is the basic thing whose 206 times is being sent?
If you are getting these attacks surely you could just disconnect from your internet and maybe turn your router off. Would that stop it?
Here's an interesting question that I just thought of. What are the chances that the average person will be the victim of a direct, intentional attack on their computer over the internet. I don't mean viruses or malwares that are designed to spread to as many networks as possible. I mean a direct attack from a hacker/generally unfriendly person onto their personal computer?
+William Sculley Rather low, if your say somebody's grandfather that uses the computer only to read news articles - virtually never. if your someone that either has a geeky, tech enthusiast as a close friend or actually engages in arguments, etc online with other individuals then your chances are significantly higher but still rather low. generally speaking though you need to be in some sort of position that holds power, this could be: big youtuber, popular game dev, facebook famous, major corporation employee, etc.. just generally anybody with a large following or connected to an enterprise with a large influence. just think of it this way - the larger your following the more likely.
NOTE: just because your chances are very low DOES NOT mean it can't and won't happen to you, it very well could.
If you're a gamer or someone who engages in online communities. You would be very familiar with ddos attacks. Every 12 year old and has mates have access to ddos tools. You can buy 20-30min duration attack tools, for a few dollars.
+William Sculley You can be sure that (Like +Vrox said) if you are a gamer, a normal DDoS will be something "normal" ... but a BIG DDoS or a REAL attack from a REAL (bad) hacker? No!, I'm not saying that you are no-one, but a REAL (bad) hacker has better stuff to do than scaring normal users... they would be working to find new way to fix this (probably because big companys pay for this kinda fixes, like Google) or going for the big stuff, or better known as "the big jackpot" :P. (I said "Real (bad) hackers" because HACKERS aren't bad, they work on finding exploits/bugs/security issues, but the bad people that get called "HACKERS" just for getting your Facebook conversations or e-mail password should be called Cyber Criminals).
My point:
:) Important people = High chances of being in Cyber Criminal's eye, anything else it's almost impossible
You can defend against it if your home connection is being targeted by just using a vpn beforehand.
You should always be using a vpn actually.
This is why the world's Internet providers must band together and implement egress filtering. I say "band together" because unless a sufficiently large proportion of carriers do it, it will be ineffectual. Malicious people will just hop to an ISP which doesn't do it, so there needs to be a united front.
I love what happened between 5:59 and 7:07.
Best video yet!! and easy even for me to understand!!!!
Amazing video, well explained and extremely interesting; subscribed.