45 Multicast Concept Review

45   Multicast Concept Review



multicast concept review if you're like me you forget the technologies that you don't use on a day-to-day basis like for most people ipv6 or for almost everybody multicast so I figured we could do a brief concept review before we dive into the multicast design principle first off unicast versus multi cast in the unicast world which were all used to we have a stream of information send from a source to a destination governed by the IP address let's just say there's a 500 terabyte file it's a it's a video recording of the last company meeting in that way way too long that is being sent from this server to this host let's just say we've got a gigabit connectivity between their thousand Meg's megabits per second and the file transfers happening is this guy says I want to see the last company meeting he drags a file or streams a file from that server and essentially you cut your bandwidth by half because you've got one device now sending to two destinations right as this guy jumps in on the file transfer a third stream ends up being sent that's just how unicast communication works now you could flip on over to broadcast communication where this guy sends one stream but that's what routers do is they stop broadcast so you're only getting as far as that router and you're flooding every single device that's attached to that switch right so that doesn't work out so what multicast allows you to do is have a routable communication that takes a single stream of information now notice I'm saying stream that's the typical use of multicast as streaming things stream of information one stream and delivers it to the specific subscribers of that stream unlike unicast communication which is a source to a destination multicast communication comes from a source to a roop and this is where it gets a little weird you get into those Class D addresses let's just say 224 . 52 dot one dot 96 is just one that I picked out of the air right now this is more like a radio frequency because all of these hosts tune into that multicast address right they all start listening to that and notifying the devices along the way that they are listening that's how the routers know to forward that stream so you're not propagating broadcasts or something like that if you do it right which it does say take some intention to do that if you do it right you can get that stream going directly to just the hosts that need it wherever they are in the network it's pretty awesome to make that happen you need pim protocol independent multicast whole nugget on that coming up but that's the routing protocol of multicast means same same concept as like OSPF or rip is two unicast traffic PIM is your multicast routing protocol right multicast traffic is always UDP base this is why it's typically used for video and audio stream that's what it's built for I mean someday we'll have multicast across the entire internet widely supported where we can tune in to our radio stations or television broadcasts or or or you know continue let your mind go video game things I don't know where we've got single streams going to millions of people around the world it's pretty amazing if you think about it as Internet connectivity becomes more pervasive literally the world could be watching the same television station at the same time everywhere in the world that's I don't know it's just like holy cow that's amazing so that's that's the idea behind multicast udp-based so now wait a sec hold the phone is it limited to just these things no it is not those of you that have been an IT for a while have probably done computer imaging at some point using I for some reason I just remember the old program Norton Ghost which I think is still around but there's now things like Acronis or things that I don't do that images computers and actually blasts a operating system to all of them so how do you do that if it's an unreliable communication method well you have to build the reliability into the application so you just since you're not using TCP there's no handshaking right you can't have handshaking three-way handshakes when you got a whole bunch of people that you don't know tuning in to this you have to build some kind of reliability into the application layer so it can check and make sure that the data is there right cool so multicast has the typical network challenges like quality of service like security like bandwidth consumption if you have a I mean moult multicast streams I had I just came yesterday I was literally at a school talking with them about this football coach who was trying to stream his team highlights and I pulled up the bandwidth of this and I said how many devices did he stream it to and they said one they were watching it on a projector and I go look at this and I pulled up the bandwidth graph 50 megabits per second this guy was using to stream this I'm like what resolution was that in wow you know so so essentially if you were multi casting that football stream good grief I mean you just chopped 50 megabits per second out from every link that is crossing along the way now you can put bounce I'm totally man I'm once I get going on multi guess I'm like oh there's so much to talk about you can put bounds TTL is a major bounder of multicast to say that's how far you go no more right so you don't flood areas of your network that will never need that multicast originally a specialty technology is kind of like wow like Norton Ghost uses this is neat becoming heavily adopted in today's world let me expand a little bit on the multicast addresses we just talked about which I said are more like radio frequencies than addresses because you tuned in to them they are from this range which is the Class D block and you literally just pick one to assign to the application now I want to make sure I feel this the sense of uneasiness in in all my CBT Nuggets friends out there because they're like well what do you mean pick one how do you give it to somebody or pick it you're not you're not getting involved in like c-sharp coding to make an application multicast compatible essentially when you're getting the application it's either gonna support multicast or it's not and that whoever's running that application probably not you maybe it is will have an option and they'll open it up and it'll say hey do you want to use multicast and it'll have typically a checkbox right there they go yes choose no and then it'll ungrate a box where they type in what multicast address they would like to use for that application I mean maybe maybe this is a music streaming application where you want to stream you know country music to your entire organization right so there will be a server side of the application and a client side of the application sometimes the client side will Auto configure itself and detect the server and all that kind of stuff other times they'll you know again come in it'll say you know do you want to use multicast and you type in that same IP address right here so from your perspective as a network architect you just need to pick the address and give it to the guys and say hey go ahead and tell them you know 230 dot one dot one dot one now I know some of you are like yeah seems a little haphazard well of course it is you have to have you know at least an Excel spreadsheet or some kind of thing so you remember what addresses that you've assigned and also realized there are reserved addresses just like in the IPV for Class A through C some addresses just can't use these are all reserved now it's not that you can't use them it's just that you may step on a routing protocol or some other protocol out there if you do don't commit those to memory but for me just a gut level I don't use anything starting with 224 just because I know there's so many different reserved things in there I usually go with 230 oh you know when somebody's like hey I need a multicast address they'll just shoot it off eight to thirty one five nine two and okay and I hope that I didn't assign that to somebody else you know there's again be a little more responsible than that but something to be aware of because you can tell just by looking at this this is like an all-night troubleshooting waiting to happen for somebody Mac filtering essentially you've got this multicast right it's coming down from the server coming down right here history our routers got multicast routing turn on know using Pam we'll talk about that later it says okay I'm gonna route it down here to these guys which are subscribe to that right well switches by default treat a multicast like a broadcast and that's not good we don't want that because then everybody's getting flooded with this multicast that they don't need so what you'll typically do is enable igmp snooping anything that has the word snooping in it it's a good feature by me what this does is listen it kind of sniffs snoops on the multicast join request you know saying I want that stream and it's like oh okay he wants that stream well what do you switch is dealing they deal in layer two addressing right so there's a MAC address that's assigned to each one of these multicast addresses this is how the magic happens it actually Maps the last 23 bits of this IP address so take that convert it to desolates just say we did two thirty one one one take the last twenty three bits of that convert it to binary but right and then convert that binary down to hexadecimal and you have the last 23 bits of the MAC address that will be automatically generated and essentially that one MAC address will be assigned to different ports and that's how a switch is able to filter it now since we're only using 23 bits with the MAC address there are some addresses that can lead to duplicate MAC address mappings meaning these guys all result in the same last 23 bits don't worry breathe easy it's not something to panic about what do we do don't use them if you're gonna use this one don't use that one or that one or that one because they're all gonna map to the same addresses now you could roll the dice like I did and honestly that's what most people do because look at the gap I mean how many addresses are there between 2 24 1 1 1 and 2 2001 good – you know – 25 1 1 1 it's you know 65,000 some addresses right so what are the chances you could you could use that or you could know this exists and be very methodical about it it really depends on the personality you have but you can see if you didn't know about this this could lead to a ton of troubleshooting because you end up having to multicasts going to the same port now I want to give you an overview of multicast routing without diving fully into PIM because that's a whole upcoming nugget but this is gonna be super important for laying the foundation multicast routing is backwards bizarro land we're used to source destination unicast where I'm saying ok this is my source I go to my default gateway it looks at the routing table and finds the best path and delivers that unicast message to the destiny not so an ulti gasps multicasts the sources can just start transmitting let's just say this guy's that music streaming device he can just start sending anytime he wants and let's just say he's assigned the multicast address to 31.5 dot nine right no we just again pick that out of the air and assigned it now this guy out here says hey I want to listen to the music so he opens his little application says I want to join radio station K u P D you know and you know that's what the client sees because the client application Pretty's it all up but behind the scenes this is actually pointing to two twenty to thirty dot one dot v dot nine because that's how we configured it right so he's like oh I want to listen to this you know it's that big ol start button click behind the scenes what happens is that client will send a join message now this switch as we just talked about we'll pick that up with igmp snooping and here okay this port right here wants to join the multicast stream it gets that MAC address generated and all that kind of stuff hits the router now in the router I'm biting my tongue so hard I'm not getting into full pim routing at this one but the router is actually going to look at its multicast routing table which is really just a whole bunch of s G pairs oh if I if I could like oh it's on fire look everybody fire right there most important concept of the day s G pair what is that it is source group now the group is this two 3159 that's the group that you want to join and in the routing table of that router will actually be the real source address that device that computer right there server my cat just stepped on the keyboard that's funny that server is actually 10.5 22.7 as its IP address that's the real source right so now the routers gonna use something called reverse path forwarding ah you see how these puzzle pieces are starting to come together right so it's gonna be like okay you wanted to join that group sweet I've got the actual source which is ten five twenty to eleven I'm gonna find the best way back reverse back to that source Bing Bing Bing that lays out the multicast path that we're gonna go now what I just described to you is is known as pim sparse mode and that's actually the old name of it but I'll expand on that more and more later on but that's how these connections are made between the source and destination it's a little backwards now the only reason I brought PIM sparse mode onto the scene was to talk about that global concept that there are more than one way there I should say there is more than one way that you can do multicast routing two major methods shortest path tree and shared tree shortest path three tree was what I just described to where the router is going to use reverse path forwarding to figure out okay based on that source group pair that we have this is the best way that you can go will maintain that session between that host and that server in our shortest path path tree now the beauty of that is it eats actually that's not the beauty the beauty of that is this it's the shortest path that will calculate the shortest path from this source to this destination actually that I should reverse that this is the source of the stream to this destination of the stream but we find that path going backwards to the source so it's going to find the shortest path but it's going to eat a bunch of resources because it has to maintain a whole bunch of unique source group pairs for all the different devices that are joining so shortest path tree is but in an enterprise that's using a whole bunch of multicast now hear me when I say this I'm gonna go a little prophetic on you multicast is gonna change the world hear me say that right now ipv6 what message goes away broadcast right all that's left is multicast and all that's gonna be going on is a whole lot of multicast routing with a whole bunch of different sources they're going to be streaming all this stuff oh my goodness if I could give you the picture that is in my head of what the world will look like in 20 years 20 years totally radically different landscape to where television broadcasting radio broadcasting people probably won't even call it radio anymore essentially everything is multicast based internet around the world sources thousands billions millions of sources ah boom my head just exploded that's I mean that's the kind of world we're going in and so when you start talking about this kind of thing where you're maintaining all these these entries and your routers for all these different sources it consumes too many resources so a lot of large enterprises that start going into a bunch of multi casting will create a shared tree other key point of the day being almost as important as that SG pair actually nowhere close but very important rendezvous point essentially what this does is eliminate all these different source group pairs and just says you know what I'm gonna say star G what does that mean it means that for whatever sources for this group we're going to go to which ever device or devices will talk more about that later I've designated as the rendezvous point rendezvous what that means is essentially all paths lead to that router now depending on the PIM mode that you use or the multicast routing mode that you use that may just be initially like okay everybody's going to that guy and he's the one that figures it out and then from there they generate the shortest path tree back to this to the source you know again it's kind of a mix of both worlds or it could be one of those things where everything always continues to go through that rendezvous I just love that word rendezvous point router all the way back to all the different sources so you have this massive hub where all the traffic is passing through right so more on that when we get into the different pin modes switches by the way will flood multicast meaning if you pull that switch out of the box slapping it in the rack configure it with some base you know layer 3 switch and all that kind of stuff and then just walk away from it it will treat every broadcast like a sari every multicast like a broadcast traffic so really easy to turn on IG MP snooping turn that on your set the switches then start filtering and save your network from disaster I can't tell you how many times in this nugget I'm going human there's so much I want to tell you but I think I've accomplished my objective of giving you a big picture multicast concept review we'll unpack that as we continue on this design series I hope this has been informative for you and I like to thank you for viewing

Leave a Reply

Your email address will not be published. Required fields are marked *