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  • So I want to finish up our discussion of end end packet forwarding through this network by focusing a little bit more on what the routers themselves they're doing.

  • But before we start with that, I want Thio.

  • Just take a closer look at what is inside an I p packet.

  • And so, of course, we have our Ethernet frame, which we've been looking at before, and the I P packet sits inside the payload of this frame.

  • So this payload is going to encompass our entire I P packet, which is gonna include the I P header as well as whatever payload is inside the I P header.

  • And so any time we have an I P packet, the ether type is going to be 0800 Um, and remember, there's either the either type or the particle field if this is PPP.

  • So we're looking at Ethernet, but I P can go over PPP as well or any other layer to data link framing mechanism.

  • Um and then that framing mechanism is gonna either have a neither type of protocol of some sort.

  • That's gonna tell you what the payload is on.

  • In those cases, the 0800 indicates that his i p then within the i p header, there's actually a protocol that tells you what isn't inside the I p packet.

  • So when we're doing the ping before, we're using the Internet control message protocol, or ICMP, and the protocol for that is actually one.

  • So you see, we have this thing where we have this Ethernet frame that tells you that uses the either type to tell you what's inside It is I pee and then an I p header that uses this protocol field to tell you what's inside the I p packet, his ICMP.

  • And so this is a pattern that you're going to see.

  • Ah, lot in networking, which is called encapsulation, where you have one protocol encapsulating another protocol which maybe encapsulates another protocol and so on.

  • And hopefully as we go through this, you'll see that each of these protocols has a specific purpose.

  • But getting back to the I p header, I just want to walk through these different fields.

  • The I P header starts off with a version and what we're talking about his i p version four, which uses 32 bit addresses that we've been that we've been looking at.

  • There's also I p version six, which is starting to gain some popularity on the Internet, although it's still a very, very small percentage.

  • The next is the Internet header length and this is the length of the I P header because down at the bottom there's some options that you can have and the length of these options is variable.

  • But in practice, you will probably never see are almost never see an I P packet with any kind of options in the header.

  • It's very rare to see that these days they were sort of added to the protocol very early on, but it turned out it wasn't wasn't very useful.

  • So you almost never see that.

  • And so the header length is always are almost always gonna be five.

  • And what that means is, it's there.

  • There's five words 32 bit words, Um, and so that you can think of this is you know, each of these rose is 32 bits.

  • And so that's five means that there are five of these 32 bit rose here.

  • Um, and if there are options, this would be six or seven or eight or so forth.

  • And then we have the type of service field, which is an eight bit number that indicates how the routers in the network should prioritize this this particular packet and oftentimes it's not used.

  • But one case where it is used is for routing protocol traffic, which is the routers communicating between each other, too.

  • Exchange routing information on DSO.

  • The routing information is, of course, very important to the network functioning properly, So the writers want to prioritize that traffic, and so they'll set the type of service field to a different value to indicate that and that that's the most common usage.

  • It can also be used to provide, you know, different levels of service in larger network.

  • So our networks that have, you know, customers that are paying more for better service, that sort of thing.

  • It could be used.

  • The next couple fields, the total length, the identification, the flags and the fragment offset.

  • These were all used in I P fragmentation, which I'm not gonna talk about just yet.

  • I may make another video where I go into that and a little bit more detail.

  • The timeto live field is just a eight bit number that is set by the sender and then each router that the packet goes through its deck, prevented by one.

  • And then if the timeto live ever gets to zero, then the router will just discard the packet.

  • And the reason for that is to try to prevent loops.

  • So if we look back up here, you could imagine if a packet comes in here and Denver thinks that to get to whatever, wherever this packet is destined, it should be sent to New York and New York Thinks that should be sent to Atlanta and then Atlanta thinks it should be sent to Denver in Denver is going to send it back to New York.

  • And so on in this packet is just gonna get caught in a loop.

  • And so to prevent this from just like looping indefinitely, the time to live field is Decker minted at each hub, and eventually the timeto live field would get to zero.

  • And one of these routers will just drop the packet, and usually this loop like this might indicate something is gone wrong.

  • But it can happen occasionally when the routing protocols between the routers are transitory state on, and so you might have loops in a network, and that might be normal, but it usually doesn't last for very long at all, but just to handle the case where they do come up, we have this time to live field that that sort of takes over and we'll drop a packet if all else fails.

  • And then finally, we have this header check some, which is works sort of like the frame check sequence in either Ethernet.

  • Remember the French Mexicans, either in the Ethernet frame or or in the PPP frame on it.

  • It's just a check some of the of the values in the I P header.

  • And so it's actually not that useful because we already have this frame check sequence on the Ethernet frame or on the PPP frame or or some some other data link level framing mechanism is usually gonna have ah, frame check.

  • So this Henry Jackson is a bit redundant, but it was It was part of the protocol, and so it it exists on, and that's what it does.

  • And then finally, there's the source address and destination address, which are just the source and destination I p addresses of You know where this packet is coming from and where it is going to and it's the destination.

  • Address is the one that the routers are looking at to figure out how to forward it.

  • And then right after the I p header, we have whatever data is in the I p packet itself and in future videos will explore some different things that we might see inside an I p packet.

  • But what we've looked at so far is just this ICMP message, which is which is the ping message and the way that we know that we haven't.

  • I see icmp message is that this protocol field is gonna be set the one that's protocol field were set to something different than we would have some different data here and again in future videos will look at what some of those things could be.

  • But for now, let's go back up and look at our network and looking exactly what these routers are doing is as we go through.

  • So remember, we have a packet that we're sending from this 1 92.1 68 9 dot to address over here, and the destination is the $20 to address over here.

  • So this is this is the destination address in our in our packet that we're sending.

  • And it's the destination address that each of these routers is looking at.

  • So all of the routing decisions are based just on the destination address.

  • And so in the previous videos, we saw how 1 90 to 1 68 9.2 is gonna is gonna forward it's packet to the San Francisco router.

  • But now let's take a look at what's actually going on inside that San Francisco router.

  • So if we look inside the San Francisco router, it's gonna have a routing table.

  • And this is a snapshot of the actual routing table that's in there so we can take a look at what's going on.

  • Um, and so again, this package is coming in with a destination of 1 92.1 68.20 dot too.

  • And so San Francisco is gonna look in its routing table and try to find an address that matches, and so we have or a prefix that matches, actually.

  • And so the prefix that finds is 1 90 to 1 68 20.0 slash 24.

  • And so the slash 24 means were only comparing the 1st 24 bits, which really means just the 1 90 to 1 68 20 part and so 1 90 to 1 68 20 matches the 1st 24 bits of 1 90 to 1 68 20 dot too.

  • And so is gonna choose this route right here.

  • And there's a couple things that we can see here.

  • So the most important part here is is the next hop.

  • And so that's what being shown here is it says to get to 1 90 to 1 68.20 slash 24 that prefix go to 10.0 that 15 dot to which is right here via E M, too.

  • So it's interface to So it's saying, Take that packet and send it out this direction towards Denver to this 10.0 15 to address.

  • So now that the package has arrived in Denver, Denver is gonna look at it and it's gonna do the same thing.

  • It's gonna look in its routing table for the 1 92 for four matching address for a matching prefix.

  • And so the matching prefects it's gonna find is the same.

  • 11 92 68 20 0.0 slash 24.

  • So the 1st 24 bits of this again matches that destination we're looking for over here, and this time it says to send it out or to send it to 10.0 21 dot to via E.

  • M.

  • Three.

  • And so Interface three is right here and then the 10.0 to 21.2 Next top is his New York, and so it's going to send it out this interface here.

  • And so if we look at New York's routing table, we see that 1 92.1 68 20 slash 24 prefix.

  • And now it says via E.

  • M three, which is Interface three over here.

  • And it doesn't give us the next top on, and it says it's directly connected.

  • Eso What this says is that this address is is directly connected to this interface.

  • It's on this same Ethernet network, and so that tells the router that it needs thio go ahead into an AARP for 1 90 to 1 68 22 And it's going to do that Samar process that we saw before and then deliver deliver that Ethernet frame once computer be gets that packet.

  • If it wants to respond, then it's going to send its response back to 1 90 to 1 68 9.2 over here.

  • And so we can actually do the same thing in reverse.

  • So Be is goingto be configured with 1 92.1 68 20.1 as its default gateway.

  • And so will our for that, although it will probably already know the Mac address because it's already received traffic from here.

  • But if it doesn't, it'll art for that and that it will send its frame to New York with the destination of 9.2.

  • And then if we look in New York's routing table, it has a route for 1 90 to 1 68 9 slash 24 which matches the 1 90 to 1 68 9 dot too, and something that I didn't point out before.

  • But which is kind of interesting, as you can see this metric three, and that, in this particular case tells us the distance to, uh to this, which is three hops.

  • So it's one hot to hops, three hops, which is just kind of an interesting little side note there.

  • But in any event, this says to go to 10.0 that 21.1 via E.

  • M.

  • One.

  • And so here's E M.

  • One and we go to 10 0 21 1 So it's taking the same path back, which it doesn't have to do.

  • It could take a different path back, but in this case, the way these routes are, it happens to be taking the same path back and then Same thing.

  • If we look in Denver, we're looking for that 1 92.1 68.9 slash 24 route, which matches our destination now of 1 90 to 1 68 9.2 on.

  • And this says to send it to 10.0 15.1 just right here via E.

  • M.

  • One, which is interface one here and you'll see here.

  • It says metric, too.

  • So now it's only two hops away, so we're getting closer.

  • And then finally, when we get to San Francisco, we look at our routing table again and we see that 1 90 to 1 68.9 slash 24 is directly connected via E.

  • M three.

  • And so we can go ahead and deliver that that packet finally on this Ethernet over here and something I want to point out is that actually, if you if you look through this entire routing table, you'll see there's a bunch of routes to all of these other little networks along here.

  • So from this, this host here, if we if we tried to send traffic to any of these routers interfaces, we should have a route that takes us there.

  • And I would encourage you actually to go through this video and kind of posited at the different spots.

  • Toe look at the writing tables in each of these routers and see, you know exactly what this routing table looks like.

  • There's a couple interesting things actually won.

  • One thing that I would point out, for example, is a Denver.

  • If we're at Denver here, the route to 10.0 dot eight slash 30 which would encompass Tenn 0.8 dot one and 10.0 tita, too.

  • We actually have two different ways to get there.

  • This one says you can go to 10.0 that 16 dot to which is over here you have to out this interface, or you can go to 10.0 21.2 a m three, which is out this interface, and both of those have a metric of two.

  • So what it's saying is that to get over here to this network right here, we can either go this way or we could go this way.

  • And either way, it's too hops.

  • Either we're going through New York or we're going through Atlanta.

  • Either way, it's the same distance.

  • And so I think it's just interesting to see that there there are two ways to get there, and in this particular instance, the router has chosen to prefer this this route.

  • But when we talk more about routing protocols, we'll see how you can influence that and how those decisions are made in more detail.

  • But I would encourage you to go back through this video and take a look a closer look at actually at the's routing tables.

So I want to finish up our discussion of end end packet forwarding through this network by focusing a little bit more on what the routers themselves they're doing.

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ホップバイホップルーティング|ネットワーキングチュートリアル (13件中11件) (Hop-by-hop routing | Networking tutorial (11 of 13))

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    林宜悉 に公開 2021 年 01 月 14 日
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