Watching 4K live broadcast on TV does not occupy bandwidth. When I captured the packet, I found that it was multicast.

Last week, I turned on IPTV and played a 4K live channel on the TV at home. I ran a speed test on my computer - 941 Mbit/s and it didn't move at all. When the same TV is switched to the live broadcast app in the box to watch the same channel, the speed immediately drops to around 910. On the same channel, one view does not occupy the bandwidth, but the other view does. The difference lies in the delivery method.: IPTV uses multicast, and the App in the box uses unicast. This article will explain the multicast issue in detail, and by the way, I will show you the package I captured.

First, distinguish the three delivery methods: unicast, broadcast, and multicast.

There are three ways to send data on the Internet. It is most intuitive to compare express delivery and newspapers.

Unicast is one-to-one express delivery. You watch the video, and the server sends a copy to your home, and your home pays for the shipping cost (which accounts for the bandwidth). The neighbor is also watching, and the server sends him a separate copy, and everyone goes his own way. Ninety percent of the traffic at home - browsing the web, watching videos, downloading - is all unicast. The advantage is accuracy, but the disadvantage is that when there are too many people, the server becomes tired and the bandwidth is crowded.

The radio is the loudspeaker in the village. When a packet is sent out, all devices in the same network segment must listen to it again. ARP is looking for people to make propaganda through the radio , The DHCP door number is also broadcast . This kind of bag is small and can be finished with just a shout, so it usually doesn't get in the way. ； What really happened was that the loudspeaker was caught in the network cable loop and looped endlessly. That was The loop pit mentioned in the switch article , not expanded here.

Multicast is subscription. The newspaper only prints one copy and delivers it by post. When it arrives at your doorstep, whoever ordered it will receive it, and any equipment that has not been ordered will be ignored. IPTV live broadcast channels and screen projection devices discover each other and follow this path.

The multicast address is very recognizable. IPv4 address 224.0.0.0 arrive 239.255.255.255 This entire section is allocated to multicast. The IP of ordinary devices will never look like this, so it can be recognized at a glance when capturing packets. Most operators' IPTV channels use addresses starting with 239, and the corresponding multicast MAC addresses uniformly start with 239. 01:00:5e At the beginning - this prefix will be mentioned later, it is the source of confusion in the switch.

Why watching live TV does not occupy bandwidth megabytes

Let’s calculate the unicast account first. The live broadcast app in the box is unicast: The platform server pushes the stream all the way to each home. If your home receives 4K all the way, the broadband pool will be less than ten or twenty megabits. The more people watching, the more crowded the servers and links along the way will be, so some stations will be stuck during the evening peak.

The multicast account is completely different. Operators put streams of every channel in town Only one recommendation , distributed to the community along its network. If anyone wants to see it, the set-top box will send an "I want to order" message, and this stream will be distributed to you nearby. A hundred households watch the Spring Festival Gala and one household watches the Spring Festival Gala. All the streams running on the metropolitan area network are those. This flow flows through the separate channel designated by the operator for IPTV and does not pass through the logical link of your home broadband. Therefore, when I measured the speed at 941 Mbps, it did not move at all - the pool did not move at all.

This is why I dared to exclude IPTV from my account when calculating the number of megabytes of broadband installed, but the OTT box I bought must be included.—— Whether the two sending methods occupy the pool or not are two different things. . The soft terminal that China Telecom later launched was a compromise: There is no need for a set-top box, but your home broadband is used, and 4K still takes up 20 to 30 MB. This account must be settled when we build two TV sets. .

IGMP Snooping: The switch secretly does the work in the middle

The protocol used by the set-top box to "subscribe" is called IGMP. When you press CCTV5, the set-top box sends an IGMP join message to the network, saying that I want to subscribe to the group 239.xxx. ； When changing channels, send another leave message and unsubscribe from the old one. This message goes all the way up, and the operation equipment along the way knows where this flow is going to be sent.

The trouble is in your home. When the multicast stream enters your home, your switch must assign it to the port on the set-top box. But the switch relies on the MAC address table for forwarding. The multicast address is "a group of people" not "a device". The corresponding port cannot be found in the table - what should I do? The processing method of the fool switch is flooding: Give one copy to all mouths. Anyway, the multicast MAC starts with 01:00:5e. It seems that the switch is not an ordinary unicast, so it might as well be broadcast throughout the house.

This is the case with my 8-port Gigabit fool switch. I captured the packets on the network port of the NAS and looked at it. When the set-top box was turned on, the command was this line.:

tcpdump -i eth0 net 239.0.0.0/8 -c 20

As a result, the NAS port can intercept packets starting with 239 every second, and 4K traffic of more than ten megabytes is flowing to a hard disk cabinet that is not watching TV at all. that's why When I first wrote IPTV single line multiplexing I mentioned IGMP Snooping: The switch that supports this function will "eavesdrop" on the IGMP messages and record which port is subscribed to which group. In the future, multicast will only be sent to the subscribed port and other ports will be clean. I moved the set-top box to my cousin's managed switch with Snooping and tried it. The NAS port immediately became silent and the flood of more than ten megabytes disappeared.

By the way, let me explain an old question: Why does IPTV have to wait a second or two to change channels? There are three stages of waiting when unpacking - the set-top box unsubscribes first, the device needs to confirm that you have not slipped your hand, and the default wait is two to three seconds for the query. ； After sending and joining again, the packets are reported step by step, and there is a delay at each hop. ； When streaming to the set-top box, it has to pre-read the buffer and wait for the next key frame before the picture can be displayed. The manufacturer's optimization is nothing more than shortening the query, speeding up the departure, and pre-adding adjacent channels, but the order of one or two seconds is determined by the mechanism. Wait for it, it will not be stuck.

Why does IPTV need a separate port and a VLAN?

Seeing this, you should be able to deduce the reason.: Once multicast leaves the operator's network and enters your home, it becomes home traffic and no one can avoid it when it floods. Therefore, the operators simply set up a separate stand for IPTV - a separate IPTV port on the optical modem, or a separate VLAN, which is separated from the Internet traffic, so that the water in the well does not interfere with the river.

This design explains several common phenomena. First, the connection with OTHER or IPTV in the backend WAN connection table of Optical Mode is the entrance to the multicast channel. I lost it when changing the bridge, and the TV screen went black. , the only way to save it is to copy the VLAN back. Second, some routers support IPTV binding, such as The IPTV switch hidden in Xiaomi’s backend , the essence is to isolate this multicast channel in the router. Third, routers do not forward multicast across network segments by default - multicast across network segments requires a special multicast routing protocol, which is not suitable for home routers. Therefore, the set-top box must be directly connected to the IPTV port or the corresponding VLAN. You cannot move it behind any router and watch it.

Who else at home is using multicast?: Why can we search for each other when casting screens?

In addition to IPTV, there are a lot of things at home that are secretly using multicast, the most common of which is screencasting.

As soon as the mobile phone casts the screen, the TV will appear - they rely on the SSDP discovery protocol, and the packets are sent to 239.255.255.250 This fixed multicast address is port 1900, and all the devices in the same network segment that support screen mirroring sit on this "channel" to listen. Apple's AirPlay and printer are found through mDNS, the address is 224.0.0.251 . In other words, every time you cast a screen in your home, there is a multicast call behind it.

This directly explains two annoying things. The first thing, The TV cannot be found when connected to the guest network or AP-isolated WiFi. ——Isolation blocks multicasting, making it difficult for calls to pass through, and devices pretending to be blind to each other. The second thing, After I divided my home into VLAN partitions, screencasting also stopped. ——Multicast does not cross network segments, and the shouts in zone A cannot be heard in zone B. Either move the device to be projected back to the same zone, or configure an mDNS reflector on the gateway to specifically forward and amplify these shouts. The principle is the same as that of IPTV but cannot be crossed by a router.: Multicast inherently cannot go out of network segments.

Three problems related to multicast, all right

The order of investigation is smooth.: First check whether the live broadcast is black or not. If the screen is black, go to the VLAN line. ； No black screen but all network cards, check the switch to see if there is flooding ； If everything is normal, don't move it. Just know that the multicast thing is working. One word at the end: Unicast is one-to-one express delivery, broadcast is a loudspeaker, and multicast is a newspaper subscription - watching live TV does not occupy the bandwidth, waiting for a second or two when changing channels, and screencasting can only be used on the same network. These are all the rules of this "newspaper".