What does multicast mean? Watching 4K live streaming on TV doesn't take up broadband—I caught a quick look and understood
last week, my home TV was running IPTV with 4K live channels, and I casually ran a speed test on my computer—941 Mbps, not moving at all. When switching to the same TV and the live streaming app in the box to watch the same channel, the speed test immediately dropped to around 910. For the same channel, one view doesn't take up bandwidth, another does; the difference lies in the delivery method: IPTV uses multicast, while the apps in the box use unicast. This article will break down the group broadcast issue and clearly explain it, while I'm also showing you the bags I caught.
first distinguish between unicast, broadcast, and multicast methods
there are three ways to deliver data online, the most direct example is to compare it to a courier or a newspaper.
single-broadcast is one-on-one delivery. you watch the video, the server sends one copy to your home, and you cover the shipping cost (which takes up broadband). The neighbor was watching too, so the server sent him a separate copy, and everyone went their separate ways. Ninety percent of the traffic at home—browsing, watching videos, downloading—is all unicast. The advantage is precision; the downside is that too many people cause server fatigue and bandwidth crowding.
broadcast is the village's loudspeaker. send out a packet, all devices on the same network segment have to listen once. ARP finding people is by broadcasting, DHCP the door number is also broadcast . These bags are small, just shout, and usually don't cause trouble; If something really goes wrong, the loudspeaker gets caught in the network cable loop and loops endlessly. That's the loop pitfall discussed in the switch article, but I won't elaborate here.
multicast is subscription. newspaper only prints one copy, only one copy is sent by mail. Once it reaches your doorstep, whoever orders it can pick it up, and you don't need to worry about equipment you haven't ordered. IPTV live channels and casting devices have discovered each other, all following this path.
multicast addresses are very recognizable. In IPv4 addresses, the entire segment from 224.0.0.0 to 239.255.255.255 is assigned to multicast. Ordinary device IPs never look like this, so packet capture is instantly recognizable. Most ISP IPTV channels use addresses starting with 239, and the corresponding multicast MAC addresses uniformly start with 01:00:5e — this prefix is also mentioned as the root cause of switch confusion.
Why doesn't watching live TV account for megabytes of broadband
first calculate the single-broadcast accounts. The live streaming app in the box is unicast: the platform server pushes the stream to every provider, and your home charges 4K per channel, so the broadband pool is reduced by ten or twenty Mbps. The more people watching, the more crowded the server and the links along the route, so some stations can lag during the evening rush hour.
multicast accounts are completely different. Operators in the city only push one from each channel's streaming , distributing it all the way to neighborhoods along their networks. Whoever wants to watch can send a "I want to subscribe" message from the set-top box, and the stream will be shared locally. A hundred households watching the Gala, or one watching the Gala, the entire metropolitan area network is running around the same stream. This stream is the channel the operator assigned to IPTV, not through your broadband logic link, so my speed test is 941 MHz and doesn't move at all—the pool hasn't moved at all.
| Comparison Items | Box Live App (Unicast) | IPTV Live (Multicast) |
| Streaming Streams | One per Person | one portion citywide, distributed locally |
| does not occupy broadband | account, 4K route about 15~25 mega | does not occupy one port, using independent channels |
| evening peak performance | the server and CDN | stable and unrelated to the viewer's level |
| which port | go is up to you. As long as you have internet | optical modem, IPTV port, or dedicated VLAN |
this is also why, when I used to calculate how many megabytes of broadband installed, I dared to exclude IPTV from the account, but the OTT box I bought myself had to be counted in— two types of freebies are two different things, whether they occupy the pool or not. Telecom later launched a compromise on soft terminals: no set-top boxes were needed, but they used your broadband. Even 4K would take up 20 to 30 Mbps, so you had to settle the when buying two TVs.
IGMP Snooping: The switch secretly does the work in the middle
the protocol used for "subscription" on set-top boxes is called IGMP. You press CCTV5, and the set-top box sends an IGMP message to the network, saying you want to subscribe to the 239.x.x.x group; when switching channels, send another leave message to unsubscribe from the old one. The message moves upward, and the operating equipment along the way knows where the stream is headed.
The trouble lies with your family. When multicast streaming reaches your doorstep, your switch must allocate it to the set-top box's port. But switch forwarding relies on the MAC address table, and the multicast address is 'a group of people' rather than 'a single device'. The corresponding port can't be found in the table—what should you do? foolproof switch's handling is flooding: every port sends a copy. anyway, multicast MACs start with 01:00:5e, and the switch saw it wasn't a regular unicast, so they just went with whole-house broadcasting.
my 8-port gigabit foolproof switch at home is just like that. I checked the packet capture on the NAS network port. When the set-top box was open, the command line was:
tcpdump -i eth0 net 239.0.0.0/8 -c 20
the NAS port can capture packets starting with 239 every second, and a continuous stream of 4K data of over ten megabytes is wasted on a hard drive cabinet that isn't watching TV at all. That's why when I first wrote about single-line multiplexing for IPTV I mentioned IGMP Snooping: switches that support this feature "eavesdrop" on IGMP messages, record which port subscribes to which group, and from then on, only send the subscribed ports, leaving other ports clean. I moved the set-top box to my cousin's managed switch with Snooping to test it. The NAS port immediately went silent, and the flood of over ten megabytes disappeared.
to explain an old question: Why does it take one or two seconds to change channels for IPTV? opening the box, there are three waiting stages—set-top box shipment first cancels order, device must confirm you haven't slipped or missed, default wait two to three seconds to query; Send again to join, reporting the packet step by step, with each hop having a delay; If it flows to the set-top box, you still need a pre-read buffer and wait for the next keyframe to release the image. Vendor optimization is just about shortening queries, speeding up exits, and pre-adding adjacent channels, but the magnitude of a second or two is determined by the mechanism. Waiting for it is not a stutter.
why does IPTV require a separate port, and also a VLAN
?reading this, you should be able to explain why: once multicast leaves the operator's network and enters your home, it's your home traffic, and when it floods, no one can escape it. So operators simply set up a separate booth for IPTV—a separate IPTV port or a separate VLAN on the optical modem, separated from internet traffic, keeping water out of business.
this design explains several common phenomena. First, the connection labeled OTHER or IPTV in the backend WAN connection table of the optical modem is the entry point to the multicast channel. you lose it during bridging changes, the TV will go black . Saving it depends on copying the VLAN back. Second, some routers support IPTV binding. For example, Xiaomi's hidden IPTV switch essentially isolates this multicast channel inside the router. Third, routers default to not forwarding multicast across subsegments—multicast requires a dedicated multicast routing protocol to cross subsegments, and home routers are not included. Therefore, the set-top box must be directly connected to the IPTV port or corresponding VLAN; you can't just move it behind any router to watch.
Who else at home is using group streaming: Why can casting be found by each other
BesidesIPTV, there is a pile of things quietly using group streams at home, most commonly casting.
the phone casts a single tap, and the TV pops up—both rely on the SSDP discovery protocol, sending messages to port 1900 at 239.255.255.250, a fixed multicast address, and all devices on the same subnet that support casting are listening on this "channel." Apple's AirPlay and printer discovery go via mDNS, address 224.0.0.251. In other words, every time you cast your screen at home, there's a group broadcast call behind it.
this directly explains two troublesome issues. First, connected to guest networks or IP isolated by AP, you can't find TV —isolation blocks multicast, making calls impossible and devices passing each other. Second, I partitioned my home into VLAN zones and the casting stopped —multicast doesn't cross subsidiaries, so calls from Area A can't be heard in Area B. You either move the device you want to cast back to the same area, or set up an mDNS reflector on the gateway to forward and amplify these messages. The principle is the same as IPTV, which cannot cross routers: multicast is inherently inherently incompatible with the network segment.
three issues related to multi-broadcasting, let's take them as they are
| symptoms | root cause | |
| Live streaming screen blacks out, on-demand replay works fine | group stream channel is disconnected: If the VLAN or IPTV port isn't connected | check the optical modem's WAN meter and the OTHER line and the VLAN number, then check and revert |
| wait one or two seconds to change channels | unsubscribe and add buffering mechanisms for delay | This is normal. Don't mess with |
| NAS or other devices that are flooding with tens of megabytes of data for nothing | foolish switch network | switch to a switch with IGMP Snooping, or connect the set-top box separately to the IPTV port |
troubleshooting order: first, check if the live stream is black, then follow the VLAN line when the screen is black; No black screen but full network card, check the switch, see if it's flooded; If everything is normal, don't move. Group broadcasting is just about knowing it's working. In the end, it comes down to one sentence: unicast is one-to-one delivery, broadcasting is loudspeaker, multicast is subscription—watching live TV doesn't take up broadband, switching channels takes a second or two, casting only works on the same network—these are all the rules of this 'newspaper.'
