Broadcast and multicast

Overview:

Broadcast and multicast, as the name implies, are passed to all systems in the subnet or to a system belonging to a multicast group. They apply only to UDP.

For IP, we know that the host number is 1 of the broadcast address, while the Class D IP (that is, from 224.0.0.0 to 239.255.255.255) is the multicast address.

For Ethernet, when the lowest bit of the highest byte in the address is set to 1, it means that the address is a multicast address, that is, 01:00:00:00:00:00 (the Ethernet broadcast address FF:FF:FF:FF:FF:FF is a special case of multicast addresses).

If the NIC receives a frame, go through 4 steps first:

1, interface card. If frame inspection and error, the frame will be discarded;

2, device driver. First look at the frame type specified to use the protocol (such as IP, ARP, etc.), and then multicast filtering to detect whether the host belongs to the multicast group;

3. When the frame type is specified as the IP datagram, it is transmitted to the IP layer. The IP layer carries out more filtering detection based on the source address and destination address in the IP address. If normal, pass to the next layer;

4, TCP, or UDP. Filtering based on the port number, discarding the datagram and generating an ICMP unreachable message if no process currently uses the destination port number.

Although there are more layers of filtering, the use of broadcasts increases the processing load of hosts that are not interested in broadcast data. So multicast can be used.

Multicast group Address:

There are some conventional multicast group addresses (similar to service and port numbers): 224.0.0.1 represents "all system groups within this subnet", 224.0.0.2 represents "all router groups within the subnet", and so on. The IP addresses of these multicast groups can also be mapped to the corresponding MAC addresses. The rule is to ignore the IP multicast group number up to 5bit, to map the low 23bit to the low 23bit in the Ethernet address (the lowest bit of the highest byte is 1). Because the high 5bit is ignored, the 32 (2 of 5) different multicast group numbers are mapped to an Ethernet address, which requires the device driver or IP layer to filter the packets.

IGMP message:

IGMP messages allow all systems on a physical network to know which multicast group the host is currently in. A process must be associated with an interface if it wants to join a multicast group. A process can join the same multicast group on multiple interfaces.

The multicast router periodically sends IGMP to see if there are any hosts that contain processes that belong to the multicast group. It does not need to know the quantity, as long as it knows if there is or not, in order to determine whether to forward the multicast datagram.

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Caprice

　　Chinese culture is extensive and profound, and it has an extraordinary aesthetic sense in literature. However, in the scientific and technological aspects of the individual think English is more advantageous. When I read this translation, I felt that there were a lot of awkward, blunt and confusing places. such as ports and interfaces, one can not be careful to see the wrong, if port and interface will not be mistaken. And the word spacing in English is easier to grasp the point. Every time I read a translation book always think: Next time must buy the original! But sometimes the original is out of stock, or shipping price from abroad is too much. So if there is a chance to go to Hong Kong to buy some original book back:)

Broadcast and multicast