Sorting out the knowledge points of multicast and broadcasting

The role of multicast

Advantages of multicast ：

1. Deliver packets to multiple destination addresses
2. The client requests the server , But there is no need to know the server address , It is suitable for scenarios where the client knows little about the network environment , A typical application is DHCP
3. Reduce the cost of broadcasting , The broadcast will be sent to all hosts in the LAN , Along the protocol stack, it will not return until the transport layer finds that there is no process on the destination port ICMP Error message , It costs a lot , After using multicast instead , Unrelated hosts , This message is discarded on the network interface , It reduces the overhead of subsequent protocol stacks

The disadvantages of multicast ：

1. It's more complicated to implement

Anysource multicast and source specific multicast

• Anysource multicast ASM

Anysource multicast means that the receiver of multicast does not pay attention to the address of the sender , Just send it , I accept both .

• Source specific multicast SSM

The receiver is sensitive to the identity of the sender , Only messages sent by the sender in the specified list , Only the receiving party receives . Nowadays, source specific multicast is used more

Multicast address and MAC Mapping between addresses

Corresponding to the broadcast address MAC The fixed address is all f, Multicast address also has its corresponding MAC Address , Just this address is not fixed , It has something to do with multicast address .

IPv4 Multicast and MAC Address mapping

step ：

1. MAC The address is 48 position , Before sharing 24 position +1 Transposition + after 23 position
2. front 24 The fixed position is 01:00:5e
3. after 23 Bit after the multicast address 23 position

Because the multicast address range is 224.0.0.0-239.255.255.255, Altogether 2^{28 An address ,MAC The address is only taken after 23 position , So every 2}5=32 Multiple multicast IP Address , Share a multicast MAC Address

IPv6 Multicast and MAC Address mapping

step ：

1. MAC Before the address 16 Bit fixation 33:33
2. MAC After the address 32 Position fetching IPv6 After multicast address 32 position

because IPv6 Before multicast address 16 Bit fixation , after 112 Bits can be arbitrary , So there is 2^80 Network layer addresses share a link layer address .

View the multicast group corresponding to the network port

Linux Use the following command to view the multicast group corresponding to the network port :

[[email protected] ~]# netstat -gn
IPv6/IPv4 Group Memberships
Interface       RefCnt Group
--------------- ------ ---------------------
lo              1      224.0.0.1
enp175s0f0      1      224.0.0.1
virbr1          1      224.0.0.1

224.0.0.1 It is a multicast address added to all network ports of all hosts by default , That means , Message with destination address of this address , Will be received by this network port . You can see , This address can ping through .

The host filters the multicast address

After the multicast message passes through the switch , Will be sent by flooding VLAN Every host inside . The host further filters the multicast packets .

The first is the filtering of the mesh port ： By default, the network interface will receive MAC Messages and broadcast messages whose address is the address of this network interface , The situation of receiving multicast messages at the network interface is different , There are two ways to implement mesh filtering ：

• Maintain the multicast address corresponding to the multicast address of interest to the host on the network interface HASH value , When the destination address of the message is in these HASH Among values , Just give the message to the protocol stack , Otherwise, discard
• Maintain a multicast table on the network interface , When the destination address of the message is in this table , Just give the message to the protocol stack .

NIC After filtering the multicast address , The message enters the protocol stack , The protocol stack will further filter the message .