Table of contents
5.3 Internet Group Management Protocol (IGMP)
5.4 Multicast routing algorithm
5.1 Concept of multicast
In order to better support one-to-many communications such as video conferencing, it is necessary that a single packet sent by the source host at one time can reach several destination hosts identified by a group address and be correctly received by them . This is multicast ( multicast) mechanism .
Compared with unicast, in one-to-many communication, multicast only needs to send one data at a time . Only when the transmission path bifurcates, the packet will be copied and forwarded , which greatly reduces the load of the network and Sender's burden. Multicast requires the support of a multicast router that can run the multicast protocol .
5.2 IP multicast address
- In IPv4, Class D addresses are used as multicast addresses . Multicast addresses can only be used as destination addresses , not source addresses.
- Each Class D address is used to identify a multicast group. All hosts that use the same IP multicast address to receive IP multicast datagrams form a multicast group.
- A host can join or leave a multicast group at any time. There are no restrictions on the number of multicast group members and their geographical location. A host can belong to multiple multicast groups.
- Non-multicast group members can also send IP multicast datagrams to the multicast group.
- Like IP datagrams, IP multicast datagrams are also " delivered on a best-effort basis " and are not guaranteed to be delivered to all members of the multicast group.
- No ICMP error messages are generated for IP multicast datagrams . Testing the multicast address using the PING command will not get a response.
IP multicast can be divided into :
- Hardware multicast only on this LAN .
- Multicasting over the Internet . At present, most hosts are connected to the Internet through LAN. Therefore, in the final stage of multicasting on the Internet, IP multicast datagrams must be delivered to all members of the multicast group using hardware multicast on the LAN.
Hardware multicast on LAN
Since the mapping relationship between IP multicast addresses and multicast MAC addresses is not unique, hosts that receive IP multicast datagrams must use software to filter them at the Internet layer and discard IP multicast datagrams that are not intended to be received by the host.
When multiple unicasts are used to simulate a multicast, the delay of the router will be very large, but the delay of processing a multicast packet is very small.
5.3 Internet Group Management Protocol (IGMP)
To let the router know the membership information of the multicast group, IGMP needs to be used .
IGMP is not a protocol for managing all multicast group members within the Internet. IGMP is only valid on this network . IGMP does not know the number of members in the IP multicast group, nor does it know on which networks these members are distributed.
IGMP allows the multicast router connected to the local LAN to know whether a host (a process on the host) on the local LAN has joined or exited a certain multicast group.
5.4 Multicast routing algorithm
Multicast routers connected to the LAN must also work together with other multicast routers on the Internet in order to transmit multicast datagrams to all group members at the minimum cost, which requires the use of multicast routing protocols .
Multicast routing selection: Find the multicast forwarding tree with the source host as the root node. Different multicast groups correspond to different multicast forwarding trees. The same multicast group will also have different groups for different source points. broadcast forwarding tree.
Each packet is transmitted only once on each link (that is, routers on the multicast forwarding tree will not receive duplicate multicast datagrams).
