IP Multicast (formerly also called multicast) has become a hot topic in the Internet. The IP Multicast on the Internet, need to rely on多播路由器来实现
Objective: To better support many communication, that is, send a source to many endpoints. (Broadcast different)
Such as news, Tencent conferencing, video, etc.
Advantages: can greatly save network resources
If the video server To 90 users to send video. If unicast, you need Unicast 90 times. If multicast data is transmitted once only use to the multicast group, the router then forwards the packet replication. Can significantly reduce the waste of network resources.
Second, a multicast IP address
IP multicast packet transmitted more than required to use multicast IP address
Written in the destination address is the multicast datagrams 多播组的地址, which belongs to the class D address (class D addresses dedicated for multicast)
Multicast addresses can only be used for the destination address, source address can not be used. (Not many to one)
Third, multicast datagrams
Multicast datagrams is 尽最大努力交付. And no ICMP error message. Therefore, if the multicast address after the command type in PING, will never be the response.
Fourth, the filter on the LAN
Part of participating in a multicast LAN, some do not participate in multicast. To prevent the site is not involved in multicast receive multicast data, you need to rely on 硬件过滤(network card, specific principles ???).
Since the mapping between IP multicast addresses and Ethernet hardware address is not unique, and therefore subject to the host multicast datagrams is even in IP层利用软件进行过滤, the host is not present to receive datagrams discarded.
Fifth, the multicast protocol requires two
IGMP Internet Group Management Protocol
A local area network connected to a router, as long as the user is a member of the multicast group of the existence of the local area network, and a router receives information multicast group, the router sends multicast data to a local area network, the user then do not accept no matter what.
Similar and ICMP, IGMP is also present in the data portion of the IP datagram. therefore,不把IGMP看成是一个单独的协议，而是属于整个网际协议IP的一个组成部分。
IGMP two phases of work
第一阶段When a host if a new multicast group, the host should multicast group of multicast address (D class address) to send IGMP packets, declare themselves to be members of the group. After the local multicast router receives the IGMP packet, send group membership to other multicast routers on the Internet.
