Foreword
There are networks of many kinds of ways to transfer data, if a device wants to send data to a few specific destinations, but these addresses belong to different network environments, if we send a unicast mode for a device just too much burden, live similar manner, if a live broadcast source sends data to all viewers by means of unicast so no matter how cattle machines consumption can not afford, but they can not broadcast the broadcast domain that is unable to escape the restrictions of the router Therefore we need to multicast transmission to help us solve this problem.
A single IP multicast traffic is sent to a group of users simultaneously, the same multicast data flow on each link is only one copy, thereby reducing bandwidth consumption. With the continuous development of the Internet, a variety of interactive data network, voice and video information more and more, while emerging e-commerce, online meetings, online auction, video on demand, distance learning and other services are also gradually on the rise. Most of these services in line with multipoint mode, information security, paid sex, high network bandwidth requirements.
IP transmission of three ways
| IP Unicast | Data to a unicast address as the destination address for transmission |
|---|---|
| IP Multicast | Data to a multicast address as the destination address for transmission |
| IP broadcast | In a data broadcast address as the destination address for transmission |
The user can recognize the IP address of the host into three categories
| Unicast IP address | A unicast IP address can identify a user equipment, a unicast IP address as the destination address of the data a user can only be accepted |
|---|---|
| Multicast IP address | A plurality of multicast IP addresses able to identify different network locations of user equipments, a multicast IP address as the destination address of the data of different users can be a plurality of receiving network locations |
| IP broadcast address | A broadcast IP address identifies a network can all user equipments, using a broadcast IP address for the destination address of the data can be accepted by all users of the network segment, but not inter-network communication. |
The traditional point to point business
Server provides service for a single user, using unicast transmission of data, there are differences between different data, similar to e-mail services, WEB page service, online banking services.
Multipoint service
Server to provide services for a group of users using multicast transmission of data, no difference in data between different users, similar to the live broadcast, IPTV, video conferencing and so on.
Unicast deployed multipoint applications
As a result of unicast way, each user needs a unicast traffic, and traffic all repeat traffic, thus leading to repeat too much traffic, a serious drain on bandwidth and device link.
Broadcast deployed multipoint applications
The server sends the service all the equipment to the corresponding network segment, whether or not need, and can not guarantee the security of the data, as long as the device is within the region can easily receive the data multicast source.
Multicast deployed multipoint applications
For a multicast mode, a single data stream to a group of users, data is copied distributed routing node as far as possible, non-repetitive traffic, bandwidth resources are saved and the device, and that only the data multicasts the multicast group members can receive the other device can not receive data, pricey security. So we use multicast way to build a multi-multipoint deployment.
Multicast Framework
| Multicast | Concerns |
|---|---|
| Host Access | Where there are multicast receivers? |
| Multicast Source Discovery Technology | Multicast data can be obtained from where? |
| Multicast addressing | The multicast data to where? |
| Multicast routing | How to transmit multicast information? |
In the actual transfer process:
| Multicast source to the router | Generating a multicast source of multicast data, and sends the package to complete the data gateway router. |
|---|---|
| Router to router | Router to selectively forward data and copy recipient according to the distribution. |
| The router to the receiving end | Router receives multicast data sent to the appropriate recipients. |
Multicast source to the router
Unicast data, we need to encapsulate the peer IP, MAC address and local IP, MAC address, which can be obtained by various means, but in our multicast, the purpose of IP, MAC address no longer a single destination address, but address a group, how to determine the multicast IP address and MAC address is a major focus.
Multicast IP address
A multicast IP address and do not indicate a specific host, but a collection of host statement host joins a multicast group that is identified itself
| range | meaning |
|---|---|
| 224.0.0.0—224.0.0.255 | Reserved for routing protocols permanent group addresses |
| 224.0.1.0—231.255.255.255 , 233.0.0.0—238.255.255.255 | Any-Source temporary multicast group address |
| 232.0.0.0—232.255.255.255 | Source-Specific Multicast group address temporary |
| 239.0.0.0—239.255.255.255 | Local management of Any-Source temporary multicast group address |
IP组播常见模型分为ASM模型和SSM模型。
ASM全称为Any-Source Multicast，译为任意源组播。在ASM模型中，任意发送者都可以成为组播源，向某组播组地址发送信息。接收者加入该组播组后，能够接收到发往该组播组的所有信息。在ASM模型中，接收者无法预先知道组播源的位置，接收者可以在任意时间加入或离开该组播组。
SSM全称为Source-Specific Multicast，译为指定源组播。在现实生活中，用户可能仅对某些源发送的组播信息感兴趣，而不愿接收其它源发送的信息。SSM模型为用户提供了一种能够在客户端指定信源的传输服务。SSM模型和ASM模型的根本区别是接收者已经通过其他手段预先知道了组播源的具体位置。SSM和ASM使用不同的组播地址范围，直接在接收者和组播源之间建立组播转发树。
组播MAC地址
组播MAC地址，第一个字节的最后一位为1。
单播MAC地址，第一个字节的最后一位为0。
IANA规定，IPv4组播MAC地址的高24位为0x01005e，第25位固定为0。
组播IP地址与MAC地址的映射
In order to multicast sources and multicast group member communication, network-layer multicast, the IP multicast address. Meanwhile, in order to achieve the correct transmission of the local multicast information on the physical network, it is necessary to provide a link level multicast, a multicast MAC address. The multicast data transmission, the destination is not a specific receiver, but a member of the group uncertain, so a technique is needed to map IP multicast address as the multicast MAC address.
Multicast address 24 fixed 01-00-5E, and bit 25 is 0, the follow-up of 23 multicast IP addresses from our interception over 23, it becomes our multicast MAC address.
But it also creates a problem, that is, as long as after 23 the same multicast IP address, their multicast MAC address will be identical, but the IETF think in two or more groups with a local area network address to generate the same MAC address chances are very low and will not cause much impact.
