IP multicast (Multicast) on the ring LAN)

Introduction

IP multicast provides a method to send messages to a group of hosts. IP addresses are described in the destination address column of IP datagram. For specific formats and usage, see IP protocol. A group address is also called a Class d ip address. Its range is 224.0.0.0 to 239.255.255. This article mainly describes how to map IP multicast addresses to the corresponding MAC addresses of the ring.

Background

The symbol ring structure provides several types of address mechanisms, including separate and Group addresses. A special subtype in A group address is called a function address, which is indicated by one of the target MAC addresses. They are widely used in card ring, such as ring monitoring, NETBIOS, bridge, and LAN Manager frames. However, there are only 31 functions, so some unrelated functions need to share the same function address. If you can use an IP address that is the same as that of the FDDI network to broadcast multiple IP addresses to a hardware multicast address, it would be better. However, the current licensing ring controller chip does not support such ing. To learn why not, let's take a look at the target MAC address format.

Destination Address format

The destination MAC address consists of 6 bytes. In the following table, the transmission sequence is from byte 0 to byte 5, from 0th bits to 7th bits. This is the standard bitwise order of IEEE 802.2 addresses. The bit order in the receiver's memory can be the opposite of that in the middle. The maximum bit is 0th bits.

7
6
5
4
3
2
1
0
　
U/L
I/g
Byte 0

Byte 1

Fai
Byte 2

Byte 3

Byte 4

Byte 5

The I/G bit indicates whether the address is an independent address (0) or a group address (1 ). It is very similar to dix In the ethernet address format. U/L indicates whether the address is globally managed (0) or locally managed (1). The Global Management address is the address specified by the Standardization Organization. If I/g = 0 and U/L = 0, this address is the address of the global management group. If I/g = 1, U/L = 1, this address is a locally managed group address or functional address. Whether it is a functional address depends on the functional address flag (FAI) bit. If Fai = 0, the address is a functional address, if FAI is not equal to 0, the address is the group address managed locally.

Different function addresses are composed of 2 bytes (only the remaining 7 bits), 3, 4, and 5 31 bits, but the function addresses can only be set to one bits at a time, therefore, the number of functional addresses is limited to 31. There are three ways to map IP multicast addresses to hardware addresses:

IP multicast address and all loop broadcast address ing
Ing between IP multicast addresses and specified functional addresses
Ing between an IP multicast address and an existing IEEE specified IP multicast address
To ensure interoperability, all systems that support IP multicast must use the same hardware address on the first ring. The bridge can be converted between the above three methods. Considering compatibility, methods 1 and 2 must be supported for method 3 and method 1 must be supported for method 2. Method 3 is recommended. If not, method 2 is used, and method 1 is used.

IP multicast address

Because the number of functional IP addresses in the ring is small, all IP multicast addresses are mapped to one functional IP address. In standard format, this address is 03-00-00-20-00-00, in non-standard format, the address is a C0-00-00-04-00-00. Because the address is limited, do not think that all the addresses sent to the above must be IP multicast. As mentioned above, many protocols share a function address, which requires attention.