What is unicast, multicast, and broadcast? BR

 What is unicast, multicast, and broadcast?

The terms unicast, multicast, and broadcast are all terms used to describe the communication methods between network nodes. So what exactly do these terms mean? What is the difference? Let's break it down.

--★Unicast★--


The communication between network nodes is like a conversation between people. If a person speaks to another person, the term network technology is used to describe "unicast". At this time, information is received and transmitted only between two nodes. See figure 1.

Figure 1 unicast: one-to-one

Unicast has been widely used in the network, and most of the data on the network is transmitted in the form of unicast, but the network users do not know it. For example, when sending and receiving emails and Browsing webpages, you must establish a connection with the mail server and web server. At this time, the unicast data transmission mode is used. However, "point-to-point communication" is usually used instead of "unicast", because "unicast" is generally used in comparison with "Multicast" and "broadcast.

--★Multicast★--

"Multicast" can be understood as a person talking to multiple people (but not all people present), which can improve the call efficiency. If you want to notify a specific person of the same thing, but do not want others to know, It is very troublesome to use the phone to notify one by one. Instead, you can use the speakers of daily life to broadcast notifications, the goal of notifying individual users is not achieved. In this case, "multicasting" is very convenient and convenient, but there are very few multicast devices in real life. Multicast 2 is shown in.

Figure 2 multicasting: One-to-multiple

"Multicasting" can also be called "Multicast". It is not widely used in network technology. It is particularly suitable for online video conferencing and on-demand video streaming. Because if the single-play mode is used for transmitting data one by one node and how many target nodes are there, there will be many transmission processes. This method is obviously extremely inefficient and is not desirable; if you use a broadcast method that does not differentiate the target and sends all data, although the data can be transferred at one time, it is clear that the purpose of distinguishing specific data receiving objects is not achieved. Multicast can be used to transmit data from all target nodes at a time, or to transmit data only to specific objects.

Multicast over an IP network is generally achieved through multicast IP addresses. Multicast IP address is a Class d ip address, that is, the IP address between 224.0.0.0 and 239.255.255.255. The DHCP manager in Windows 2000 supports automatic allocation of multicast IP addresses.

--★Broadcasting★--

"Broadcast" can be understood as a person who speaks to the audience through a broadcast horn. The advantage of this is that the call efficiency is high, and the information can be transmitted to the whole at once, as shown in 3.

"Broadcast" has many applications in the network. For example, the client automatically obtains the IP address through DHCP through broadcast. However, compared with unicast and multicast, broadcast occupies almost all the bandwidth of the network in the subnet. Let's take a meeting as an example. Only one person can speak at the venue. Imagine that if everyone uses a microphone to speak at the same time, it will turn into a box of porridge.

A large number of broadcast packets cannot appear in the network for a long time. Otherwise, the so-called "broadcast storm" may occur ". Broadcast storm means that the network is occupied by a large number of broadcast data packets for a long time. Normal point-to-point communication cannot proceed normally, and the external performance is that the network speed is extremely slow. There are many reasons for a broadcast storm. A faulty Nic may send broadcast packets to the network for a long time, resulting in a broadcast storm.

Figure 3 broadcast: One-to-all

The working principle of the hub determines that it is impossible to filter broadcast storms. General switches do not have this function. However, some network switches (such as omnidirectional QS series switches) as well as the broadcast storm filter function, the router itself has the function of isolating the broadcast storm.

Broadcast storms cannot be completely eliminated, but they can only be transmitted within the same subnet, just as the sound of speakers can only be transmitted within the same venue, therefore, in a large and medium-sized LAN consisting of hundreds or even thousands of computers, subnets are generally divided, just like isolating a hall from many small halls by walls, to isolate broadcast storms.

In an IP address, the broadcast address is represented by the IP address "255.255.255.255". This IP address represents all IP addresses in the same subnet.

Unicast, multicast, and broadcast are the basic knowledge of network technology. Readers interested in network technology must understand it!

 

The terms unicast, multicast, and broadcast are used to describe the communication methods between network nodes. So what exactly do these terms mean? What is the difference ?.

1. unicast: communication between network nodes is like a conversation between people. If a person speaks to another person, the network technology is used to describe "unicast". At this time, information is received and transmitted only between two nodes. Unicast has been widely used in the network, and most of the data on the network is transmitted in the form of unicast, but the network users do not know it. For example, when sending and receiving emails and Browsing webpages, you must establish a connection with the mail server and web server. At this time, the unicast data transmission mode is used. However, "point-to-point communication" is usually used instead of "unicast", because "unicast" is generally used in comparison with "Multicast" and "broadcast.

2. multicasting: "multicasting" can also be called "multicast". There are not many applications in network technology, and online video conferences and on-demand videos are particularly suitable for multicasting. Because if the single-play mode is used for transmitting data one by one node and how many target nodes are there, there will be many transmission processes. This method is obviously extremely inefficient and is not desirable; if you use a broadcast method that does not differentiate the target and sends all data, although the data can be transferred at one time, it is clear that the purpose of distinguishing specific data receiving objects is not achieved. Multicast can be used to transmit data from all target nodes at a time, or to transmit data only to specific objects. Multicast over an IP network is generally achieved through multicast IP addresses. Multicast IP address is a Class d ip address, that is, the IP address between 224.0.0.0 and 239.255.255.255. Windows
The DHCP manager in 2000 supports automatic allocation of multicast IP addresses.

3. Broadcasting: "broadcasting" has many applications in the network. For example, the client can automatically obtain the IP address through DHCP through broadcasting. However, compared with unicast and multicast, broadcast occupies almost all the bandwidth of the network in the subnet. Let's take a meeting as an example. Only one person can speak at the venue. Imagine that if everyone uses a microphone to speak at the same time, it will turn into a box of porridge. The working principle of the hub determines that it is impossible to filter broadcast storms. General switches do not have this function. However, some network switches (such as omnidirectional QS series switches) as well as the broadcast storm filter function, the router itself has the function of isolating the broadcast storm. Broadcast storms cannot be completely eliminated, but they can only be transmitted within the same subnet, just as the sound of speakers can only be transmitted within the same venue, therefore, in a large and medium-sized LAN consisting of hundreds or even thousands of computers, subnets are generally divided, just like isolating a hall from many small halls by walls, to isolate broadcast storms.
In an IP address, the broadcast address is represented by the IP address "255.255.255.255". This IP address represents all IP addresses in the same subnet.

 

There are three communication modes in the current network: unicast, broadcast, and multicast. Among them, multicast occurs at the latest time, but it also has the advantages of unicast and broadcast, it has the most promising future.

I. unicast:

One-to-one communication mode between hosts. vswitches and routers in the network only forward data and do not copy the data. If 10 clients need the same data, the server needs to transmit the data one by one and repeat the same work for 10 times. However, because it can respond to each customer in a timely manner, all current web browsing uses the IP Unicast protocol. Vrouters and vswitches in the network select a transmission path based on the target address, and transmit the IP Unicast data to the specified destination.
Advantages of unicast:

1. The server responds to client requests in a timely manner.

2. The server can easily implement personalized services by sending inaccessible data to requests of each customer.

Disadvantages of unicast:

1. the server sends data streams to each client. The server traffic is equal to the number of clients × client traffic. The server is overwhelmed in streaming media applications with a large number of customers and high traffic per client.

2. The existing network bandwidth is a pyramid structure. The inter-city backbone bandwidth is only equivalent to 5% of the total bandwidth of all its users. If all the unicast protocols are used, the network trunk will be overwhelmed. Now, P2P applications have frequently congested the trunk. As long as 5% of customers use the network at full speed, no one else needs to play. It is almost impossible to expand the trunk by 20 times.

Ii. Broadcast:

One-to-all communication mode between hosts. The network copies and forwards signals from each host unconditionally, all Hosts can receive all the information (whether or not you need it). Because you do not need to select a path, the network cost can be very low. A cable TV network is a typical broadcast network. Our TV actually receives signals from all channels, but only restores signals from one channel to a screen. Broadcast is also allowed in the data network, but it is restricted within the LAN of a layer-2 switch. Broadcast Data is prohibited from passing through the router to prevent broadcast data from affecting hosts in a large area.
Broadcast advantages:

1. Simple network equipment, simple maintenance, and low network deployment cost

2. Because the server does not need to send data separately to each client, the server traffic load is extremely low.

Disadvantages of broadcast:

1. Unable to provide personalized services in a timely manner based on the requirements and time of each customer.

2. The bandwidth allowed by the server to provide data is limited. The maximum bandwidth of the client = the total bandwidth of the service. For example, the cable TV client line supports 100 channels (IF digital compression technology is used, 500 channels can be provided theoretically ), even if the service provider has more financial resources to configure more sending devices and change them to the fiber trunk, the limit cannot be exceeded. That is to say, it is impossible to provide more diversified and personalized services to many customers.

3. Broadcasting is prohibited from being transmitted over the Internet broadband network.

Iii. Multicast:

One-to-one communication mode between hosts, that is, hosts added to the same group can accept all data in the group, vswitches and vrouters in the network only copy and forward the required data to the demanders. A host can send a request to a vro to join or exit a group. The vro and vswitch in the network can copy and transmit data, that is, only the data in the group is transmitted to those hosts in the group. In this way, data can be transmitted to multiple hosts that need to be added to the group at a time, and other communications between hosts that do not need to be added to the group are not affected.
Advantages of Multicast:

1. clients with the same data stream need to join the same group to share a data stream, saving the server load. It has the advantages of broadcast.

2. Because the multicast protocol replicates and forwards data streams based on the receiver's needs, the total service bandwidth of the server is not limited by the bandwidth of the client access end. The IP protocol supports over 0.2 billion 268435456 () multicast packets, so it provides a wide range of services.

3. This protocol is the same as the unicast protocol that allows transmission over the internet broadband network.

Disadvantages of Multicast:

1. Compared with the unicast protocol, there is no error correction mechanism and it is difficult to make up for packet loss errors, but it can be compensated through a certain Fault Tolerance Mechanism and QoS.

2. although the current network supports multicast transmission, it still needs to be improved in terms of customer authentication and QoS. These shortcomings have mature solutions theoretically, it only needs to be gradually applied to existing networks.

 

A layer-2 switch is a device at the data link layer. It can read the MAC address information in the data packet and exchange it based on the MAC address. There is an address table inside the vswitch, which indicates the relationship between the MAC address and the vswitch port. The Layer 2 vswitch does not limit the broadcast package, and copies the broadcast package to all ports.

Broadcast is divided into two-layer broadcast and three-layer broadcast, and two-layer broadcast is FFFF. FFFF. FFFF (MAC address), when a L2 Switch encounters such a packet, it will flood to all ports of the same VLAN and will not be filtered out, because if a L2 Switch filters this packet, how can ARP broadcast work normally? Without ARP, how do I communicate with PCs in the same network segment? The router can filter the layer-2 broadcast because the router is a layer-3 device. If the router does not filter such packets, the layer-2 broadcast packet initiated by anyone will reach the entire Internet range. What else do the router need to do? The router is used to divide the broadcast domain.

For layer-3 broadcast, the local broadcast 255.255.255.255 and the specific subnet broadcast such as 192.168.1.255/24, the local broadcast 255.255.255.255 certainly did not span the router, because once such broadcast can span the router, such broadcasts initiated by anyone will run to any corner of the Internet. However, the broadcast of a specific subnet can be configured to allow cross-vro or cross-vro. If allowed, I can initiate a layer-3 broadcast to a specific subnet across vro. Generally, do not allow this type of broadcast, because once you allow it, it can easily cause ICMP attacks.
Sumrf attacks.

The replication function of a vswitch or vro is assumed to receive, process, and forward the data to the corresponding interface (store one copy and multiple copies for replication)

 

Process for layer-3 switches to receive data:

 

Device A with IP address ---------------------- layer-3 Switch ---------------------- Device B with IP Address

For example, if a wants to send data to B and the target IP address is known, A uses the subnet mask to obtain the network address and determine whether the destination IP address is in the same network segment as itself.

If a is in the same network segment but does not know the MAC address required for data forwarding, A sends an ARP request, B returns its MAC address, and a uses this mac encapsulation packet to send it to the switch concurrently, the second-layer switch module is used to find the MAC address table and forward data packets to the corresponding port.

If the destination IP address is not displayed in the same CIDR block, a needs to communicate with B, and there is no corresponding MAC address entry in the stream cache entry, send the first normal data packet to a default gateway. This default gateway is usually set in the operating system and corresponds to the layer-3 routing module. Therefore, we can see that for data not in the same subnet, the MAC address of the default gateway is first placed in the Mac table. Then, the layer-3 module receives the packet and queries the route table to determine the route to B. A new frame header is constructed, the MAC address of the default gateway is the source MAC address, and the MAC address of host B is the target MAC address. Through a certain identification trigger mechanism, it establishes the MAC address and forwarding port correspondence between host a and host B, and records the data from host a to host B in the stream cache entry table, it is directly submitted to the layer-2 Switching Module. This is generally referred to as one-time route forwarding.

On the surface, layer-3 switches are the combination of layer-2 switches and routers. However, this combination is not a simple physical combination, but a logical combination of strengths. When the first data stream of a certain information source performs layer-3 switching, the routing system generates a Mac ing table between the MAC address and IP address, the table is stored. When subsequent data streams from the same information source enter the switching environment again, the switch will map the table based on the address generated and saved for the first time, the second layer is directly transmitted from the source address to the destination address, which is no longer processed by the third route system. This eliminates the network delay caused by Route Selection and improves the packet forwarding efficiency, this solves the speed bottleneck caused by routes when inter-network information is transmitted. Therefore, the layer-3 switch can complete both the port switching function of the layer-2 switch and the routing function of some routers. That is, the switch solution of the layer-3 switch is actually a solution that supports multi-layer dynamic integration, although this multi-layer dynamic integration function can also be carried out by traditional routers and second-layer switches to some extent, compared with the three-layer switch, not only does it require more device configurations, more space, more wiring, and higher costs, but also significantly lower data transmission performance, because in massive data transmission, the router in the carrying solution cannot overcome the bottleneck of the route transmission rate.

 

When the host starts the multicast receiving program, it registers a multicast IP address to the TCP/IP protocol. Therefore, when the host receives the IP address of the multicast group, it receives the IP address. At the same time, it also registers a layer-2 MAC address to the NIC working on the data link layer. In this way, when the host receives a group broadcast, it can
Determine whether the multicast packets are needed on the network card.
IGMP: An Internet group management protocol used for interaction between a host and a router. IGMP must be used for all the hosts to be added to the multicast group and all the routers connected to subnets with multicast hosts. IGMP messages cannot be forwarded by routes and can only be placed within the ingress network. The TTL parameter of IGMP is always 1, which ensures the use of IGMP.
Range.
The main function of the multicast routing protocol is to transmit multicast data from one vro over one network to another.
Multicast Routing Protocols are classified into intra-domain multicast routing protocols and Inter-Domain multicast routing protocols.
Http://wenku.baidu.com/view/b30740553c1ec5da50e27015.html

 

 

Multicast forwarding technology:
In unicast, data is forwarded based on the destination address of the data packet. The destination address clearly indicates the location of a host, but multicast data packets are not the destination address of the IP data packet, it is not unique, so it needs to use a reverse path forwarding method to forward data packets, it is to check when forwarding, this check determines whether to forward or discard the input information package. What is the check process?
First, you must understand that the RPF single-play protocol is closely related. The so-called reverse path Forwarding is based on viewing where the multicast source is located, and the multicast source is an IP Unicast address, therefore, the router checks the source address of the arrived data packet, and then checks the route table to see which interface the source address can be reached through. If the information package arrives on the interface that can return to the original site, this indicates that the multicast data stream is in the correct direction, then the RPF check is successful and the information packet is forwarded. However, if the original address of this data packet is not from this interface, it indicates that this multicast packet is not obtained from the optimal path and should be discarded.
Because RPF must be combined with the unicast route table, the implementation of the IP Unicast route table should be completed in advance before the multicast of the component.

No multicast function switch forwards multicast data packets:
For some bridges and L2 devices, they cannot implement routing or RPF. For a L2 Switch that cannot identify multicast data streams, it receives a multicast packet and processes it according to the processing method of the broadcast packet.

Implement the multicast function of a layer-2 Switch:
Because the vswitch without the multicast function is the same as the broadcast when forwarding multicast data packets, it cannot achieve the purpose of multicast. Therefore, many L2 switches now have added the multicast function. After the multicast function is added, you can recognize multicast streams, you can also map multicast addresses and ports for multicast streams.
A vswitch that can recognize multicast adds a MAC address in its forwarding table that is a multicast address, and the port contains all the ports connected to the host that wants to receive multicast data.