Broadcast and multicast (tcp/ip details volume 1/2)

One 、 brief introduction

         Three IP Address ： Unicast address 、 Broadcast address and multicast address .

         Broadcast and multicast only apply to U D P, They are very important for applications that need to send messages to multiple recipients at the same time .T C P It's a connection oriented protocol , It means running on two hosts （ from I P Address determination ） Two processes within （ Determined by the port number ） There is a connection between .

         Consider a shared channel network with multiple hosts such as Ethernet . Each Ethernet frame contains the Ethernet address of the source host and the destination host （4 8 b i t）. Usually Each Ethernet frame is only sent to a single destination host , The destination address indicates a single receiving interface , So it's called unicast ( u n i c a s t ). In this way , The communication between any two hosts will not interfere with other hosts in the network （ Except for the cases that may cause contention for the shared channel ）.
         However , Sometimes a host sends frames to all other hosts on the network , This is radio . adopt A R P and R A R P You can see the process . multicast (multicast) Between unicast and broadcast ： The frame is only transmitted to multiple hosts belonging to the multicast group .
In order to understand radio and multicast , It is necessary to understand the filtering process of frames transmitted from the channel by the host . chart 1 2 - 1 Explain the process .

          First , The network card views the frames transmitted by the channel , Determine whether to receive the frame , If you receive it and then send it to the device driver . Usually, the network card only receives those destination addresses Physical address of network card or Broadcast address Frame of . in addition , Most interfaces are Set to mixed mode , This mode can receive a copy of each frame . As an example , t c p d u m p Use this mode .

         at present , Most network cards are configured to receive a destination address of Multicast address or Some subnet multicast addresses Frame of . For Ethernet , When the lowest bit of the highest byte in the address is set to 1 Indicates that the address is a multicast address , It can be expressed in hexadecimal as 0 1 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0（ Ethernet broadcast address ff : ff : ff : ff : ff : ff It can be regarded as a special case of Ethernet multicast address ）.

         If the network card receives a frame , This frame will be sent to the device driver （ If Frame check and error , The network card will discard the frame ）. The device driver will perform additional frame filtering . First , The protocol to be used must be specified in the frame type （ I P、A R P wait ）. secondly , Perform multicast filtering to detect whether the host belongs to the multicast group described by the multicast address . The device driver then transmits the data frame to the next layer , such as , When the frame type is specified as I P The data tell the time , Just send it to I P layer .I P according to I P The source address and destination address in the address are more filtered and detected . If normal , Send the datagram to the next layer （ Such as T C P or U D P）.

         Every time U D P Received by I P The datagram sent , According to the destination port number , Sometimes active port number is used for datagram filtering . If no process currently uses the destination port number , Just discard the datagram and create a I C M P Unreachable message （T C P Make similar filtering according to its port number ）. If U D P There are tests and errors in data reports , Will be discarded . The problem with using broadcast is that it increases the processing load of hosts that are not interested in broadcast data . Take one and use U D P Broadcast application as an example . If there is 5 0 Host computer , But only 2 0 Participate in the app , Every time 2 0 One of the hosts sent U D P When broadcasting data , rest 3 0 A host has to process these broadcast datagrams . Until U D P layer , received U D P Broadcast datagram will be discarded . this 3 0 Hosts discarded U D P Broadcast datagram because these hosts do not use the destination port .

         The emergence of multicast reduces the processing load of hosts that are not interested in applications . Use multicast , The host can join one or more multicast groups . such , The network card will know which multicast group the host belongs to , Then only those multicast frames of the multicast group where the host is located are received .

Two 、 radio broadcast

        

          Introduce 7 A special I P Address , Pictured 3 - 9 Shown . In this picture , 0 Indicates that all bits are 0;- 1 Indicates that all bits are 1;n e t i d、s u b n e t i d and h o s t i d They are not all 0 Or all 1 The corresponding field of . If the subnet number column is empty, it means that the address is not divided into subnets .

          Divide the table into three parts . The first two entries in the table are special source addresses , The middle term is a special loopback address , The last four items are broad   Broadcast address .
         The first two items in the table , The network number is 0, If the host uses B O O T P The protocol determines the native I P Address can only appear as the source address during initialization .

1. Limited broadcasting

         The restricted broadcast address is 2 5 5 . 2 5 5 . 2 5 5 . 2 5 5. The address Used during host configuration I P Destination address of datagram , here , The host may not yet know the netmask of its network , Even its I P I don't know the address .

         In any case , Routers do not forward datagrams whose destination address is a restricted broadcast address , Such datagrams only appear in the local network .

         An unsolved problem is ： If a host has multiple interfaces , When a process sends a datagram to the broadcast address of the network , For broadcasting , Whether datagrams should be sent to each connected interface ？ If not , Applications that want to broadcast to all interfaces of the host must determine all interfaces in the host that support broadcasting , Then send a datagram copy to each interface . majority B S D The system will 2 5 5 . 2 5 5 . 2 5 5 . 2 5 5 It is regarded as the broadcast address of the first interface after configuration , And it does not provide the function of transmitting datagrams to the interface with broadcasting capability . however , r o u t e d（ see 1 0 . 3 section ） and r w h o d（B S Dr w h o Client's server ） Is to send... To each interface U D P Two applications of datagram . Both applications use a similar startup process to determine all interfaces in the host , And know which interfaces have broadcast capability . meanwhile , Take the broadcast address pointing to the network corresponding to that interface as the destination address of the datagram sent to the interface .

        Host Requirements RFC There is no further reference to whether a multi interface host should send a restricted broadcast to all its interfaces .

2. Broadcast to the Internet

         The broadcast address pointing to the network is the host number of all 1 The address of .A Class webcast address is n e t i d . 2 5 5 . 2 5 5 . 2 5 5, among netid by A The network number of the class .

         A router must forward broadcasts to the network , But it also has to have a choice not to forward .

3. Broadcast to subnet

         The broadcast address to the subnet is the host number of all 1 And the address with a specific subnet number . As a direct broadcast address of the subnet I P The address needs to know the mask of the subnet . for example , If the router receives a message to 1 2 8 . 1 . 2 . 2 5 5 Datagram , When B Class network 1 2 8 . 1 The subnet mask of is 2 5 5 . 2 5 5 . 2 5 5 . 0 when , This address is the broadcast address pointing to the subnet ; But if the mask of the subnet is 2 5 5 . 2 5 5 . 2 5 4 . 0, This address is not a broadcast address pointing to a subnet .

4. Broadcast to all subnets

         Broadcast also needs to know the subnet mask of the destination network , So as to distinguish it from the broadcast address pointing to the network . The subnet number and host number of the broadcast address pointing to all subnets are all 1. for example , If the destination subnet mask is 2 5 5 . 2 5 5 . 2 5 5 . 0, that I P Address 1 2 8 . 1 . 2 5 5 . 2 5 5 Is a broadcast address to all subnets . However , If the network is not divided into subnets , This is a broadcast to the Internet .

3、 ... and 、 multicast

         Points to all subnets I P Multicast provides two types of services ：

•   Send data to multiple destination addresses . There are many applications that send information to multiple recipients ： For example, interactive conference system and distributing mail or news to multiple recipients . If multicast is not used , At present, most of these applications use T C P To complete （ A separate data copy is sent to each destination address ）. However , Even with multicast , Some applications may continue to use T C P To ensure its reliability .
• The client's request to the server . for example , The diskless workstation needs to determine to start the boot server . at present , This service is provided through broadcasting （ Just like No 1 6 Chapter B O O T P）, However, using multicast can reduce the burden of hosts that do not provide this service .

 1. Multicast group address

         The multicast group address includes 111 0 The highest 4 bit And multicast group number . They can usually be expressed as dotted decimal numbers , Range from 2 2 4 . 0 . 0 . 0 To 2 3 9 . 2 5 5 . 2 5 5 . 2 5 5.

         A set of hosts that can receive address data sent to a specific multicast group is called a host group (host group). A host group can span multiple networks . Members of the host group can join or leave the host group at any time . There is no limit on the number of hosts in the host group , At the same time, hosts that do not belong to a host group can send information to the group .

          Some multicast group addresses are I A N A Identified as a well-known address . They are also referred to as permanent host groups , This sum T C P And U D P The familiar ports in are similar . Again , These well-known multicast addresses are in R F C In the latest allocation figures . Note that the groups represented by these multicast addresses are permanent groups , And their members are not permanent .

         for example ,2 2 4 . 0 . 0 . 1 representative “ All system groups in the subnet ”,2 2 4 . 0 . 0 . 2 representative “ All router groups in this subnet ”. Multicast address 2 2 4 . 0 . 1 . 1 Used as network time protocol N T P,2 2 4 . 0 . 0 . 9 Used as a R I P - 2 ,2 2 4 . 0 . 1 . 2 use do S G I The company's d o g f i g h t application .

2. Multicast group address to Ethernet address translation

        I A N A Have an Ethernet address block , That's high 24 bit by 0 0 : 0 0 : 5 e（ Hexadecimal representation ）, This means that the address block has an address range from 0 0 : 0 0 : 5 e : 0 0 : 0 0 : 0 0 To 0 0 : 0 0 : 5 e : ff : ff : ff.I A N A Assign half of them as multicast addresses . To specify a multicast address , The first byte of any Ethernet address must be 0 1, It means with I P The Ethernet address range corresponding to multicast is from 0 1 : 0 0 : 5 e : 0 0 : 0 0 : 0 0 To 0 1 : 0 0 : 5 e : 7 f : ff : ff.

         Here to C S M A / C D Or token network uses I n t e r n e t Standard bit order , In the same order as the bits that appear in memory . This is also the order adopted by most programmers and system administrators . I E E E The document adopts this bit transmission order .Assigned Numbers RFC The differences between these representations are given .

         This kind of address assignment will make the Ethernet multicast address 2 3 b i t And I P The multicast group number corresponds to , By placing the low bit in the multicast group number 2 3 b i t Mapped to the low order in the Ethernet address 2 3 b i t Realization , This process is shown in the figure 1 2 - 3 Shown . Due to the highest multicast group number 5 bit Ignored in the mapping process , Therefore, the multicast group corresponding to each Ethernet multicast address is not unique . 3 2 Different multicast group numbers are mapped to an Ethernet address . for example , Multicast address 2 2 4 . 1 2 8 . 6 4 . 3 2（ Hexadecimal e 0 . 8 0 . 4 0 . 2 0） and 2 2 4 . 0 . 6 4 . 3 2（ Hexadecimal e 0 . 0 0 . 4 0 . 2 0） Both map to the same Ethernet address 0 1 : 0 0 : 5 e : 0 0 : 4 0 : 2 0.

         Since address mapping is not unique , Then the device driver or I P layer （ See the picture 1 2 - 1） The datagram must be filtered . Because the network card may receive multicast data frames that the host does not want to receive . in addition , If the network card does not provide enough multicast data frame filtering function , The device driver must receive all multicast data frames , And then filter them .

         LAN cards tend to have two types of processing ： One is that the network card implements multicast filtering according to the hash value of the multicast address , This means that unwanted multicast data will still be received ; The other is that the network card only receives a fixed number of multicast addresses , This means that when the host wants to receive multicast addresses other than those previously supported by the network card , The network card must be set to “ Multicast hybrid (multicast promiscuous)” Pattern . therefore , Both types of network cards still need the device driver to check whether the received frame is really what the host needs .

         Even if the network card realizes perfect multicast filtering （ be based on 48 bit The hardware address of ）, As a result of D class I P Address to 48 bit The mapping of hardware addresses is not one-to-one , The filtration process is still necessary . Despite the imperfections of address mapping and the need for hardware filtering , Multicast is still better than broadcasting .

         Multicast over a single physical network is simple . The multicast process will aim to I P The address is specified as a multicast address , The device driver converts it to the corresponding Ethernet address , Then send the data out . These receiving processes must notify their I P layer , The datagrams they want to receive sent to a given multicast address , And the device driver must be able to receive these multicast frames . This process is “ Join a multicast group ”（ Use “ Receiving process ” The reason for the plural form is that for a certain multicast information , There are multiple recipients on the same host or multiple hosts , That's why we should use multicast first ）. When a host receives a multicast datagram , It must send a copy to each process belonging to that multicast group . This is similar to the unicast received by a single process U D P The data report U D P Different . Use multicast , There may be multiple processes belonging to the same multicast group on a host .

         When multicast is extended beyond a single physical network, multicast data needs to be forwarded through the router , Complexity increases . A protocol is needed to let the multicast router know any host belonging to the multicast group in the network . This agreement is I n t e r n e t Group management protocol （I G M P）

Four 、 unicast 、 radio broadcast 、 Multicast advantages and disadvantages

unicast ： Communication between network nodes is like a conversation between people . If one person speaks to another , So in terms of network technology, it's “ unicast ”, At this point, information is received and transmitted only between the two nodes . Unicast has been widely used in the network , Most of the data on the network is transmitted in the form of unicast , It's just that ordinary network users don't know . for example , You're on email 、 While browsing the web , Must work with mail server 、Web Server establishes connection , In this case, unicast data transmission mode is used . But usually it uses “ Point to point communication ”（Point to Point） Instead of “ unicast ”, because “ unicast ” Generally speaking, it is related to “ multicast ” and “ radio broadcast ” Use... Correspondingly .

The advantages of unicast ：

• The server responds to the client's request in time
• The server sends impassable data for each customer's impassable request , Easy to realize personalized service .

The disadvantages of unicast ：

• The server sends a data stream for each client , Server traffic ＝ Number of clients × Client traffic ; When the number of customers is large 、 In streaming media applications with large traffic per client, the server is overwhelmed .
• The existing network bandwidth is a pyramid structure , The inter city and inter provincial backbone bandwidth is only equal to the sum of the bandwidth of all users 5％. If all unicast protocols are used , The network backbone will be overwhelmed . current P2P The application has often blocked the trunk . And expand the trunk 20 Times is almost impossible .
   

multicast ：“ multicast ” It can also be called “ Multicast ”, There are not many applications of network technology , Online video conference 、 Online video on demand is particularly suitable for multicasting . Because if you do unicast , Node-by-node transmission , How many target nodes are there , How many times will there be transfers , This approach is obviously highly inefficient , Is not desirable ; If the target is not distinguished 、 Broadcast mode for all transmissions , Although the data can be transmitted at one time , But obviously, it can not achieve the purpose of distinguishing specific data receiving objects . Adopt multicast mode , The data of all target nodes can be transmitted at once , You can also achieve the goal of passing data only to specific objects . 　　IP Network multicast is generally through multicast IP Address to achieve . multicast IP The address is D class IP Address , namely 224.0.0.0 to 239.255.255.255 Between IP Address .Windows 2000 Medium DHCP The manager supports multicast IP Automatic address assignment .

Advantages of multicast ：

• Clients that need the same data stream join the same group and share a data stream , Save the load of the server . Have the advantages of broadcasting .
• Because the multicast protocol copies and forwards the data stream according to the needs of the recipient , Therefore, the total service bandwidth of the server is not limited by the bandwidth of the customer's access end .IP The agreement allows for 2 Billion 6 More than ten million multicast , So its services can be very rich .
• This protocol is the same as unicast protocol, which allows you to Internet Broadband network transmission .

The disadvantages of multicast ：

• Compared with unicast protocol, there is no error correction mechanism , It is difficult to make up for packet loss and error , However, some fault tolerance mechanisms and QOS To make up for .
• Although the current networks support multicast transmission , But in customer certification 、QOS And other aspects need to be improved , These shortcomings have mature solutions in theory , It just needs to be gradually popularized and applied to the existing network .

radio broadcast ：“ radio broadcast ” It is widely used in the network , If the client passes DHCP Automatically get IP The process of address is realized by broadcasting . But compared with unicast and multicast , Broadcasting takes up almost all the bandwidth of the network in the subnet . Take a meeting for example , Only one person can speak at the meeting , Imagine if everyone spoke with a microphone at the same time , There will be a mess in the meeting place . Because of the working principle of the hub, it is impossible to filter broadcast storms , General switches do not have this function , But now some network switches （ As omnidirectional QS Series switches ） It also has the function of filtering broadcast storms , The router itself has the function of isolating broadcast storms . 　　 Broadcast storms cannot be completely eliminated , But it can only spread in the same subnet , Just like the sound of the horn can only be transmitted in the same field , Therefore, in the large and medium-sized LAN composed of hundreds or even thousands of computers , Generally, subnets are divided , It's like separating a hall into many small halls with walls , In order to isolate the broadcast storm . stay IP In the network , For broadcast address IP Address “255.255.255.255” To express , This IP The address represents all in the same subnet IP Address .

The advantages of broadcasting ：

• Network devices are simple , Simple maintenance , The cost of the net is low
• Because the server doesn't have to send data to each client separately , So the server traffic load is very low .

The disadvantages of broadcasting ：

• Unable to provide personalized service in time according to the requirements and time of each customer .
• The bandwidth that the network allows the server to provide data is limited , The maximum bandwidth of the client ＝ Total service bandwidth . For example, cable TV client line support 100 Channels （ If we use digital compression technology , In theory, it can provide 500 Channels ）, Even if the service provider has more financial resources to allocate more transmitting devices 、 Change to fiber backbone , You can't go beyond that limit . In other words, it is impossible to provide more diversity to many customers 、 More personalized service .
• Broadcast prohibition is allowed in Internet Broadband network transmission .