IGMP message (tcp/ip details volume 1/ Volume 2)

Created by QQ：851301776, mailbox ：[email protected], Welcome to technical exchange , This blog is mainly my own learning experience , Just to make a little progress every day ！

Personal motto ：
1. No one was born , As long as it is thick, it will happen .
2. You can have a low degree , You can skip school , But you have to learn

One 、 sketch

        IGMP： Used to support host and router multicast I n t e r n e t Group management protocol

It lets all systems on a physical network know the multicast group that the host is currently in . Multicast routers need this information to know which interfaces multicast datagrams should be forwarded to .

Two 、IGMP message

    

         This is version 1 Of I G M P.I G M P The type is 1 Description is the query message sent by the multicast router , by 2 Description is the report message sent by the host . Check the calculation of sum and I C M P The agreement is the same .
         The group address is D class I P Address . Set the group address to in the query message 0, In the report message, the group address is the address of the group to participate . 

3、 ... and 、IGMP agreement

1. Join a multicast group

         Multicast is based on the concept of a process （ The term process is used to refer to a program executed by the operating system ）, The process joins a multicast group on a given interface of a host . The members of a multicast group on a given interface are dynamic — It changes at any time as processes join and leave the multicast group .

         The process referred to here must join a multicast group on a given interface in some way . A process can also leave a previously joined multicast group . These are any of the hosts that support multicast A P I Necessary parts . Use the qualifier “ Interface ” Because the members in the multicast group are associated with interfaces . A process can join the same multicast group on multiple interfaces .

         This implies that a host identifies a multicast group by group address and interface . The host must keep a table , This table contains all multicast groups containing at least one process and the number of processes in the multicast group .        

2.IGMP Reports and queries

         Multicast routers use I G M P Message to record the changes of group members in the network connected to the router . The rules of use are as follows ：

• When the first process joins a group , The host sends a I G M P The report . If multiple processes of a host join the same group , Send only one I G M P The report . This report is sent to the same interface where the process joined the group .
• When a process leaves a group , The host does not send I G M P The report , Even if the last process in the group leaves . After the host knows that there are no more group members in the determined group , Received later I G M P The report message will not be sent in the query .
• Multicast router sends messages regularly I G M P Query to see if any hosts contain processes belonging to the multicast group . The multicast router must send one to each interface I G M P Inquire about . Because the router wants the host to send back a report for each multicast group it joins , therefore I G M P The group address in the query message is set to 0.
• The host sends I G M P Report in response to a I G M P Inquire about , Send back to each group that contains at least one process I G M P The report .

         Use these query and report messages , The multicast router maintains a table for each interface , The table records the multicast group of at least one host on the interface . When the router receives the multicast datagram to be forwarded , It only forwards the datagram to （ Use the corresponding multicast link layer address ） It also has interfaces belonging to that group of hosts .
         chart 1 3 - 3 It shows two I G M P message , One is the report sent by the host , The other is the query sent by the router . The router is asking each host on that interface to specify each multicast group it joins .

3. Implementation details  

         To improve the efficiency of the agreement , There are many implementation details to consider . First , When a host first sends I G M P The report （ When the first process joins a multicast group ） when , There is no guarantee that the report will be received reliably （ Because it uses I P Delivery of services ）. The next report will be sent after an interval . This time interval is determined by the host at 0 ~ 1 0 Random selection within seconds .

         secondly , When a host receives a query from the router , Do not respond immediately , Instead, some responses are sent after a certain time interval （ use “ Respond to ” Because the host must send a response to each group it participates in ）. Since multiple hosts participating in the same multicast group can send a report , Their transmission interval can be set to random delay . All hosts in a physical network will receive all reports sent by other hosts in the same group , Because it's like the picture 1 3 - 3 The destination address in the report shown is the group address . This means that if a host is waiting to send a report , Received the same report from other hosts , Then the response of the host does not need to be sent . Because the multicast router doesn't care how many hosts belong to this group , The only concern is whether the group still has at least one host . You bet , A multicast router doesn't even care which host belongs to a multicast group . It just wants to know if there is at least one host in the multicast group on a given interface .

         In a single physical network without any multicast routers , Only I G M P Traffic is when the host joins a new multicast group , Support I P Reports from multicast hosts .

4. Time to live field

         In the figure 1 3 - 3 in , We noticed that I G M P Lifetime of reports and queries ( T T L ) All set to 1, This involves I P In the first part T T L Field . An initial T T L by 0 Of multicast datagrams will be restricted to the same host . By default , To transmit multicast datagrams T T L Set to 1, This will restrict multicast datagrams to the same subnet . Bigger T T L Value can be forwarded by multicast router .

         Datagrams destined for a multicast address are never generated I C M P A mistake . When T T L The value is 0 when , Multicast routers also do not generate I C M P“ Overtime ” A mistake .

         Under normal circumstances , The user process does not care about the T T L. However , One exception is Tr a c e r o u t e Program , It mainly depends on the settings T T L Value to complete . Since the multicast application must be able to set up the TTL value , This means that the programming interface must provide this capability to the user process .

         By increasing the T T L The method of value , An application program can realize the extended ring search for a specific server ( e x p a n d i n gring search). The first multicast datagram is T T L be equal to 1 send out . If there is no response , Just try to T T L Set to 2, then 3, wait . In this way , The application can find the nearest server measured in hops .

         from 2 2 4 . 0 . 0 . 0 To 2 2 4 . 0 . 0 . 2 5 5 The special address space of is intended for multicast with a range not exceeding 1 Application of skip . No matter T T L What is the value , Multicast routers do not forward datagrams whose destination addresses are any of these addresses .

5. all Host group

         In the figure 1 3 - 3 in , We saw the router I G M P The query is sent to the destination I P Address 2 2 4 . 0 . 0 . 1. This address is called all host group addresses . It involves all hosts and routers with multicast capability in a physical network . After the interface is initialized , All hosts on multicast capable interfaces automatically join this multicast group . Members of this group do not need to send I G M P The report .