Overview of IP Multicast for H3C multicast series

First, the basic introduction of multicast

Multicast refers to the way that a sending source sends a single IP packet over the network to a specific set of recipients. Multicast combined with the advantages of unicast and broadcast, in the point-to-multipoint transmission, the sending source does not need to care about the number of recipients, only send a message, the router is only concerned about the interface under the recipient, also do not need to care about the number of receivers, so on the link between routers only a report is sent.

Multicast reduces the burden on the sending source compared to unicast, and improves the effective utilization of the link. In addition, the sending source can send messages to multiple receivers at the same time, which can meet the needs of low latency applications.

In contrast to broadcast, multicast routers only replicate packets on the recipient's interface, and the message is ultimately delivered to the recipient instead of all hosts in the network, which can save a significant amount of network bandwidth. In addition, broadcasts can only be performed in the same network segment, and multicast enables the transmission of cross-network segments.

advantages of multicast:

(1) Multicast can enhance the efficiency of message delivery, control network traffic, reduce server and CPU load

(2) Multicast can optimize network performance and eliminate traffic redundancy.

(3) Multicast can adapt to distributed applications, and when the number of receivers changes, the fluctuation of network traffic is stable.

At the same time, because the multicast application is based on UDP instead of TCP, this determines that the multicast application has the corresponding disadvantage of UDP, as listed below.

(1) Multicast data is based on best Effort, which can not guarantee the priority transmission of voice, video and other applications. When the message is lost, the application layer retransmission mechanism cannot guarantee the low-latency demand of the real-time application.

(2) No congestion control mechanism is provided. When the network is congested, broadband is not retained for high-priority applications.

(3) Unable to implement multicast packet duplicate detection. When the network topology changes, the receiver may receive duplicate messages, which need to be removed by the application layer.

(4) Unable to correct the problem of multicast packet scrambling arrival.

Multicast technology is mainly used in multimedia conferencing, IP video surveillance, real-time data multicast, game and simulation.

Second, multicast technology architecture

Implementing multicast needs to address the following issues

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(1) The recipient of the multicast is an indefinite set of receivers that cannot be identified by using the host IP address as a unicast, and all the first to resolve how to identify a group of recipients in the network.

(2) If the identity of the group is implemented, it is also necessary to resolve how the recipient joins and leaves the group, and how the routing device maintains group membership information.

(3) The multicast receiver may be scattered in any corner of the network, then the forwarding path between the multicast source and the multicast receiver is based on what model, how the multicast data is forwarded on the path.

(4) How to set up and maintain multicast data forwarding path

These technical requirements are implemented through some important mechanisms in multicast architecture, including multicast address , multicast Group Management Protocol , multicast distribution tree model , multicast forwarding mechanism and multicast routing protocol .

Third, multicast address

Multicast traffic is used to identify a group of receivers using a multicast address, which is known as a multicast group using the multicast address identity.

The IANA (Internet Assigned Numbers Authority, internet numbering distribution board) assigns the class D address space to the IPV4 multicast use, the address range is 224.0.0.0-- 239.255.255.255, the classification and specific meanings of multicast addresses are as follows.

(1)224.0.0.0-224.0.1.255: protocol reserved multicast address . In addition to 224.0.0.0 reserved, other addresses are used for routing protocols, topology lookups, and protocol maintenance.

(2)224.0.2.0-238.255.255.255: user Group address , the whole network is valid.

(3)239.0.0.0-239.255.255.255: Local management Group address , valid only within the local admin domain.

Multicast addresses address the problem of IP packets addressing the network layer, but ultimately the communication depends on the data link layer and the physical layer, so as with unicast, multicast also needs to consider how the data is addressed at the link layer.

When the Ethernet transmits the unicast IP message, the destination MAC address uses the recipient's MAC address. Because the multicast destination is no longer a specific recipient, it is a group of member uncertainties, so a specific multicast MAC address is required at the link layer to identify a group of recipients.

IANA definition IPv4 multicast MAC address format is 01-00-5e-xx-xx-xx

As shown, the high 24 bits in the multicast MAC address are fixed to 0x01005e, the 25th bit is 0, and the low 23 bits come from the low 23 bits of the multicast IP address.

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Because the high 4 bits of the multicast IP address are 1110, which represents the multicast identity, only 23 bits in the low 28 bits are mapped to the multicast MAC address, so that 5 bits of information is lost in the multicast address. Thus, there are 32 multicast IP addresses mapped to the same multicast MAC address , so that in the two-tier process, the device may receive some multicast data outside the multicast group, and these redundant multicast data need to filter the upper layer of the device.

For example, the multicast IP address is 228.128.128.128, the corresponding multicast MAC address is 01-00-5e-00-80-80; The multicast IP address is 229.128.128.128, and its corresponding multicast MAC address is still 01-00-5e-00-80-80 。

IV. Multicast Group Management Protocol

Resolves the issue of how to identify the multicast group, and also to consider how the recipient joins the multicast group, how to maintain the multicast group, and who is maintaining the multicast group and so on. Use the Multicast Group Management protocol in the multicast architecture to achieve these requirements.

The Multicast Group Management Protocol is the protocol that runs between the host and the router. The host notifies the router to join or leave a multicast group through the Multicast Group Management protocol, and the router responds to the host join request through the Multicast Group Management Protocol, establishes the corresponding multicast table entry, and maintains the multicast group information through the query message.

Common Multicast Group Management protocol for IGMP(internet group Management Protocol, Internet Groups Management Protocol)

Five, multicast forwarding mechanism

In unicast communication, the path between the sending source and the receiver is a point-to-points line, where the starting point is the sending source and the destination is a receiver, and the path is established by the Unicast routing protocol.

In multicast communication, because the destination is an indefinite set of receivers, the multicast distribution tree is used, the receiver is located in the tree structure leaves, and the multicast distribution tree is established by the Multicast routing protocol.

Depending on the location of the multicast distribution tree root, the multicast distribution tree model is divided into the shortest path tree (shortest path TREE,SPT) model and the shared tree (Rendezvous point tree,prt) model.

The SPT root is the sending source, so SPT is also called the "source tree", and its path from the source to each receiver is optimal.

The RPT model tree root is a device in the network called the aggregation point, and the multicast data from the sending source to the receiver must first pass through the aggregation point and then be sent by the aggregation point to each receiver. Therefore, the path from the sending source to the receiver is not necessarily the optimal path in the RPT model.

In unicast communication, the IP message forwarding is based on the destination IP address of the message, the router in the network receives the unicast IP message only through the destination IP address to find the unicast routing table, determine the message corresponding to the next hop address, the message out of the interface can be obtained.

In multicast communication, the optimal path of message transmission and the corresponding interface can not be obtained simply by checking the destination IP address of the reading paper, because the destination address of the group broadcast text is not a definite host, and it is possible that each interface of the router has a receiver.

Therefore, the multicast using reverse path forwarding method, determine whether the group broadcast text from the root of the multicast distribution tree to reach the shortest path, only from the optimal path of the group broadcast WENCAI will be forwarded, from the non-optimal path of the group broadcast text will be discarded. and the multicast path loop can be eliminated.

Unicast forwarding is mainly concerned about where the message goes, and when multicast is forwarded, the main concern is where the message comes from.

Six, multicast routing protocol

Multicast routing protocol runs between three-tier multicast devices, which is used to establish and maintain multicast routing and to forward multicast packets correctly and efficiently. Multicast routing protocol establishes a non-loop (loop-free) data transfer path from one data source to a plurality of receivers, that is, the multicast distribution tree.

Multicast routing protocol According to the scope of the group can be divided into

Intra-Domain Multicast routing protocol: DVMRP (Distance vector multicast Routing Protocol, distance vector Multicast routing protocol), MOSPF (multicast Extensions to OSPF, multicast extended OSPF protocol) and PIM (Protocol Independent Multicast, protocol Independent Multicast)

Inter-domain Multicast Routing protocol: MSDP (multicast source Discovery Protocol, Multicast Origin Discovery Protocol), MBGP (multicast BGP, multicast BGP)

The multicast routing protocol in the domain can be divided into the following multicast distribution tree:

SPT-based Multicast routing protocol: PIM-DM (Protocol independent multicast-dense mode, protocol independent multicast-dense mode), DVMRP, MOSPF

RPT-based Multicast routing protocol: PIM-SM (Protocol independent multicast-sparse mode, protocol-independent multicast-sparse mode)

VII. Multicast Protocol System

The multicast protocol mainly consists of the protocol between the host and the router, the protocol between the router and the route, and the protocol between the multicast domains.

The protocol between host and router is the multicast Group Management protocol, and IGMP is commonly used in IPv4. With IGMP, routers can understand which multicast groups exist in the local network segment, and maintain group membership information.

The protocol between router and router is for multicast routing protocol, the common Multicast routing protocol is PIM. With PIM, group membership information can be spread across the network, creating a multicast distribution tree from the sending source to the receiver.

The boundary of the multicast domain is usually the boundary of the unicast domain, because the multicast routing information between domains may not interact directly, and the receiver cannot receive multicast data across domains, it is necessary to run inter-domain multicast routing protocol between domains to solve the problem of multicast communication between domains, the common inter-domain multicast routing protocol is MSDP

VIII. Multicast model

Depending on how the recipient handles the multicast source, the multicast model is divided into the following two classes.

(1) ASM (Any-source multicast, Arbitrary source multicast) model. In the ASM model, the multicast recipient cannot specify a multicast source, and any multicast source sent to the same multicast group will be transmitted to the multicast receiver by the network device.

(2) SSM (source-specific multicast, designated source multicast) model. In real life, users may be interested in only multicast information sent by certain multicast sources, rather than receiving information sent from other sources. The SSM model provides the user with a transport service that can specify a multicast source on the client.

The fundamental difference between the SSM model and the ASM model is that the receiver in the SSM model knows the specific location of the multicast source in advance by other means.

ASM, when the recipient joins a multicast group through the Multicast Group Management protocol, the sending source of the packet multicast data is not the region.

The ASM model cannot satisfy the host receiving multicast data sent by the specified sending source, and if the host receives multiple copies of the same multicast group from different sending sources, it needs to be differentiated by the upper application.

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Overview of IP Multicast for H3C multicast series