Igmp là gì

PIM source-specific multicast (SSM) uses a subset ofPIM sparse mode and IGMP version 3 (IGMPv3) to allow a clientto receive multicast traffic directly from the source. PIM SSM usesthe PIM sparse-mode functionality to create an SPT between the receiverand the source, but builds the SPT without the help of an RP.

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Any Source Multicast (ASM) was the Original Multicast

RFC 1112, the original multicast RFC, supported both many-to-manyand one-to-many models. These came to be known collectively as any-sourcemulticast (ASM) because ASM allowed one or many sources for a multicastgroup"s traffic. However, an ASM network must be able to determinethe locations of all sources for a particular multicast group wheneverthere are interested listeners, no matter where the sources mightbe located in the network. In ASM, the key function of is a required function of the network itself.


Source Discovery in Sparse Mode vs Dense Mode

Multicast source discovery appears to be an easy process, butin sparse mode it is not. In dense mode, it is simple enough to floodtraffic to every router in the whole network so that every routerlearns the source address of the content for that multicast group.However, the flooding presents scalability and network resource useissues and is not a viable option in sparse mode.

PIM sparse mode (like any sparse mode protocol) achieves therequired source discovery functionality without flooding at the costof a considerable amount of complexity. RP routers must be added andmust know all multicast sources, and complicated shared distributiontrees must be built to the RPs.


PIM SSM is a Subset of PIM Sparse Mode

PIM SSM is simpler than PIM sparse mode because only the one-to-manymodel is supported. Initial commercial multicast Internet applicationsare likely to be available to (thatis, receivers that issue join messages) from only a single source(a special case of SSM covers the need for a backup source). PIM SSMtherefore forms a subset of PIM sparse mode. PIM SSM builds shortest-pathtrees (SPTs) rooted at the source immediately because in SSM, therouter closest to the interested receiver host is informed of theunicast IP address of the source for the multicast traffic. That is,PIM SSM bypasses the RP connection stage through shared distributiontrees, as in PIM sparse mode, and goes directly to the source-baseddistribution tree.


Why Use PIM SSM

In an environment where many sources come and go, such as fora videoconferencing service, ASM is appropriate. However, by ignoringthe many-to-many model and focusing attention on the one-to-many source-specificmulticast (SSM) model, several commercially promising multicast applications,such as television channel distribution over the Internet, might bebrought to the Internet much more quickly and efficiently than iffull ASM functionality were required of the network.

An SSM-configured network has distinct advantages over a traditionallyconfigured PIM sparse-mode network. There is no need for shared treesor RP mapping (no RP is required), or for RP-to-RP source discoverythrough MSDP.

PIM SSM is simpler than PIM sparse mode because only the one-to-manymodel is supported. Initial commercial multicast Internet applicationsare likely to be available to (thatis, receivers that issue join messages) from only a single source(a special case of SSM covers the need for a backup source). PIM SSMtherefore forms a subset of PIM sparse mode. PIM SSM builds shortest-pathtrees (SPTs) rooted at the source immediately because in SSM, therouter closest to the interested receiver host is informed of theunicast IP address of the source for the multicast traffic. That is,PIM SSM bypasses the RP connection stage through shared distributiontrees, as in PIM sparse mode, and goes directly to the source-baseddistribution tree.


PIM Terminology

PIM SSM introduces new terms for many of the concepts in PIMsparse mode. PIM SSM can technically be used in the entire 224/4 multicastaddress range, although PIM SSM operation is guaranteed only in the232/8 range (232.0.0/24 is reserved). The new SSM terms are appropriatefor Internet video applications and are summarized in Table 1.

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Table 1: ASM andSSM Terminology

Term

Any-Source Multicast

Source-Specific Multicast

Address identifier

G

S,G

Address designation

group

channel

Receiver operations

join, leave

subscribe, unsubscribe

Group address range

224/4 excluding 232/8

224/4 (guaranteed only for 232/8)


Although PIM SSM describes receiver operations as and , the samePIM sparse mode join and leave messages are used by both forms ofthe protocol. The terminology change distinguishes ASM from SSM eventhough the receiver messages are identical.


How PIM SSM Works

PIM source-specific multicast (SSM) uses a subset of PIM sparsemode and IGMP version 3 (IGMPv3) to allow a client to receivemulticast traffic directly from the source. PIM SSM uses the PIM sparse-modefunctionality to create an SPT between the receiver and the source,but builds the SPT without the help of an RP.

By default, the SSM group multicast address is limited to theIP address range from 232.0.0.0 through 232.255.255.255. However,you can extend SSM operations into another Class D range by includingthe ssm-groups statement at the hierarchy level. The default SSM address range from232.0.0.0 through 232.255.255.255 cannot be used in the ssm-groups statement. This statement is for adding other multicast addressesto the default SSM group addresses. This statement does not overridethe default SSM group address range.

In a PIM SSM-configured network, a host subscribes to an SSMchannel (by means of IGMPv3), announcing a desire to join group Gand source S (see Figure 1). The directlyconnected PIM sparse-mode router, the receiver"s DR, sends an (S,G)join message to its RPF neighbor for the source. Notice in Figure 1 that the RP is not contacted in this processby the receiver, as would be the case in normal PIM sparse-mode operations.


Figure 1: Receiver Announces Desire to JoinGroup G and Source S
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The (S,G) join message initiates the source tree and then buildsit out hop by hop until it reaches the source. In Figure 2, the source tree is built across the networkto Router 3, the last-hop router connected to the source.


Figure 2: Router 3 (Last-Hop Router) Joinsthe Source Tree
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Using the source tree, multicast traffic is delivered to thesubscribing host (see Figure 3).


Figure 3: (S,G) State Is Built Between theSource and the Receiver
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Using PIM SSM

You can configure Junos OS to accept any-source multicast (ASM)join messages (*,G) for group addresses that are within the defaultor configured range of source-specific multicast (SSM) groups. Thisallows you to support a mix of any-source and source-specific multicastgroups simultaneously.

Deploying SSM is easy. You need to configure PIM sparse modeon all router interfaces and issue the necessary SSM commands, includingspecifying IGMPv3 on the receiver"s LAN. If PIM sparse mode is notexplicitly configured on both the source and group member interfaces,multicast packets are not forwarded. Source lists, supported in IGMPv3,are used in PIM SSM. As sources become active and start sending multicastpackets, interested receivers in the SSM group receive the multicastpackets.

To configure additional SSM groups, include the ssm-groups statement at the hierarchylevel.