Multicast traffic distribution in the network. Applicability advantages and limits
If it is necessary to transmit traffic from one source to several recipients, group transmission mechanism is recommended to use. This data distribution form is called multicast and an example of such a scenario is the video transmission from a surveillance camera to several user devices.
In the example below PC1 and PC2 receive video from Camera 1 using unicast (see Figure): PC1 and PC2 establish sessions with Camera 1, and Camera 1 transmits data separately for PC1 and separately for PC2. Thus, two traffic copies are transmitted between Camera 1 and Switch 1. If number of traffic recipients increases, the link load will increase significantly, which will lead to inefficiency of bandwidth using.
Figure 6.1 - Unicast traffic distribution
Using IGMP allows to use bandwidth more efficiently. IGMP (Internet Group Management Protocol) is a network protocol for the interaction of multicast clients and the nearest router.
The multicast technology is shown in the figure below. PC1 and PC2 receive video from Camera 1, but, unlike the previous example, traffic is distributed as multicast. Thus, only one copy of the traffic is transmitted between Camera 1 and Switch 1. To perform this the router must be used, which will distribute traffic between sources and destinations.
The group traffic distribution procedure is following:
Step 1: user starts a program to view the surveillance cameras on PC1, PC1 generates an IGMP Membership Report message and sends it to Router 1.
Step 2: Router 1, having received the IGMP Membership Report message, adds the interface through which the message was received to the multicast distribution list. Multicast traffic will be transmitted via interfaces from this list.
Step 3: Router 1 processes the traffic from the multicast source and sends it to the recipients using generated list of interfaces.
Step 3a: if user starts a program for viewing the surveillance cameras on PC2, then PC2 generates and sends IGMP Membership Report to Router 1. Since PC1 and PC2 are connected to the same Router 1 interface, this interface will not be duplicated in the interfaces list for multicast.
Step 4: Router 1 generates IGMP Query messages periodically, checking for active multicast traffic recipients in the network segment. Router 1 will continue the multicast transmission to the segment as long as receives responses to the generated messages.
Step 4a: a client device can opt out of receiving multicast traffic ahead of schedule by sending IGMP Leave message to the Router.
Thus, only one copy of multicast traffic is transmitted to the network segment. A device which perform traffic distribution in multicast networks is called IGMP Querier. Only one IGMP Querier router is enabled in the network segment. Even if there are two routers configured to redirect multicast traffic to one network segment, one of them will be active and the other is set as backup.
The disadvantage of using the multicast is the complexity of network management, since the network should now be viewed in two planes: as unicast and as multicast. Thus, unicast and muticast packets will be distributed on the network simultaneously, i.e. logically, the router will be divided in two parts: the first part deals with unicast packets, the second - with multicast, and for each logical component its own routing table will be formed.
Figure 6.2 - Multicast traffic distribution
One more disadvantage of this scheme will appear if connecting another switch to Switch 1 (see figure).
Figure 6.3 - Multicast traffic distribution
The task coincide with the previous one: PC1 and PC2 should receive the stream from Camera 1, but PC3 and PC4 should not receive this traffic.
The traffic distribution algorithm is following:
Step 1: Camera 1 transmits traffic over a radio network to Router 1.
Step 2: the multicast traffic routing table is formed using the requests and responses of clients. Since PC1 and PC2 are camera stream consumers, they form responses to requests from Router 1, and are added to the routing table of Router 1 to receive the Camera 1 traffic through the interface to which Switch 1 is connected.
Step 3: Router 1, using the multicast traffic routing table transmits the broadcast data to Switch 1.
Step 4: Switch 1 receives the multicast frame and sends it to all ports, except the one through which it was received. A multicast frame is processed by the switch same way as a broadcast frame.
Step 4a: A multicast stream is transmitted to Switch 2, PC1 and PC2.
Step 5: Switch 2 receives the multicast frame and sends it to all ports, except the one through which it was received.
Step 5a: A multicast stream is transmitted to PC3 and PC4.
The mechanism that allows L2 devices to manage multicast streams is called IGMP snooping. The device on which IGMP snooping is enabled monitors the IGMP service messages exchange between the router and the clients. Service message analysing allows the device to assign roles to ports:
- Member ports - ports to which clients are connected. Connections through intermediate devices are also allowed.
- Router port - port to which the multicast router is connected. A router also can be connected through intermediate devices.
Port roles are fixed for each streams. This allows to pass multicast traffic only between clients and sources.
Figure 6.4 - Multicast traffic distribution using IGMP snooping
IGMP snooping in radio
In previous lessons, the switch groups concept on InfiNet equipment was reviewed. The switch groups using allows to represent the radio network as a single switch, i.e. processing multicast traffic by switch groups will be similar to processing on a common switch. Thus, described scheme will have one more disadvantage - in the case of using same switch groups on CPEs, traffic will be duplicated (see Figure). Camera 1 traffic will be sent to Router 1 wired network segment and to wireless segment to another CPEs on which the switch group with the same number is configured.
Figure 6.5 - Multicast traffic distribution in radio
IGMP snooping mechanism enabling on the BS sector allows to solve this problem. Same as in the example above, the BS sector analyzes multicast service traffic and assigns the roles to the switch group interfaces. This prevent duplication Camera 1 traffic in the direction of Camera 2, traffic is passed only to Router 1.
Figure 6.6 - Multicast traffic distribution in radio using IGMP snooping
To configure IGMP snooping on BS proceed with the following steps:
Step 1: enable Polling or use firmware with TDMA technology support. This will reduce jitter and packet loss, which is important for multicast.
Figure 6.7 - Polling configuration
Step 2: Go to the "Basic Settings → MAC Switch" section. Enable IGMP support for the switch group which is used for multicast traffic transmission.
Figure 6.8 - IGMP support configuraion
Step 3: Go to the "Basic Settings → MAC Switch → IGMP Snooping" section and check the "Router Port Forwarding" box.
Figure 6.9 - "Router Port Forwarding" option