Multimedia traffic

Multimedia traffic uses one of the following methods of transmission, each of which has a different effect

on network bandwidth:

• Unicast: With a unicast design, an application sends one copy of each packet to every client’s

unicast address. As a result, unicast transmission has significant scaling restrictions. If the group

is large, the same information must be carried multiple times, even on shared links.

• Broadcast: In a broadcast design, an application sends only one copy of each packet using a

broadcast address. Broadcasting a packet to all devices is inefficient, except in the case where

only a small subset of the network actually needs to see the packet. Broadcast multimedia is

dispersed throughout the network similarly to normal broadcast traffic in that every host device

must process the broadcast multimedia data frame. However, unlike standard broadcast frames,

which are generally small, multimedia broadcasts can reach data rates as high as 13 Mbps or

more. Even if an end station is not using multimedia applications, the device still processes the

broadcast traffic. This requirement might use most, if not all, of the allocated bandwidth for each

device. For this reason, Cisco highly discourages broadcast implementation for applications

delivering data, voice, or video to multiple receivers.

• Multicast: The most efficient solution to transmit multimedia applications is multicast, which falls

between unicast and broadcast. In multicast transmission, a multimedia server sends one copy of

each packet to a special address that represents multiple clients. Unlike the unicast environment,

a multicast server sends out a single data stream to multiple clients. Unlike the broadcast

environment, the client device decides whether to listen to the multicast address. Multicasting

saves bandwidth and controls network traffic by forcing the network to forward packets only when

necessary. By eliminating traffic redundancy, multicasting reduces network bandwidth

consumption and host processing. Furthermore, Catalyst switches can process IP multicast

packets and deliver those packets only to receivers that request receipt of those packets at both

Layer 2 and Layer 3.

IP multicast is the transmission of an IP data packet to a host group that is defined by a single IP address

called a multicast IP address. A host can join or leave the multicast IP group dynamically regardless of

the location or number of members. An important characteristic of the multicast is its capability to limit

variation in delivery time (jitter) of IP multicast frames along the complete server-to-client path.

In Figure 7-23, the video server transmits a single video stream for each multicast group. A multicast, or

host, group is defined as a set of host devices listening to a specific multicast address. Cisco routers and

switches replicate the video stream as required to the host groups that are in the path. This technique

enables an arbitrary number of clients to subscribe to the multicast address and receive the stream. One

host can be a part of one or more multicast groups for multiple applications. In addition, routers can

transmit multiple data streams for different applications for a single group address.

Figure 7-23. Multicast Traffic

In the multicast scenario shown in Figure 7-23 only 1.5 Mbps of server-to-network bandwidth is utilized,

leaving the remaining bandwidth free for other uses.

Note

In a unicast scenario, the server sequences through a transmission of multiple copies of the data, so

variability in delivery time is large, especially for large transmissions or large distribution lists.

IP multicast relies on the concept of a virtual group address called the multicast IP address. In IP unicast

routing, a packet is routed from a source address to a destination address, traversing the IP network on a

hop-by-hop basis. In IP multicast, the packet’s destination address is not assigned to a single destination.

Instead, receivers join a group; when they do, packets addressed to that group begin flowing to them. All

members of the group receive the packet; a host must be a member of the group to receive the packet.

Multicast sources do not need to be members of that group. In Figure 7-24, packets sent by group

member 3 (represented by the solid arrows) are received by group members 1 and 2 but not by the

nonmember of the group. The nonmember host can also send packets (represented by the dotted

arrows) to the multicast group that are received by all three group members because all the new hosts

are members of the multicast group. Group members 1 and 2 do not send multicast packets; they are

just the receivers.

Figure 7-24. Multicast Group Membership

IP multicast traffic uses UDP as the transport layer protocol. Unlike TCP, UDP adds no reliability, flow

control, or error-recovery functions to IP. Because of the simplicity of UDP, data packet headers contain

fewer bytes and consume less network overhead than TCP.

Because the location of hosts in the group is widely spread in the network, the multicast router sends the

packets to respective multiple interfaces to reach all the hosts. This makes the multicast forwarding more

complex. To avoid duplication, several multicast routing protocols use reverse path forwarding (RPF),

discussed in the section “Reverse Path Forwarding,” later in this chapter.

This section discusses the following fundamentals of IP multicast in more detail:

• Multicast IP address structure

• Multicast MAC address structure

• Reverse path forwarding

• Multicast forwarding tree

Multicast IP Address Structure

The range of IP addresses is divided into classes based on the high-order bits of a 32-bit IP address. IP

multicast uses the Class D addresses, which range from 224.0.0.0 to 239.255.255.255. These addresses

consist of binary 1110 as the most significant bits (MSB) in the first octet, followed by a 28-bit group

address, as shown in Figure 7-25. Unlike with Class A, B, and C IP addresses, the last 28 bits of a Class D

address are unstructured.

Figure 7-25. Multicast IP Address Structure

These remaining 28 bits of the IP address identify the multicast group ID, which is a single address that

is typically written as decimal numbers in the range 224.0.0.0 to 239.255.255.255. The Internet

Assigned Numbers Authority (IANA) controls the assignment of IP multicast addresses. The Class D

address range is used only for the group address or destination address of IP multicast traffic. The source

address for multicast datagrams is always the unicast source address.

IP multicast addresses specify a set of IP hosts that have joined a group and are interested in receiving

multicast traffic designated for that particular group. Table 7-5 outlines the IP multicast address

conventions.

Table 7-5. Multicast IP Address Ranges

Description Range

Reserved link local addresses 224.0.0.0 to 224.0.0.255

Globally scoped addresses 224.0.1.0 to 238.255.255.255

Source-specific multicast addresses 232.0.0.0 to 232.255.255.255

GLOP addresses 233.0.0.0 to 233.255.255.255

Limited-scope addresses 239.0.0.0 to 239.255.255.255

Applications allocate multicast addresses dynamically or statically. Dynamic multicast addressing provides

applications with a group address on demand. Because dynamic multicast addresses have a specific

lifetime, applications must request this type of address only for as long as the address is needed.

Statically allocated multicast addresses are reserved for specific protocols that require well-known

addresses, such as Open Shortest Path First (OSPF). IANA assigns these well-known addresses, which are

called permanent host groups and are similar in concept to the well-known TCP and UDP port numbers.

The following sections discuss the details of the multicast addresses listed in Table 7-5.

Reserved Link Local Addresses

IANA has reserved addresses in the range 224.0.0.0 to 224.0.0.255 (link local destination addresses) to

be used by network protocols on a local network segment. Routers do not forward packets in this address

range, because these packets are typically sent with a Time-To-Live (TTL) value of 1. Network protocols

use these addresses for automatic router discovery and to communicate important routing information.

For example, OSPF uses the IP addresses 224.0.0.5 and 224.0.0.6 to exchange link-state information.

Address 224.0.0.1 identifies the all-hosts group. Every multicast-capable host must join this group when

initializing its IP stack. If you send an ICMP echo request to this address, all multicast-capable hosts on

the network answer the request with an ICMP echo reply.

Address 224.0.0.2 identifies the all-routers group. Multicast routers join this group on all multicastenabled

interfaces.

Globally Scoped Addresses

Addresses in the range 224.0.1.0 to 238.255.255.255 are called globally scoped addresses. Companies

use these addresses to multicast data between organizations and across the Internet. Multicast

applications reserve some of

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