many-to-many multicast concept as a foundation, a whole new range of applications may be built (for example, collaboration, concurrent processing, and distributed interactive simulations).
  • Many-to-one, where many receivers are sending data back to one sender.
    • Many new multicast applications are emerging as demand for them grows.
    Content 7.1 Explaining Multicast 7.1.5 IP Multicast Addresses Routers distinguish multicast traffic from unicasts or broadcast traffic by using the reserved Class D IP address space. Network devices can quickly pick out Class D multicast IP addresses by looking at the four most significant, high-order bits, which are always 1110. The following 28 bits are referred to as the group address. Therefore, the range of IP multicast addresses is from 224.0.0.0 through 239.255.255.255. The multicast IP address space is separated into the following address groups: Within an autonomous system or domain, the limited scope address range can be further subdivided so that local multicast boundaries can be defined. This subdivision is called address scoping and allows for address reuse between smaller domains. The administratively scoped multicast address space is divided into the following scopes:
    Content 7.1 Explaining Multicast 7.1.6 Layer 2 Multicast Addressing How does a router or a switch relate a multicast IP address with a MAC address? Normally, network interface cards (NICs) on a LAN segment only receive packets destined for their burned-in MAC address. However, there is no Address Resolution Protocol (ARP) equivalent for multicast address mapping. Instead, IANA has set aside the vendor code portion of the reserved Organizationally Unique Identifier (OUI) value to identify multicast MAC addresses. Multicast MAC addresses always begin with the low-order bit (0x01) in the first octet. Specifically, the 0x01005e prefix (plus the next lower bit, which is zero) has been reserved for mapping Layer 3 IP multicast addresses into Layer 2 MAC addresses. The complete multicast MAC address range is from 0100.5e00.0000 through 0100.5e7f.ffff. This makes the first 25 bits of the MAC address fixed (24 bits plus the zero bit) and allows for the last 23 bits of the MAC address to correspond to the last 23 bits in the IP multicast group address. The translation between IP multicast and MAC address is achieved by the mapping of the low-order 23 bits of the IP (Layer 3) multicast address into the low-order 23 bits of the IEEE (Layer 2) MAC address. There are 28 bits of unique address space for an IP multicast address (32 minus the first four bits containing the 1110 Class D prefix), and there are only 23 bits mapped into the IEEE MAC address. Therefore, five bits of the IP address are unused and not transferred into the MAC address, which means that there are five bits of overlap. The result is that two (or more) different IP multicast addresses may map to the same MAC multicast address. For example, 224.1.1.1 and 225.1.1.1 map to the same multicast MAC address. If one user subscribed to Group A (as designated by 224.1.1.1) and the other user subscribed to Group B (as designated by 225.1.1.1), they would both receive both A and B streams at Layer 2. At Layer 3, however, only the packets associated with the IP address of the selected multicast group would be viewable, because the port ranges used within the address is different between aliased streams. This gives the possibility that 32 different multicast IP addresses could all correspond to a single multicast MAC address. For example, all the IP multicast addresses in Figure map to the same Layer 2 multicast of 01-00-5e-0a-00-01. Network administrators should consider this when assigning IP multicast addresses.
    Content 7.1 Explaining Multicast 7.1.7 Multicast Sessions Whenever a multicast application is started on a receiver, the application has to know which multicast group to join. The application has to learn about the available sessions or streams, which typically map to one or more IP multicast groups. There are several ways that applications can learn about multicast sessions: The session directory (sd) application acts as a guide and displays multicast content. A client application runs on a PC and lets the user know what content is available. This directory application uses either Session Description Protocol (SDP) or Session Announcement Protocol (SAP) to learn about the content. Note
    Both the sd application and SDP are sometimes called SDR or sdr. In Cisco documentation, SDP/SAP is referred to as sdr. The original sd application served as a means to announce available sessions and to assist in creating new sessions. The initial sd tool was revised, resulting in the SDP tool (referred to in this course as SDR), which is an application tool that allows the following: On the receiver side, SDR learns about available groups or sessions. If a user clicks an icon describing a multicast stream listed via SDR, a join to that multicast group is initiated. When SDR is used on the sender side, it creates new sessions and avoids address conflicts. Senders at the time of session creation consult their respective SDR caches (senders are also receivers) and choose one of the unused multicast addresses. When the session is created, the senders start announcing it with all the