Multicast Operation


As noted above, the backbone may consist of (i) a pure IP network or (ii) a mixed satellite transmission link to a metropolitan headend that, in turn, uses a metropolitan (or regional) telco IP network. Applications such as video are very sensitive to end-to-end delay, jitter, and (uncorrectable) packet loss; QoS considerations are critical. These networks tend to have fewer hops and pruning may be somewhat trivially implemented by a making use of a simplified network topology.

At the logical level, there are three types of communication between systems in a(n IP) network:

  • Unicast: Here, one system communicates directly to another system.
  • Broadcast: Here, one system communicates to all systems.
  • Multicast: Here, one system communicates to a select group of other systems.

In traditional IP networks, a packet is typically sent by a source to a single destination (unicast); alternatively, the packet can be sent to all devices on the network (broadcast). There are business- and multimedia (entertainment) applications that require a multicast transmission mechanism to enable bandwidthefficient communication between groups of devices where information is transmitted to a single multicast address and received by any device that wishes to obtain such information. In traditional IP networks, it is not possible to generate a single transmission of data when this data is destined for a (large) group of remote devices. There are classes of applications that require  distribution of information to a defined (but possibly dynamic) set of users. IP Multicast, an extension to IP, is required to properly address these communications needs. As the term implies, IP Multicast has been developed to support efficient communication between a source and multiple remote destinations.

Multicast applications include, among others, datacasting—for example, for distribution of real-time financial data—entertainment digital television over an IP network (commercial-grade IPTV), Internet radio, multipoint video conferencing, distance-learning, streaming media applications, and corporate communications. Other applications include distributed interactive simulation, cloud/grid computing, and distributed video gaming (where most receivers are also senders). IP Multicast protocols and underlying technologies enable efficient distribution of data, voice, and video streams to a large population of users, ranging from hundreds to thousands to millions of users. IP Multicast technology enjoys intrinsic scalability, which is critical for these types of applications.

As an example in the IPTV arena, with the current trend toward the delivery of HDTV signals, each requiring the 12 Mbps range, and the consumers’ desire for a large number of channels (200–300 being typical), there has to be an efficient mechanism of delivering a signal of 1–2 Gbps1 aggregate to a large number of remote users. If a source had to deliver 1 Gbps of signal to, say, 1 million receivers by  transmitting all of this bandwidth across the core network, it would require a petabit per second network fabric; this is currently not possible. On the other hand, if the source could send the 1 Gbps of traffic to (say) 50 remote distribution points (for example, headends), each of which then makes use of a local distribution network to reach 20,000 subscribers, the core network only needs to support 50 Gbps, which is possible with proper design. For such reasons, IP Multicast is seen as a bandwidth-conserving technology that optimizes traffic management by simultaneously delivering a stream of information to a large population of recipients, including corporate enterprise users and residential customers. IPTV uses IP-based basic transport (where IP packets contain MPEG-4 TSs) and IP Multicast for service control and content acquisition (group membership). See Fig. 5.1 for a pictorial example.

One important design principle of IP Multicast is to allow receiver-initiated attachment (joins) to information streams, thus supporting a distributed informatics model. A second important principle is the ability to support optimal pruning such that the distribution of the content is streamlined by pushing replication as close to the receiver as possible. These principles enable bandwidth-efficient use of underlying network infrastructure.

The issue of security in multicast environments is addressed via Conditional Access Systems (CAS) that provide per-program encryption (typically, but not always, symmetric encryption; also known as inner encryption) or aggregate IP-level encryption (again typically, but not always, symmetric encryption; also known as outer encryption).

Carriers have been upgrading their network infrastructure in the past few years to enhance their capability to provide QoS-managed services, such as IPTV. Specifically, legacy remote access platforms, implemented largely to support basic DSL service roll-outs—for example, supporting ATM aggregation and DSL termination—are being replaced by new broadband network gateway access technologies optimized around IP, Ethernet, and VDSL2 (Very High Bitrate

Bandwidth advantage of IP Multicast.

Digital Subscriber Line 2). These services and capabilities are delivered with multiservice routers on the network edge. Viewer-initiated program selection is achieved using the IGMP, specifically with the Join Group Request message. (IGMP v2 messages include Create Group Request, Create Group Reply, Join Group Request, Join Group Reply, Leave Group (LG) Request, LG Reply, Confirm Group Request, and Confirm Group Reply.) Multicast communication is based on the construct of a group of receivers (hosts) that have an interest in receiving a particular stream of information, be it voice, video, or data. There are no physical or geographical constraints, or boundaries to belong to a group, as long as the hosts have (broadband) network connectivity. The connectivity of the receivers can be heterogeneous in nature, in terms of bandwidth and connecting infrastructure (for example, receivers connected over the Internet), or homogeneous (for example, IPTV or DVB-H users). Hosts that are desirous of receiving data intended for a particular group join the group using a group management protocol: hosts/receivers must become explicit members of the group to receive the data stream, but such membership may be ephemeral and/or dynamic. Groups of IP hosts that have joined the group and wish to receive traffic sent to this specific group are identified by multicast addresses.

Multicast routing protocols belong to one of two categories: Dense-Mode (DM) protocols and Sparse-Mode (SM) protocols.

  • DM protocols are designed on the assumption that the majority of routers in the network will need to distribute multicast traffic for each multicast group. DM protocols build distribution trees by initially flooding the entire network and then pruning out the (presumably small number of) paths without active receivers. The DM protocols are used in LAN environments, where bandwidth
    considerations are less important, but can also be used in WANs in special cases (for example, where the backbone is a one-hop broadcast medium such as a satellite beam with wide geographic illumination, such as in some IPTV applications).
  • SM protocols are designed on the assumption that only few routers in the network will need to distribute multicast traffic for each multicast group. SM protocols start out with an empty distribution tree and add drop-off branches only upon explicit requests from receivers to join the distribution. SM protocols are generally used in WAN environments, where bandwidth considerations are important.

For IP Multicast there are several multicast routing protocols that can be employed to acquire real-time topological and membership information for active groups. Routing protocols that may be utilized include the Protocol-Independent Multicast (PIM), the Distance Vector Multicast Routing Protocol (DVMRP), the MOSPF (Multicast Open Shortest Path First), and Core-Based Trees (CBT). Multicast
routing protocols build distribution trees by examining routing a forwarding table that contains unicast reachability information. PIM and CBT use the unicast forwarding table of the router. Other protocols use their specific unicast reachability routing tables; for example, DVMRP uses its distance vector routing protocol to determine how to create source-based distribution trees, while MOSPF utilizes its link state table to create source-based distribution trees. MOSPF, DVMRP, and PIM-DM are dense-mode routing protocols, while CBT and PIM-SM are sparse-mode routing protocols. PIM is currently the most-widely used protocol.

As noted, IGMP (versions 1, 2, and 3) is the protocol used by Internet Protocol Version 4 (IPv4) hosts to communicate multicast group membership states to multicast routers. IGMP is used to dynamically register individual hosts/receivers on a particular local subnet (for example, LAN) to a multicast group. IGMP

IGMP v2 message format.

version 1 defined the basic mechanism. It supports a Membership Query (MQ) message and a Membership Report (MR) message. Most implementations at press time employed IGMP version 2; it adds LG messages. Version 3 adds source awareness, allowing the inclusion or exclusion of sources. IGMP allows group membership lists to be dynamically maintained. The host (user) sends an IGMP “report,” or join, to the router to be included in the group. Periodically, the router sends a “query” to learn which hosts (users) are still part of a group. If a host wishes to continue its group membership, it responds to the query with a “report.” If the host does not send a “report,” the router prunes the group list to delete this host; this eliminates unnecessary network transmissions. With IGMP v2, a host may send an LG message to alert the router that it is no longer participating in a multicast group; this allows the router to prune the group list to delete this host before the next query is scheduled, thereby minimizing the time period during which unneeded transmissions are forwarded to the network.

The IGMP messages for IGMP version 2 are shown in Fig. 5.2. The message comprises an eight octet structure. During transmission, IGMP messages are encapsulated in IP datagrams; to indicate that an IGMP packet is being carried, the IP header contains a protocol number of 2. An IP datagram includes a Protocol Type field, that for IGMP is equal to 2 (IGMP is one of many protocols that can be specified in this field). An IGMP v2 PDU consists of a 20-byte IP header and 8 bytes of IGMP.

Some of the areas that require consideration and technical support to develop and deploy IPTV systems include the following, among many others:

  • content aggregation;
  • content encoding (e.g., AVC/H.264/MPEG-4 Part 10, MPEG-2, SD, HD, Serial Digital Interface (SDI), Asynchronous Serial Interface (ASI), Layer 1 switching/routing);
  • audio management;
  • digital right management/CA: encryption (DVB-CSA, AES or Advanced Encryption StandardAdvanced Encryption Standard); key management schemes (basically, CAS); transport rights;
  • encapsulation (MPEG-2 transport stream distribution);
  • backbone distribution such as satellite or terrestrial (DVB-S2, QPSK, 8-PSK, FEC, turbo coding for satellite—SONET (Synchronous Optical Network)/SDH/OTN (Synchronous Digital Hierarchy/Optical Transport Network) for terrestrial);
  • metro-level distribution;
  • last-mile distribution (LAN/WAN/optics, GbE (Gigabit Ethernet), DSL/FTTH);
  • multicast protocol mechanisms (IP multicast);
  • QoS backbone distribution;
  • QoS, metro-level distribution;
  • QoS, last-mile distribution;
  • QoS, channel surfing;
  • Set-Top Box (STB)/middleware;
  • QoE;
  • Electronic Program Guide (EPG);
  • blackouts;
  • service provisioning/billing, service management;
  • advanced video services (e.g., PDR and VOD);
  • management and confidence monitoring;
  • triple play/quadruple play.