ZTE´s IP Multicast Technology

The recent years have seen growing multimedia applications that require wide bandwidth over the Internet. As we all know, traditional networks were originally designed for data transmission and in conventional unicast transmission the network bandwidth varied linearly with the number of subscribers. The point-to-point communication may lead to the rapid rise in bandwidth consumption and the problem of network congestion, because the transmission load and network delay increase along with the growing of subscribers. However, for multicast networks, even if the number of subscribers doubles, it is unnecessary to increase the backbone bandwidth. Therefore, the multicast technology has its unique advantages in one-to-many and many-to-many multimedia service applications. The growing demands for broadband and multimedia services boost the popularity and development of IP multicast technology. Consequently, IP multicast becomes an important technology necessary for the next-generation networks. In return, the mature IP multicast technology helps operators and enterprises find new killer applications and generate more revenues.

1 Applications of Multicast Technology
The applications of multicast technology fall into two categories: one-to-many and many-to-many multicast applications.
    (1) One-to-Many Multicast Applications
    These include Video on Demand (VoD), business TV, Push media (including advertisement, news on line, online weather forecast, web category, file-based online update and notice), network time synchronization, meeting planning, key distribution, configuration update, online stock monitoring, and
remote monitoring.

    (2) Many-to-Many Multicast Applications
    The applications involve multimedia conference, data sharing distributed database system, distributed parallel processing system, shared file editing, online education, online chat, distributed interactive simulation, and multi-user network game.

    The multicast technology is an integration of a series of multicast protocols and a set of corresponding control management models, authentications and billing methods. From the point of view of operators, total solutions to the multicast technology are more necessary. Described below are features necessary for wide applications of the multicast technology.

    (1) Technical Support of 
    Network-wide Multicast TechnologyMulticast services shall be deployed on a network-wide basis, i.e., they shall be supported by a vast majority of network devices available in the current network including routers and L2/L3 switches. The network-wide multicast technology falls into two categories: intra-domain multicast and inter-domain multicast, and it consists of various protocols such as the multicast management protocol, multicast routing protocol and application-layer multicast protocol.

    (2) Controllable, Manageable and Safe Multicast Technology Support
    Multicast should have perfect control models and authentication methods.

    (3) Support of Operable and 
    Value-added Multicast Technology Monitoring, billing and IP multicast technology make possible the stable and reliable operation for services at the upper level. Meanwhile, a perfect billing system for multicast services should
be provided.

2 Network-wide Multicast Technology
The network-wide multicast technology is based on a series of multicast-related protocols. Given below is an introduction to multicast protocols and technologies used in ZTE´s data products in terms of application environments.

2.1 Multicast Protocols between Router and Client
Internet Group Management Protocol (IGMP) is the multicast Group Management Protocol (GMP) in IPv4 protocol stack. It is adopted between the multicast routers that are connected directly between the hosts. On one hand, the host notifies the local router of its intention to add a specified multicast group and receive the information about it via IGMP. On the other hand the router collects and maintains the relations of all the group members connected to the network. This latter one is done by periodically inquiring about the active status of the members in an assigned group of Local Area Network (LAN) via IGMP. Currently IGMP has three versions: V1, V2 and V3. In IPv6 protocol stack, the multicast GMP is Multicast Listener Discovery (MLD) that has two versions: V1 and V2.

2.2 Multicast Protocols between Routers
Multicast routing protocols between routers are divided into intra-domain routing protocols and inter-domain routing protocols.

    For an IP network with multicast applied only in a certain domain, such as a Metropolitan Area Network (MAN), the router only needs to support intra-domain multicast routing protocols. The routing protocols that are used for intra-domain multicast include Distance Vector Multicast Routing Protocol (DVMRP), Protocol-Independent Multicast-Dense Mode (PIM DM), Protocol-Independent Multicast-Sparse Mode (PIM SM), Protocol Independent Multicast-Source Specific Multicast (PIM SSM), and Bidirectional Protocol-Independent Multicast
(Bidir PIM).

    DVMRP is the first multicast routing protocol based on Routing Information Protocol (RIP). It is widely used in Multicast Backbone on the Internet (Mbone). Driven and triggered by data, DVMRP establishes the multicast routing table. The process of setting up a routing tree is characterized by "Broadcasting and Pruning", while the data forwarding process features "Passive Acceptance and Active Exit".

    Due to its reliance on RIP, DVMRP is not suitable for the current application of unicast protocols that requires multiple protocols. Thus Protocol-Independent Multicast (PIM) is introduced, and it includes PIM DM and PIM SM.

    The process of setting up a routing tree and forwarding multicast data in PIM DM is similar to that in DVMRP. Driven by data, PIM DM sets up the routing tree, with the setup process featuring
"Broadcasting and Pruning" while the forwarding process featuring "Passive Acceptance, Active Exit and Requirement Graft".

    Compared with the previous two multicast protocols, PIM SM focuses on establishment and maintenance of the routing as well as efficiency of the forwarding. It makes use of protocol messages to complete the establishment and maintenance of the multicast distribution tree via explicit join/pruning mechanism. The multicast tree falls into two types: group G RP-based Shared Tree (RPT) and source-based Shortest Path Tree (SPT). In addition, PIM SM provides RP selection mechanism to set up the RPT.  Compared with PIM SSM and Bidir PIM that will be introduced later, the PIM SM has wide range of applications. It can be used for either one-to-many or one-to-one
multicast services.

    PIM SSM, a simplified PIM SM, can set up the SPT directly when the receiver knows specific multicast source of the group G, while PIM SM needs to set up the RPT before switching to the SPT. For one-to-many multicast application, PIM SSM enables a simpler way of protocol implementation than PIM SM, but it shall be supported by IGMPv3.

    On the contrary, Bidir PIM is chiefly used for many-to-many multicast service. It adopts shared multicast tree between the multicast source and the RP and between the RP and the receiver. Through the shared tree, multicast data stream can be forwarded from the source to the RP and then to the receiver. Bidir PIM has fewer multicast routings to be maintained than any other PIM protocols. Therefore, it enables the simplest protocol implementation and maintenance, and supports the largest traffic of multicast services under the same conditions of system resources.
For the cross-domain transfer of multicast services over IPv4, the backbone router should support Multi-protocol Border Gateway Protocol (MBGP) and Multicast Source Discovery Protocol (MSDP).

    The most common solution to multi-domain multicast over IPv4 is the combination of PIM SM, MSDP and MBGP. The MBGP is applied for multicast service by expanding BGV4 to route reachable information and the operator´s policy to transfer inter-domain multicast protocol. The location information of the multicast source is sent by MSDP to its internal member through peer interaction between RPs no matter where that RP is. Therefore, no matter whether the multicast source is in the domain, PIM SM can obtain its location information via MSDP, thus avoiding the cross-domain problem of the multicast source.

    In IPv6, it is not necessary for MSDP to provide cross-domain multicast for PIM SM. IPv6 supports acquisition of both RPs in the full PIM SM domain and multicast sources in the full PIM SSM domain.

2.3 Multicast Protocols for Switching Environment
Typically, the L2 switch distribution tree treats multicast data stream as unknown or broadcasting data stream, which may lead to flooding of multicast traffic. To solve this problem, ZTE´s data products provide IGMP Snooping, PIM Snooping and MLD Snooping. By XSnooping of IGMP, PIM SM and MLD messages over the L2 switching equipment, it can be decided which ports are available for transferring multicast data stream.

3 ZTE´s Data Products Support Controllable, Manageable and Safe Multicast Technology
Multicast protocols themselves are unable to provide controllable, manageable and safe mechanism. However, it is necessary for the multicast technology that is closely related to a wide range of applications to solve the problem of control, management and security. ZTE´s data products deliver solutions to the problem in the following ways.

    (1) Access Authentication of Multicast Source
    The multicast service control equipment determines whether a multicast source is a legal (or registered) media server before the multicast service stream flows into the network. Only from legal multicast sources can the multicast service stream be forwarded by the access equipment. Meanwhile, in order to control the deployment, type and bandwidth of multicast services, the access rate of multicast traffic sent from the legal multicast source shall
be controlled.

    (2) Access Authentication of Multicast Receiver
    User control refers to authority authentication for users receiving of the multicast services by L2 access equipment at the receiver side based on Web, 802.1X, PPPoE authentication. If the user passes the authentication, the L2 access equipment will receive the user´s IGMP join/withdraw information, and set up related forwarding tables to allow the user to receive the multicast traffic. Otherwise, it will discard the user´s IGMP messages to prevent the user from receiving the multicast traffic.

    (3) Security Control of Multicast Information
    For a standard multicast service, a receiver can join any multicast group. As the multicast source is unable to know and control the scope and direction of the multicast tree, this service has relatively low security. In order to protect important information, ZTE´s data products can deliver a variety of multicast security solutions by using static multicast, multicast VPN and IPSec to isolate and encrypt multicast services.

4 ZTE´s Data Products Support Operable and Value-added Multicast  Technology
Based on the controllable and manageable features, the operability of multicast services involves various technologies. These technologies include billing, service quality management, service level monitoring, multicast status monitoring, multicast session and member monitoring, multicast routing and traffic monitoring, multicast protocol monitoring, multicast topology monitoring, and geographical location monitoring. ZTE´s data products adopt the above technologies to plan and balance multicast load and services on the entire network. Therefore, they provide highly reliable and quality multicast services for users, as well as accurate service billing and management methods for operators. Moreover, they help operators to analyze, diagnose, prevent and correct errors or failures occurring in the operation of multicast services.

5 Conclusions
With a view to good performance and promising market prospect of the multicast technology, many service providers take the lead in applying the technology and benefit a lot from it. Italian FastWeb, Hong Kong Broadband Network (HKBN) and Swedish Bredbandsbolaget (B2) have become leading providers of multicast services in the industry.
The IP multicast supplied by ZTE data products provides a simple and intuitive way for the people to enjoy feature-rich multimedia services over the information superhighway. Moreover, it provides a powerful technical platform for operators to deploy broadband value-added services such as streaming media and videoconference and to perform related management and monitoring.

Manuscript received: 2005-06-06