Hello everyone,
This post will introduce IPv6 multicast technologies.
1. IPv6 Generation
The Internet Protocol (IP) was born in the 1970s. After years of development, it has become the mainstream network layer protocol. But many of its designs have constrained the further development of the Internet.
As early as 1992, the development of the Next Generation Internet Protocol (IPng) was discussed. In 1995, the Internet Engineering Task Force (IETF) adopted SIPP as the basis for IPng. IPng is officially granted version number 6 by the Internet Numbering Authority (IANA). In 1996, the basic IPv6 protocol specifications were published and the revised version was published in 1998. IPv6 expands the address space and supports data transmission integrity and security. It also supports large-scale network structures, automatic network configuration, faster route selection, more efficient route aggregation, and enhanced multicast technologies. Currently, the industry widely recognizes the advantages of IPv6 and starts to evolve from IPv4 to IPv6. Major router vendors have started to provide IPv6 routing support, and various operating systems have started to embed IPv6 protocol stacks. Due to the commercial operation of IPv4 networks, multiple types of hybrid networks will emerge during evolution. Hosts will have both IPv4 and IPv6 dual stacks and then evolve to pure IPv6 networks.
2. Generation and Development of Multicast Technology
In recent years, the rapid development of the Internet has produced many new applications, especially multimedia applications with high bandwidth requirements. To alleviate the network bottleneck, the industry provides the following four major solutions:
Increase network bandwidth.
Use the QoS mechanism to control the bandwidth usage of different services.
Distributed and clustered servers
IP Multicast Technology
IP multicast technology is one of the most popular network technologies because of its unique advantages. On an IP multicast network, even if the number of users increases exponentially, the bandwidth of the backbone network does not need to be increased accordingly.
Multicast allows one or more senders (multicast sources) to send the same packet to multiple receivers. A multicast source sends a packet to a specific multicast address. Different from a unicast address, a multicast address does not belong to a single host but to a group of hosts. A multicast address indicates a group. The recipients that need to receive multicast packets join the group. In this way, no matter how many multicast packet receivers are, any link on the entire network transmits only a single packet, greatly saving bandwidth.
2.1 Multicast group management
The IGMP (Group Management Protocol) is used to dynamically manage the joining and leaving of group members. When a host wants to join a group, it sends an IGMP message to the local multicast router. The multicast router listens to IGMP messages and performs corresponding operations. It also periodically sends queries to maintain the currently active group.
There are multiple IGMP versions. IGMPv1 has two types of messages: Member Query and Member Report. The two types of messages are used to complete basic group joining and group maintenance. IGMPv2 is basically the same as IGMPv1. The difference is that IGMPv2 adds a message for leaving a group. In this way, IGMPv2 leaves a group in advertisement mode. In IGMPv1, the multicast router sends a query request. If no response is received, the multicast router determines whether a group member leaves the group. IGMPv3, which is currently being developed, adds the source-based filtering function. It allows a group member to notify the multicast router of the multicast source that it wishes to receive, that is, only the data from this source is received.
2.2 Multicast routing
In multicast, packets are sent from the multicast source to a group of hosts. To ensure that all hosts in a group can receive packets, a multicast distribution tree (MDT) is required to describe the path of packets on the network. There are three basic types of multicast trees: flooding, active tree, and shared tree. Multicast routing determines the multicast distribution tree from the multicast source to the receiver of a group. To improve the distribution efficiency, the multicast routing method must ensure that all members in the receiving group can receive multicast packets, and minimize the network resource occupation and the multicast sending range. Multicast routing protocols are classified into the dense mode and sparse mode.
3 Inheritance and Enhancement of IPv6 Multicast
Due to the advantages of multicast technology, the IETF retains multicast while cancels broadcasts when formulating the IPv6 protocol. To better use and manage multicast applications, IPv6 enhances multicast in the following aspects:
3.1 Multicast Address
Figure 1 shows the IPv6 multicast address format.
The first eight bits of an IPv6 multicast address is 11111111. The four-bit multicast flag is used to distinguish the well-known multicast address (the value is 0) from the temporary multicast address (the value is 1). The three most significant bits of this field are reserved. The four-bit range field determines the range that a multicast packet can travel. The assigned range field values are as follows (other values are either not allocated or reserved):
1: local node-specific packets cannot be output from the port.
2: Link partial packets cannot be forwarded by routers.
5: Branch: The definition of a branch is determined by the multicast router administrator of the branch.
8: organization part - The definition of an organization is determined by the multicast router administrator of the organization.
14: global
Compared with IPv4, IPv6 has the following advantages:
(1) Sufficient address space is available. The address space defined by IPv4 is equivalent to 16 Class A addresses, which is far from enough for global multicast applications. Theoretically, the IPv6 address space can reach 2,120.
(2) Application of the scope field. Different from unicast addresses, multicast addresses do not belong to a specific host or application. Except for a few addresses reserved for protocol implementation, other addresses are dynamically allocated to multicast users as required. In this case, a multicast address may be used by multiple multicast applications at the same time. Therefore, the transmission scope of the multicast addresses must not overlap. Although IPv4 uses the TTL to control the range of multicast packets to be transmitted, the TTL is not accurate, and the range of packets in different applications may overlap. The IPv6 address format specifies the range field, which facilitates the division of multicast domains and controls the propagation range of multicast applications based on the multicast domain.
Each multicast domain has its own multicast address space. The multicast packets in this address space are forwarded only in the local multicast domain, and the border router of the domain does not forward the multicast packets in this address space outside the domain. In this way, multiple multicast domains with a smaller range at the same level form a multicast domain with a larger range at a higher level. The multicast address space of multicast domains at different layers cannot overlap. Multicast domains at the same layer can have the same multicast address space. The advantage of this feature is that users can select multicast addresses in a proper multicast domain as required. In this way, multicast packets are forwarded within the expected range, ensuring the orderly running of multicast applications.
3.2 MLD Protocol
The Multicast Listener Discovery (MLD) protocol is derived from the IGMPv2 protocol and is used for managing IPv6 multicast groups. IPv6 routers use MLD to discover whether multicast group members exist on directly connected links and which multicast addresses are being listened to by neighboring routers. The multicast routing protocol running on the IPv6 router ensures that multicast packets can be sent to correct receivers based on the information.
Although MLD is developed from IGMPv2, MLD uses the new format of ICMPv6 instead of IGMP packets. ICMPv6 is a set of control protocols defined for IPv6 networks, MLD is a subset of ICMPv6.
4 Research Direction of IPv6 Multicast Technology
With the gradual evolution of IPv6 networks and the rapid growth of multicast applications, multicast services will become a major network service on the next-generation IPv6 Internet. Currently, many multicast protocol standards are formulated to support IPv6 on the basis of serving IPv4. Although their current research focus is on IPv4, IPv6 researchers need to focus more on the new features of multicast in IPv6.
4.1 Multicast Address Management
Multicast applications require a multicast address management method to obtain addresses. The multicast service can obtain the multicast address in the following three ways:
(1) Coding mode: The multicast address is written into the program code so that the multicast address becomes a part of the program or fixed in the ROM. This method is applicable to the multicast addresses that are statically allocated by the IANA.
(2) Notification mode: The multicast service randomly selects a multicast address and notifies the network of the address to be used before the address is used. Currently, multicast services in the Mbone (multicast backbone network) use this mode. The SDR is the conference directory tool.
(3) Algorithm derivation mode: A programmable algorithm is used to allocate a multicast address that does not conflict with other services worldwide to the multicast service.
Another major problem in multicast address management is the discovery of multicast domains. A multicast domain is an effective propagation range of multicast services. Different multicast domains have different multicast address spaces. Therefore, the discovery of a multicast domain is a process in which users select multicast address spaces.
At present, the IETF has set up a MALLOC working group for multicast address management. The working group focuses on formulating a series of protocol specifications related to multicast addresses to ensure global multicast address allocation. Its main achievements include Internet Multicast Address Allocation Architecture (MAAA), Multicast Address Dynamic Allocation Protocol (MADCAP), Multicast Address Allocation Protocol (AAP), Multicast Address Set Application Protocol (MASC), and so on. In addition, the Mbone Working Group of IETF developed the MZAP protocol for multicast domain discovery. The MZAP protocol fully considers the characteristics of IPv6 multicast addresses and formulates the related IPv6 multicast address allocation standard.
4.2 Multicast Network Management
Network management has become a hot topic of network technology, and there are many research achievements and products in this field. Currently, the NMS focuses on the management of unicast traffic, but seldom on the management of multicast traffic. Although some technologies are applicable to the NMS in unicast mode and the NMS in multicast mode, the fundamental difference between unicast communication and multicast communication in mechanism requires that the NMS in multicast mode adopt different technologies. As a result, network management products that mainly manage unicast traffic cannot effectively manage multicast traffic. In addition, as multicast technologies are widely used, multicast traffic accounts for an increasing proportion of network traffic, and a requirement for multicast traffic management becomes more and more urgent. Therefore, the research of network management technology for multicast traffic is becoming a new hot topic.
Although SNMP defines managed objects through the management information base (MIB), it is independent of the network transmission protocol and can be applied to IPv6 environments (the IETF has developed IPv6-related MIBs), the multicast network management in IPv6 environments has more new features. For example, a larger IPv6 network requires more robust network management, and SNMP information processing requires higher security. Currently, most router vendors provide IPv6 routing, which provides good conditions for the research of multicast network management in the IPv6 environment.
4.3 Reliable Multicast Transmission
Multicast packets are transmitted through UDP. Therefore, it lacks the reliable transmission function provided by the Transmission Control Protocol (TCP). Some multicast services, such as video and audio services, can tolerate a certain packet loss rate, while others, such as Multicast FTP (MFTP), require reliable multicast services. Currently, there are many reliable multicast protocols, such as SRM, RMTP, and TMTP, and some of them have been commercialized, but they have not yet become standards.
The RMT working group of IETF and the RMRG working group of Internet Research Institute (IRTF) are also developing related protocol standards. It includes: how to ensure reliable multicast transmission in a large-scale multicast environment, including the design of reliable multicast protocol based on negative acknowledgment mechanism(NACK), design of reliable multicast protocol based on tree acknowledgment mechanism, design of asynchronous layered coding protocol using forward error correction mechanism, etc. Recipients that support delayed joining; Security issues of reliable multicast.
4.4 Multicast Network Security
Network security is critical to the normal running of unicast and multicast networks. Multicast network security and unicast network security have similar requirements, such as user authentication, data consistency check, data encryption, and user authorization. However, considering features of multicast technologies, for example, multipoint-to-multipoint transmission, data can be sent to a group without joining a group, it is more difficult to implement network security in a multicast network. Multicast network security includes: restricting senders, restricting receivers, restricting access, verifying multicast content, protecting receivers, and traversing firewalls.
The IETF MSEC working group and the ITF GSEC working group are also developing multicast security-related protocol standards. Its work includes: defining the overall structure of multicast security (including the overall functional modules and their relationships), defining the multicast key management method, and so on.
5. Conclusion
IPv6 and multicast technologies are hot research fields. Protocols and standards are still being developed, discussed, and changed. All these require our active participation in research, development, and practice.
That is all! Thank you!
