Multicast

Multicast refers to a message in the telecommunications transmission from one point to a group (also called multi-point connection ). This technique is used in accordance with the OSI model to the network layer are used. The advantage of multicast is that the same messages to multiple users or to a closed user group can be transferred, without the purpose data transmission rate used multiplied by the sender with the number of receivers. The transmitter needs multicasting only the same data transfer rate as a single receiver. If it is packet-oriented data transmission, ( switches, routers ) on the route is the duplication of packets rather than to each distributor.

A special form of multicast is Geocast, is sent in which, in a spatially defined area.

The difference to broadcast is that the broadcast content is distributed, anyone can view with the appropriate receiving equipment, whereas must be done before a logon on the transmitter with multicast.

IP multicast

Multicast is the usual designation for IP Multicast, which allows you to send in IP networks efficiently packages to many recipients at the same time. This happens with a specific multicast address. In IPv4, this is the address range 224.0.0.0 to 239.255.255.255, reserved each beginning with FF00 :: / 8 address in IPv6. In addition, is used to coordinate with the IPv4 protocol IGMP or CGMP (Cisco components). IPv6 ICMPv6 assumes the control function.

When transmitted over Ethernet, IPv4 or IPv6 multicast addresses are mapped to certain pseudo - MAC addresses to already allow through the network card filtering for relevant traffic. The mapping is done according to the following rules:

• In IPv4, the lowest 23 bits of the IP address are used in the MAC address 01 -00- 5e -00- 00-00, resulting in addresses from the range 01 -00- 5e -00 -00- 00-01 - can result in 00- 5e - 7f -ff -ff. Here is consciously accepted that more IPv4 addresses are mapped to a MAC address (for example, 224.0.0.1 and 233.128.0.1 ).
• IPv6 multicast addresses are mapped to MAC addresses by the last four bytes of the address are used in the MAC 33-33-00-00-00-00. Again, different multicast addresses are mapped to identical MAC addresses.

An increased importance to know the multicast addresses in IPv6. An IPv6 multicast address has a prefix of FF00 :: / 8 (1111 1111). The second nibble of the address FF00 :: / 16 defines the lifetime and scope ( scope). In this case, a permanent address has a value of 0 is a temporary address from a 1 (flag). The scope of a multicast address varies from node, link, site, organization to globally and is displayed with the parameters 1, 2, 5, 8, and e.

Example: prefix FF02 :: / 16 is a permanent multicast address for a link

Multicasting makes the broadcast addresses superfluous. If, for example, a packet is sent to all hosts on a segment, a special multicast address with the meaning "ALL Nodes" will be used.

Multicast is also used in connection with audio and video transmissions. Protocols such as RTP use this mechanism. Furthermore, also supported routing protocols such as the (RIP ) version 2 or OSPF ( Open Shortest Path First ) multicasting. OSPF uses the address 224.0.0.5 or 224.0.0.6 to distribute information.

In addition, multicast is necessary for a working AppleTalk network. Also, it is used with Service Location Protocol and Multicast DNS as part of implementation of Zeroconf multicast. In addition to these currently are preferably used in the Apple world multicast protocols used in Windows systems for SSDP.

Because multicast packets are not processed by most routers in the Internet, multicast-capable subnetworks are connected by tunnel to the multicast backbone ( MBone ).

In the context of Mobile IP Multicast has inherent problems.

Security

IPSec realizes safe communication in point-to -point communication over the Internet Protocol. For multicast however, there is a problem with the Internet Key Exchange, since the receiver sets the authentication and encryption algorithm. With multicast, which must be done by the sender, since it sends IP packets with the same algorithm to multiple recipients. For solving this problem, there are several methods.

Centralised procedures

• Logical Key Hierarchy
• One-way Function Tree

Distributed method

• Ingemarsson - Tang Wong
• Burmester - Desmedt
• Tree -Based Group Diffie -Hellman

More multicast protocols

A critical aspect in the handling of multicast traffic is the efficient delivery of packets at all stations on the basis of routing protocols. IP multicast transfers data from the source to a group of recipients via a tree structure. Different IP multicast routing protocols use different techniques here to construct this tree. Is this distribution tree constructed once, all traffic flowing over him. Three multicast routing protocols have been established:

• Distance Vector Multicast Routing Protocol ( DVMRP ) according to RFC 1075
• Multicast OSPF ( MOSPF ) as an extension of the routing protocol according to RFC 1584
• Protocol - Independent Multicast (PIM )

IP multicast routing protocols can be divided into two general approaches:

• The dense mode, which assumes that the receiver stations are very close together in the network and have sufficient throughput and
• The sparse mode, which assumes that the receivers are distributed very widely over the network and have as in a WAN environment, only limited bandwidth.

While DVMRP and MOSPF are attributable to the first class, there are forms for both types of PIM.

Internet Relay Chat forms networks which implement a simple TCP-based multicast tree. The messaging protocol PSYC uses a similar principle, whereby for each chat room or communications channel has its own optimized multicast tree is generated. For XMPP is about how multicast can be retrofitted, but this is very difficult due to the existing structure. For distributed chat networks was now generally realized that they can not be implemented using IP multicast. The use of other multicast protocols is essential.

With Multimedia Broadcast Multicast Service since 2005 there is a technology available that is capable of distributing multimedia content over multicast channels for the mobile system UMTS.