Information about Ip Multicast
IP Multicast is a method of forwarding IP datagrams to a group of interested receivers. See the article on multicast for a general discussion of this subject - this article is specifically about IP Multicast.
Another widespread use of multicast within campus and commercial networks is for file distribution, particularly to deliver operating system images and updates to remote hosts.
IP multicast has also seen deployment within the financial sector for applications such as stock tickers and hoot-n-holler systems.
While IP multicast has seen some success in each of these areas, IP multicast is not widely deployed and is generally not available as a service for the average end user. There are at least two primary factors for the lack of widespread deployment, both somewhat related to the other. On the one hand, forwarding multicast traffic, particularly for two-way communication, requires a great deal of protocol complexity. On the other hand, there are a number of additional operational concerns in being able to run a multicast network successfully, largely stemming from the complexity of a widely deployed implementation, not the least of which is the enabling of additional avenues of failure, particularly from denial-of-service attacks. Many of these issues are covered in further detail below.
For more info see RFC3170 - IP Multicast Applications: Challenges & Solutions
Most of the existing application protocols that use multicast run on top of the User Datagram Protocol, UDP. In many applications, the Real Time Protocol, RTP, is used for framing of multimedia content over multicast; the RSVP protocol may be used for bandwidth reservation in a multicast distribution.
On the local network, multicast delivery is controlled by IGMP; inside a routing domain, PIM or MOSPF are used; between routing domains, one uses inter-domain multicast routing protocols, such as MBGP.
A number of errors can happen if packets intended for unicast are accidentally sent to a multicast address; in particular, sending ICMP packets to a multicast address has been used in the context of DoS attacks as a way of achieving packet amplification.
There are a number of current general assignment strategies and we will highlight just a few of them here. For general information with pointers to other documents, see RFC 3171.
The 224.0.0.0/24 block is for link local multicast only. Here you find a number of things such as routing protocols. Datagrams to these destinations should never be forwarded by a router.
Much of the remaining address space within 224/8 has either been assigned to a handful of disparate applications and uses over the years or is simply IANA reserved. This /8 block is sometimes referred to disparagingly as the multicast swamp.
The 232.0.0.0/8 block is reserved for use by Source-specific multicast (SSM).
233.0.0.0/8 is set aside for GLOP addresses. In a nutshell, the middle two octets of this block are formed from assigned ASNs, allowing any operator assigned an ASN 256 globally unique multicast group addresses per ASN. This block has been one of the most successful addressing schemes. Unfortunately, it does not scale well.
239.0.0.0/8 is currently an administratively scoped address space. Some operators have treated this entire block according to the RFC 1918 specification. A careful read of the RFC 2365 shows that only a subset of this address space should be treated this way. There are portions of this address space, the relative assignment region, that are very similar to a private unicast addressing space.
The remainder of the Class D address range is currently marked as reserved by the IANA.
In unicast routing, each router examines the destination address of an incoming packet and looks up the destination in a table to determine which interface to use in order for that packet to get closer to its destination. The source address is irrelevant to the router.
However, in multicast routing, the source address (which is a simple unicast address) is used to determine data stream direction. The source of the multicast traffic is considered upstream. The router determines which downstream interfaces are destinations for this multicast group (the destination address), and sends the packet out through the appropriate interfaces. The term reverse path forwarding is used to describe this concept of routing packets away from the source, rather than towards the destination.
For IPv6 Multicast addresses, the Ethernet MAC is derived by the four low-order octets OR'ed with the MAC 33:33:00:00:00:00, so for example the IPv6 address FF02:DEAD:BEEF::1:3 would map to the Ethernet MAC address 33:33:00:01:00:03
One such scheme, proposed by Cisco, is PGM (originally Pretty Good Multicasting, but changed for trademark reasons to Pragmatic General Multicast), documented in RFC 3208. In this scheme, multicast packets have sequence numbers and when a packet is missed a recipient can request that the packet be re-multicast with other members of the Multicast group ignoring the replacement data if not needed. More recently, an expanded version, PGM-CC has attempted to make IP Multicasting more "TCP friendly" by stepping the entire group down to the bandwidth available by the worst receiver.
Independent groups like the Internet Protocol Multicast Standards Initiative (IPMSI) have claimed that the lack of a truly scalable Secure Reliable IP Multicast protocol like the proposed SMART Multicast (SMART) (Secure Multicast for Advanced Repeating of Television) have hampered the adoption of IP Multicast in inter-domain routing. The lack of a widely adopted system that has AES level security and scalable reliability have kept mass media transmissions of sporting events (like the Super Bowl) and/or breaking news events from being transmitted on the Public Internet.
Reliable IP Multicasting protocols like PGM and SMART (mentioned above) are experimental.
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IP Multicast Uses
Many US cable TV operators and some educational institutions with significant on-campus student housing have deployed IP multicast to successfully deliver one-way streaming media such as high-speed video to large groups of receivers. Additionally, there have been some uses of audio and video conferencing using multicast. These are far less prevalent and are most often relegated to research and education institutions, which often have a greater degree of network capacity to handle the demands. Some technical conferences and meetings are transmitted using IP multicast. Until recently many of the sessions at the IETF meetings were delivered using multicast.Another widespread use of multicast within campus and commercial networks is for file distribution, particularly to deliver operating system images and updates to remote hosts.
IP multicast has also seen deployment within the financial sector for applications such as stock tickers and hoot-n-holler systems.
While IP multicast has seen some success in each of these areas, IP multicast is not widely deployed and is generally not available as a service for the average end user. There are at least two primary factors for the lack of widespread deployment, both somewhat related to the other. On the one hand, forwarding multicast traffic, particularly for two-way communication, requires a great deal of protocol complexity. On the other hand, there are a number of additional operational concerns in being able to run a multicast network successfully, largely stemming from the complexity of a widely deployed implementation, not the least of which is the enabling of additional avenues of failure, particularly from denial-of-service attacks. Many of these issues are covered in further detail below.
For more info see RFC3170 - IP Multicast Applications: Challenges & Solutions
History and Milestones
The MBONE was a long-running experimental approach to enabling multicast between sites through the use of tunnels. While the MBONE is no longer operational, there are renewed interests in tunnelling multicast once again in order to make the service available to a wide array of end users.Addressing
There are four forms of IP addressing, each with its own unique properties.- Unicast: The most common concept of an IP address is a unicast address. It normally refers to a single sender or a single receiver, and can be used for both sending and receiving. Usually, a unicast address is associated with a single device or host, but it is not a one-to-one correspondence. Some individual PCs have several distinct unicast addresses, each for its own distinct purpose. Sending the same data to multiple unicast addresses requires the sender to send all the data many times over, once for each recipient.
- Broadcast: Sending data to all possible destinations (an "all-hosts broadcast") permits the sender to send the data only once, and all receivers can copy it. In the IP protocol, 255.255.255.255 represents a limited local broadcast. In addition, a directed (limited) broadcast can be made by combining the network prefix with a host suffix composed entirely of binary 1s. For example, to send to all addresses within a network with the prefix 192.0.2, the directed broadcast IP address is 192.0.2.255.
- Multicast: A multicast address is associated with a group of interested receivers. According to RFC 3171, addresses 224.0.0.0 to 239.255.255.255 are designated as multicast addresses. This range was formerly called "Class D." The sender sends a single datagram (from the sender's unicast address) to the multicast address, and the routers take care of making copies and sending them to all receivers that have registered their interest in data from that sender.
- Anycast: Like broadcast and multicast, anycast is a one-to-many routing topology. However, the data stream is not transmitted to all receivers, just the one which the router decides is the "closest" in the network. Anycast is useful for balancing data loads. It is used in DNS and UDP.
Protocols and Applications
Since multicast is a different transmission mode from unicast, only protocols designed for multicast can be sensibly used with multicast.Most of the existing application protocols that use multicast run on top of the User Datagram Protocol, UDP. In many applications, the Real Time Protocol, RTP, is used for framing of multimedia content over multicast; the RSVP protocol may be used for bandwidth reservation in a multicast distribution.
On the local network, multicast delivery is controlled by IGMP; inside a routing domain, PIM or MOSPF are used; between routing domains, one uses inter-domain multicast routing protocols, such as MBGP.
A number of errors can happen if packets intended for unicast are accidentally sent to a multicast address; in particular, sending ICMP packets to a multicast address has been used in the context of DoS attacks as a way of achieving packet amplification.
IP Multicast Addressing Assignments
The Class D address range, which is still associated with multicast group addresses, is not allocated as traditional unicast addresses. In fact, allocating multicast group addresses has been an ongoing problem. It can result in multiple, mostly unsatisfactory solutions.There are a number of current general assignment strategies and we will highlight just a few of them here. For general information with pointers to other documents, see RFC 3171.
The 224.0.0.0/24 block is for link local multicast only. Here you find a number of things such as routing protocols. Datagrams to these destinations should never be forwarded by a router.
Much of the remaining address space within 224/8 has either been assigned to a handful of disparate applications and uses over the years or is simply IANA reserved. This /8 block is sometimes referred to disparagingly as the multicast swamp.
The 232.0.0.0/8 block is reserved for use by Source-specific multicast (SSM).
233.0.0.0/8 is set aside for GLOP addresses. In a nutshell, the middle two octets of this block are formed from assigned ASNs, allowing any operator assigned an ASN 256 globally unique multicast group addresses per ASN. This block has been one of the most successful addressing schemes. Unfortunately, it does not scale well.
239.0.0.0/8 is currently an administratively scoped address space. Some operators have treated this entire block according to the RFC 1918 specification. A careful read of the RFC 2365 shows that only a subset of this address space should be treated this way. There are portions of this address space, the relative assignment region, that are very similar to a private unicast addressing space.
The remainder of the Class D address range is currently marked as reserved by the IANA.
Routing
Each host (and in fact each application on the host) that wants to be a receiving member of a multicast group (i.e. receive data corresponding to a particular multicast address) must use the Internet Group Management Protocol (IGMP) to join. Adjacent routers also use this protocol to communicate.In unicast routing, each router examines the destination address of an incoming packet and looks up the destination in a table to determine which interface to use in order for that packet to get closer to its destination. The source address is irrelevant to the router.
However, in multicast routing, the source address (which is a simple unicast address) is used to determine data stream direction. The source of the multicast traffic is considered upstream. The router determines which downstream interfaces are destinations for this multicast group (the destination address), and sends the packet out through the appropriate interfaces. The term reverse path forwarding is used to describe this concept of routing packets away from the source, rather than towards the destination.
Layer 2 Delivery
Unicast packets are delivered to a specific recipient on an Ethernet or IEEE 802.3 subnet by setting a specific layer 2 MAC address on the Ethernet packet address. Broadcast packets make use of a broadcast MAC address (FF:FF:FF:FF:FF:FF), which includes setting the broadcast/multicast bit in the address. Multicast packets are delivered by using the Ethernet MAC address range 01:00:5e:00:00:00 - 01:00:5e:7f:ff:ff. This is 23 bits of available address space. The first octet (01) includes the broadcast/multicast bit. The lower 23 bits of the 28-bit multicast IP address are mapped into the 23 bits of available ethernet address space. This means that there is ambiguity in delivering packets. If two hosts on the same subnet each subscribe to a different multicast group whose address differs only in the first 5 bits, Ethernet packets for both multicast groups will be delivered to both hosts, requiring the network software in the hosts to discard the unrequired packets.For IPv6 Multicast addresses, the Ethernet MAC is derived by the four low-order octets OR'ed with the MAC 33:33:00:00:00:00, so for example the IPv6 address FF02:DEAD:BEEF::1:3 would map to the Ethernet MAC address 33:33:00:01:00:03
Reliable Multicast
Multicast, by its very nature, is not a connection-oriented mechanism, so protocols such as TCP, which allows for retransmission of missing packets, are not appropriate. For applications such as streaming audio and video, the occasional dropped packet is not a problem. But for distribution of critical data, a mechanism is required for requesting retransmission.One such scheme, proposed by Cisco, is PGM (originally Pretty Good Multicasting, but changed for trademark reasons to Pragmatic General Multicast), documented in RFC 3208. In this scheme, multicast packets have sequence numbers and when a packet is missed a recipient can request that the packet be re-multicast with other members of the Multicast group ignoring the replacement data if not needed. More recently, an expanded version, PGM-CC has attempted to make IP Multicasting more "TCP friendly" by stepping the entire group down to the bandwidth available by the worst receiver.
Independent groups like the Internet Protocol Multicast Standards Initiative (IPMSI) have claimed that the lack of a truly scalable Secure Reliable IP Multicast protocol like the proposed SMART Multicast (SMART) (Secure Multicast for Advanced Repeating of Television) have hampered the adoption of IP Multicast in inter-domain routing. The lack of a widely adopted system that has AES level security and scalable reliability have kept mass media transmissions of sporting events (like the Super Bowl) and/or breaking news events from being transmitted on the Public Internet.
Reliable IP Multicasting protocols like PGM and SMART (mentioned above) are experimental.
IP Multicast protocols
- Internet Group Management Protocol (IGMP)
- Protocol Independent Multicast (PIM)
- Distance Vector Multicast Routing Protocol (DVMRP)
- Multicast BGP (MBGP)
- Multicast Source Discovery Protocol (MSDP)
- Multicast Listener Discovery (MLD)
- GARP Multicast Registration Protocol (GMRP)
- Multicast DNS (mDNS)
See also
- dense multicast
- sparse multicast
- source-specific multicast
- core-based trees
- Multicast address
- Multicast
Multicast is the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the destinations split.
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Stock Ticker is a now out of print board game that was popular upon its release and is still played today. It was released by Copp-Clark Publishing, a venerable Canadian publisher.
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Game play
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Hoot-n-holler is a type of telecommunication where you have a permanent open circuit between two or more parties, where anyone can speak at any time, without having to pick up a phone or press a button. It is used, among other places, in stockbroker and warehouse applications.
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Mbone (short for "multicast backbone") is an experimental backbone for IP Multicast traffic across the Internet. Since most Internet routers do not yet support IP Multicast, the Mbone has evolved to connect multicast-capable networks over the existing Internet infrastructure.
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In computer networks, unicast is the sending of information packets to a single destination. "Unicast" is derived from the word broadcast, as unicast is the extreme opposite of broadcasting.
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In computer networking, broadcasting refers to transmitting a packet that will be received (conceptually) by every device on the network. In practice, the scope of the broadcast is limited to a broadcast domain. Contrast unicasting and multicasting.
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Multicast is the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the destinations split.
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Anycast is a network addressing and routing scheme whereby data is routed to the "nearest" or "best" destination as viewed by the routing topology.
The term is intended to echo the terms unicast, broadcast and multicast.
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The term is intended to echo the terms unicast, broadcast and multicast.
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On the Internet, the Domain Name System (DNS) associates various sorts of information with so-called domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. en.wikipedia.
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User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. Using UDP, programs on networked computers can send short messages sometimes known as datagrams (using Datagram Sockets) to one another.
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User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. Using UDP, programs on networked computers can send short messages sometimes known as datagrams (using Datagram Sockets) to one another.
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Protocol-Independent Multicast (PIM) is a family of multicast routing protocols that can provide one-to-many and many-to-many distribution of data over the Internet. The "protocol-independent" part refers to the fact that PIM does not include its own topology discovery mechanism,
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Source-specific multicast (SSM) is a method of delivering multicast packets originating in only a specific source address S to any receiver that requests it. It limits the original multicast model to be originated from just one host, simplifiying the requirements on the network and
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In the Internet, an autonomous system (AS) is a collection of IP networks and routers under the control of one entity (or sometimes more) that presents a common routing policy to the Internet. See RFC 1930 for additional detail on this updated definition.
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The Internet Group Management Protocol (IGMP) is a communications protocol used to manage the membership of Internet Protocol multicast groups. IGMP is used by IP hosts and adjacent multicast routers to establish multicast group memberships.
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Internet Protocol version 6 (IPv6) is a network layer protocol for packet-switched internetworks. It is designated as the successor of IPv4, the current version of the Internet Protocol, for general use on the Internet.
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The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. TCP provides reliable, in-order delivery of a stream of bytes, making it suitable for applications like file transfer and e-mail.
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SMART Multicast is an experimental method of Secure Reliable IP Multicast. It allows a user to forward IP datagrams to an unlimited group of receivers. See the article on multicast for a general discussion of this subject - this article is specifically about SMART IP Multicast.
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The Internet Group Management Protocol (IGMP) is a communications protocol used to manage the membership of Internet Protocol multicast groups. IGMP is used by IP hosts and adjacent multicast routers to establish multicast group memberships.
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Protocol-Independent Multicast (PIM) is a family of multicast routing protocols that can provide one-to-many and many-to-many distribution of data over the Internet. The "protocol-independent" part refers to the fact that PIM does not include its own topology discovery mechanism,
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The Distance Vector Multicast Routing Protocol (DVMRP) is used to share information between routers to transport IP Multicast packets among networks.
It is based in the RIP protocol to forward packets: the router generates a routing table with the multicast group that
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It is based in the RIP protocol to forward packets: the router generates a routing table with the multicast group that
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Multiprotocol Extensions for BGP (MBGP), defined in IETF , is an extension to the BGP unicast routing protocol that allows different types of addresses (known as address families) to be distributed in parallel.
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Multicast Source Discovery Protocol (MSDP) is a computer network protocol in the Protocol Independent Multicast (PIM) family of multicast routing protocols.
MSDP interconnects multiple IPv4 PIM Sparse-Mode (PIM-SM) domains.
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MSDP interconnects multiple IPv4 PIM Sparse-Mode (PIM-SM) domains.
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Multicast Listener Discovery is the protocol used in the IPv6 protocol suite by a router to discover listeners for a specific multicast group, much as IGMP is used in IPv4. The protocol is embedded in ICMPv6 instead of using a separate protocol.
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GMRP stands for GARP Multicast Registration Protocol. It is a Layer 2 network protocol and is defined in the IEEE 802.1p specification.
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Zeroconf or Zero Configuration Networking is a set of techniques that automatically create a usable IP network without configuration or special servers. This allows inexpert users to connect computers, networked printers, and other items together and expect them to work
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Dense mode multicast is one mode that multicast can use to construct a tree for sending packets to the multicast subscribers. It is the opposite of sparse multicast.
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protocol independent because it is not dependent on any particular unicast routing protocol for topology discovery, and sparse-mode because it is suitable for groups where a very low percentage of the nodes (and their routers) will subscribe to the multicast session.
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Source-specific multicast (SSM) is a method of delivering multicast packets originating in only a specific source address S to any receiver that requests it. It limits the original multicast model to be originated from just one host, simplifiying the requirements on the network and
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