XCAST
Encyclopedia
The explicit multi-unicast (Xcast) is a variation of multicast
that supports a great number of little multicast sessions. It is done by adding all the IP address
es in the destination field of the IP header, instead of using a multicast address. The traditional multicast schemes over IP
are scalable for big multicast groups, but they have got scalability problems for a great number of groups.
The multicast schemes can be used to minimize the bandwidth
consumption. Xcast can be used to minimize the bandwidth consumption for little groups, but the great advantage is that it eliminates the signalization and the state information for every session of a traditional multicast scheme. Thanks to this it is capable to support a great number of little sessions.
channel through which packets will be sent. The source codifies the destinations list in the Xcast header and send the packet to the router. Each router looks up a routing table
to determine the following jumps of each packet, analyzes its header, parses the destination field basing on the following jump of every destination and copies the packets as many different paths as they have to follow. After that, the router pulls the copy packets with its correct Xcast header to every following jump. When the last jump, there is no need of making a new copy, there is just one address in the destination field. Then the packet is just like a unicast
packet, and the network
will treat it as it is. That is called X2U (Xcast to Unicast).
multicast
schemes were designed to treat with huge multicast groups. It works well when doing a distribution similar to broadcast
ing, but it has scalability problems when there is a great number of groups. The multicast protocols keep routing tables that relate multicast group addresses with the nodes. This tables might become huge, and that is why some alternative schemes try to reduce the quantity of information to keep. There are also protocols to announce a source or to route between domain routers. Implementing all these protocols has a big cost that does not worth when the groups are reduced.
Xcast keeps on of the bases that have worked so well on Internet
and have allowed it to grow so much: keep the center of the network simple and do the complicated operations in the sides.
schemes since it supports a great number of little sessions. Allows efficient applications such as VoIP, videoconferencing, collaborative meetings... Maybe these applications could be done using just unicast
, but in some cases, where the bandwidth is limited it becomes really useful. In general it supposes more efficiency.
On the other hand, as Xcast is not affordable for great groups, it has no chance of substituting the traditional multicast models.
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
that supports a great number of little multicast sessions. It is done by adding all the IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
es in the destination field of the IP header, instead of using a multicast address. The traditional multicast schemes over IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
are scalable for big multicast groups, but they have got scalability problems for a great number of groups.
The multicast schemes can be used to minimize the bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
consumption. Xcast can be used to minimize the bandwidth consumption for little groups, but the great advantage is that it eliminates the signalization and the state information for every session of a traditional multicast scheme. Thanks to this it is capable to support a great number of little sessions.
How it works
In Xcast, the source node keep all destinations of the multicastMulticast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
channel through which packets will be sent. The source codifies the destinations list in the Xcast header and send the packet to the router. Each router looks up a routing table
Routing table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...
to determine the following jumps of each packet, analyzes its header, parses the destination field basing on the following jump of every destination and copies the packets as many different paths as they have to follow. After that, the router pulls the copy packets with its correct Xcast header to every following jump. When the last jump, there is no need of making a new copy, there is just one address in the destination field. Then the packet is just like a unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
packet, and the network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
will treat it as it is. That is called X2U (Xcast to Unicast).
Xcast vs. Multicast
Traditional IPInternet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
schemes were designed to treat with huge multicast groups. It works well when doing a distribution similar to broadcast
Broadcast
Broadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
ing, but it has scalability problems when there is a great number of groups. The multicast protocols keep routing tables that relate multicast group addresses with the nodes. This tables might become huge, and that is why some alternative schemes try to reduce the quantity of information to keep. There are also protocols to announce a source or to route between domain routers. Implementing all these protocols has a big cost that does not worth when the groups are reduced.
Xcast keeps on of the bases that have worked so well on Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
and have allowed it to grow so much: keep the center of the network simple and do the complicated operations in the sides.
Advantages
- The routers do not need to keep information for every session or channel. This makes Xcast very scalable about the number of sessions it can support.
- There is no need to make a direction assignment.
- They don't need protocols for multicastMulticastIn computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
routing. They are routed correctly thanks to the common unicastUnicastright|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
protocols. - There is no critic node. Xcast minimizes the networkComputer networkA computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
latencies and maximizes efficiency. - Symmetric paths are not required.
- With traditional IPInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
multicast routing protocols it is necessary to establish a communication between unicast and multicast routing protocols. That means a slow error recovery. Xcast reacts immediately to unicast routing changes. - Easier securitySecuritySecurity is the degree of protection against danger, damage, loss, and crime. Security as a form of protection are structures and processes that provide or improve security as a condition. The Institute for Security and Open Methodologies in the OSSTMM 3 defines security as "a form of protection...
and registerRegisterRegister or registration may refer to:-Music:* Register , the relative "height" or range of a note, melody, part, instrument, etc* Registration , the art of combining the different sounds of a pipe organ to produce the desired sound...
. With Xcast all sources know the channel members and all routers are able to know the number of times each packet has been duplicated in its domain. - The receptors can be heterogeneous since Xcast allows that every receptor is able to have its own requirements of service in a single multicast channel.
- Simplicity when implementing reliable protocols over Xcast.
- Flexibility: unicast, multicast and Xcast represent costs of bandwidth, signalization and processing respectively. Depending on how the network is built or how it is in a certain moment, it may be better to use a system or another. Xcast is just another alternative.
- Easy transition between different mechanisms.
Disadvantages
- They have got big headers. Each packet contains all the remaining destinations.
- It requires a more complex header processing. Every direction needs a look up to the routing tableRouting tableIn computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...
, so it is needed the same number of consults as it was unicastUnicastright|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
, furthermore, a new header must be generated after every jump. But on the other hand:- Xcast is designed for sessions with few users, so in many routers the headers will only have just one address.
- The header building can become a very easy operation, overwrite a bit map.
- When the packet reaches a region where the bandwidth is not limited, the packet can become a premature X2U.
- Limits the session to just a few users.
Applications
Xcast is an important compliment to the existing multicastMulticast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
schemes since it supports a great number of little sessions. Allows efficient applications such as VoIP, videoconferencing, collaborative meetings... Maybe these applications could be done using just unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
, but in some cases, where the bandwidth is limited it becomes really useful. In general it supposes more efficiency.
On the other hand, as Xcast is not affordable for great groups, it has no chance of substituting the traditional multicast models.
External links
- RFC 5058
- XCAST: A commercial MediaWiki-powered website based in Japan (English-language), devoted to Explicit Multicast technology.