Chapter21 Multicast Routing

Multicast Routing은 Unicast Routing을 활용한 것인데 특별한 규칙이 있는 것이 아니라 주먹구구식으로 구현을 했기 때문에 학부과정에서 깊게 배우기는 매우 힘들다. 따라서 개념 정도만 간단히 외우도록 하자.

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Unicasting

• In unicasting, there is one source and one destination network. The relationship between the source and the destination network is one to one.
• Each router in the path of the datagram tries to forward the packet to one and only one of its interfaces.

Unicast에서는 한 개의 source와 한 개의 destination만 있다.

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Multicasting

• In multicasting, there is one source and a group of destination. The relationship is one to many.
• In this type of communication, the source address is a unicast address, but the destination address is a group addresss.
• The group address defines the members of the group.

multicasting에는 한 개의 source와 group의 destination이 있다. 여기서 group은 all이 아니라는 것을 명심하자.source는 unicast address이지만 destination은 group address이다.

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Broadcasting

• Broadcasting means one-to-all communication : a host sends a paket to all hosts in an internet. Broadcasting in this sense is not provided at the Internet level for the obvious reason that it may create a huge volume of traffic and use a huge amount of bandwidth.
• Partial broadcasting is done in the Internet; some peer-to-peer applications may use broadcasting to access all peers. Controlled broadcasting may also be done in a domain (area of AS) mostly as a step to achieve multicasting.

Broadcasting은 one-to-all communication인데 당연히 Internet level에서는 사용되지 못한다. Peer-to-peer application에서는 broadcasting을 사용할 수 있다.

Partial broadcasting은 Internet에서 사용가능하다. 

Controlled broadcasting은 multicasting을 하기 위해서 사용된다.

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Multicast Basic

• Some multicasting basics : multicast addressing, collecting information about multicast groups, and multicast optimal trees.
• In multicast communication, the sender is only one, but the receiver is many, sometimes thousands or millions spread all over the world.
• It should be clear that we cannot include the addresses of all recipients in the packet.

Multicasting을 할 때, 모든 receiver의 주소를 담을 수는 없다.

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Multicast Addresses

• The destination address of a packet, as described in the Internet Protocol(IP) should be only one.
• For this reason, we need multicast addresses. A multicast address defines a group of recipients, not a single one.
• In other words, a multicast address is an identifier for group.

packet 안에 destination은 한 개만 있을 수 있으므로 Group끼리는 같은 address로 묶어준다.

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Delivery at Data-Link Layer

• In multicasting, the delivery at the Internet level is done using network-layer multicast addresses. However, data-link layer multicast addresses are also needed to deliver a multicast packet encapsulated in a frame.
• In the case of unicasting, this task is done by the ARP protocol,but,because the IP packet has a multicast IP address, the ARP protocol cannot find the corresponding MAC(physical) address to forward a multicast packet at the data-link layer.

IPv4에서 뒤에가 multicast address라면 앞에를 25개의 bit로 바꿔서 multicast address라는 것을 알린다.

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Collecting Information

• Creation of forwarding tables in both unicast and multicast routing involves two steps : 
  1. A router needs to know to which destinations it is connected.
  2. Each router needs to propagate information obtained in the first step to all other routers so that each router knows to which destination each other router is connected.

Unicast와 multicast의 collecting information하는 방법은 같다.

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Multicast Forwarding

• Another improtant issue in multicasting is the decision a router needs to make to forward a multicast packet.
• Forwarding in unicast and multicast communication is different.

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Two Approaches to Multicasting

• In multicast routing, we need to create routing trees to optimally route the packets from their source to their destination.
• However, the multicast routing decision at each router depends not only on the destination of the packet, but also on the source of the packet

multicasting에서는 packet의 destination 뿐만 아니라, packet의 source까지 고려해야한다.

 

multicast routing에는 source-based trees를 사용하는 방법과 group-shared trees를 사용하는 방법 두가지가 있다.

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DVMRP

• The Distance Vector Multicast Routing Protocol(DVMRP) is the extension of the Routing Information Protocol (RIP) which is used in unicast routing.
• It uses the source-based tree approach to multicasting. It is worth mentioning that each router in this protocol that each router in this protocol that receives a multicast packet to be forwarded implicitly creates a source-based multicast tree.

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Multicast Link State (MOSPF)

• Multicast Open Shortest Path First (MOSPF) is the extension of the Open Shortest Path First (OSPF) protocol, which is used in unicast routing.
• It also uses the source-based tree approach to multicasting. If the Internet is running a unicast link-state routing algorithm, the idea can be extended to provide a multicast link-state routing algorithm.

• To extend unicasting to multicasting, each router needs to have another database to show which interface has an active member in a particular group.

위에 구조를 보고 Multicasting Protocols의 종류를 외워두면 좋을 듯 하다.

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PIM

 

• Protocol Independent Multicast (PIM) is a family of multicast routing protocols for IP networks. It is termed protocol-independent because PIM does not include its own topology discovery mechanism, but instead uses routing information supplied by other routing protocols.
• PIM is not dependent on a specific unicast routing protocol. It can make use of any unicast routing protocol, and does not build its own routing tables, but uses the unicast routing table.

Protocol Independent Multicast(PIM)은 기존에 쓰던 routing에서 쓰는 것을 그대로 가져온다.

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BGMP and IGMP

• The Border Gateway Multicast Protocol (BGMP) is a true inter-domain multicast routing protocol.
• The Internet Group Management Protocol(IGMP) is used today for collecting information about group membership. It is a protocol defined at the network layer. IGMP messages are encapsulated in an IP datagram.