Delivery Categories of Messaging

Messaging Categories Unicast: Messages are sent to a single, specific recipient Multicast: Messages are sent to a group of recipients Broadcast: Messages are sent to all recipients on the network Anycast: Messages are sent to any member of a group of recipients

Unicast This is the normal, most common form of messaging Unicast messages are addressed using a specific address of a recipient node Network routers choose the best path (one path) for the message to travel Eventually, the routers direct the message to the correct node

Anycast Anycast messages also involve groups Anycast groups are groups of nodes where any node in the group can receive any of the messages intended for that group Anycast (rarely) is useful in situations such as sending a message to a router, which typically has several addresses You don’t care which network port is used to receive the message by the router, as long as the router gets the message Normally, the message is delivered to the node that has the shortest path from the sender

Broadcast Broadcasting can be considered a specific example of multicasting All nodes on a network are the members of the multicast group The big differences are that these nodes to not explicitly join the broadcast group, nor can they leave it Broadcasting is often implemented in hardware in LANs, which means broadcast messages use approximately the same bandwidth as a unicast message In fact, most LANs use broadcast technology to implement multicasting also

LAN Broadcasting Most LANs use broadcast technology All nodes on the network read all messages, and determine (by examining the address) if the message is intended for that node or not Broadcasting uses a specific address to indicate that the node should keep the message Broadcast-enabled NICs read messages addressed for the NIC (specifically) as well as messages addressed to the broadcast address The only concern that must be taken, is that the message should be placed back onto the network after it has been read Some LANs remove the message from the network medium when it has been received

LAN Broadcasting Broadcast LANs support broadcast delivery by transmitting a single packet This packet is received by all nodes e.g. Ethernet, Token Bus, Token Ring Daisy chain LANs send broadcast messages across the network sequentially The sender transmits a broadcast packet, which is received by the next node The next node transmits the broadcast packet again, and it is received by the next sequential node e.g. FDDI

Transmit LAN Broadcasting: Physical M M M MMM M M This is the messaging pattern in LANs that use broadcast technology

LAN Broadcast Addresses Most networks that use MAC addresses, use FF:FF:FF:FF:FF:FF for a broadcast address IP-based networks use for a broadcast address

LAN Broadcasting Efficiency LANs that use broadcast technology The time to send a broadcast message is identical to the time to send a unicast message E = O(1) LANs that use daisy-chaining The time to send a broadcast message is more than (or equal to) the time to send a unicast message The time to send a broadcast message is less than (or equal to) the time to send a unicast message to each node on the network O(1) ≤ E ≤ O(N)

WAN Broadcasting In WANs such as the Internet, a broadcast message would be received by millions of machines This is inefficient This is somewhat of an invasion of privacy This has no practical purpose Other WANs may choose to implement broadcast by sending a unicast or multicast message to all nodes on the network Even this is highly unlikely, but possible

Multicast Multicast messages are intended for a group of recipients Multicast messages are not addressed to each recipient, but addressed to the group of recipients Multicast groups are associated with specific addresses, called multicast addresses In IP networks, these are Class D addresses

Multicast Groups Messages sent to multicast group addresses are received by all members of the multicast group Therefore, in order to receive multicast group messages, a node must join the multicast group Messages can be sent to a multicast group without being a member The message is simply addressed to the multicast group’s address

LAN Multicasting LAN multicasting is often implemented in hardware Multicasting can be entirely implemented in the nodes Each node’s NIC can be configured to accept packets addressed to a multicast address Each multicast group is assigned a specific address (MAC, IP, etc…) Since all nodes normally receive (in broadcast LANs) all messages, the nodes which are configured for a particular multicast address will accept packets sent to that address Messages can be sent to a multicast group by addressing them with the multicast address for that group

LAN Multicasting In LANs that use broadcast technology, all messages are received by all nodes on a network For multicast delivery to occur, the message must simply be addressed so that the multicast group members accept the packets and non-members reject them In LANs that do not use broadcast, messages are transmitted onto the network The first multicast group member accepts the packet The group member then retransmits the packet, where it is received by the next group member

LAN Multicast Addresses In networks that use MAC addresses, multicast packets are addressed with: MAC addresses with the 8 th bit set to 1 e.g. 01.5E Unicast addresses have the 8 th bit set to 0 In IP-based networks, multicast packets are addressed with Class D addresses: – e.g

LAN Multicast Efficiency In LANs that use broadcast technology: The time to transmit a multicast message is the same as the time to transmit a unicast message E = O(1) In LANs that do not use broadcast: The time to transmit a multicast message is more than (or equal to) the time to transmit a unicast message The time to transmit a multicast message is less than (or equal to) the time to transmit a unicast message to each multicast group member O(1) ≤ E ≤ O(N)

Transmit LAN Multicasting: Physical M M MM M This is the messaging pattern in LANs that use broadcast technology Multicast Group A

WAN Multicasting WAN multicast cannot be implemented entirely at node-level (as LAN multicast can) This is because routers must forward the multicast messages to other LANs Some LANs a router can access will not contain any multicast group members It would be inefficient to send the multicast message to LANs such as these Routers must know where (on which of its ports) there are members of each multicast group

Multicast Tunneling Some WANs do not support multicast When a multicast message is sent across a network that does not support multicast, it must tunnel through that network Multicast datagrams are encapsulated into a larger datagram which is transmitted (using unicast) from one part of the network, to another Assumedly, the second part of the network is connected to another network which supports multicast or else a network that should also use multicast tunneling

WAN Multicast Efficiency LAN multicast is often identical (in efficiency) to LAN unicast WAN multicast, however, is typically less efficient The total number of messages present on the network is typically more than one for WAN multicast