[BC] Satellite Inet issues

[RichardBJohnson at comcast.net]

UDP is "User Datagram Protocol." It is the lowest common denominator of all network protocols. It consists of "Datagrams" only.

A Datagram is a packet that contains a source address, destination, address, some characterization bits, a time-to-live value, the data length, the data (called the payload), and a checksum. It is the basic communications component for everything having to do with TCP/IP Networking.

There is nothing about UDP that means, "Broadcast." UDP, in its primary form, does not expect an acknowledgment from the recipient. It is for this reason that it is efficient for high latency communications links such as satellite. However, these datagrams are not guaranteed to arrive at the destination.  Furthermore, there may be multiple packets generated by various routers so the destination receiver should expect multiple as well as missing packets.

Stream protocols on the other hand guarantee delivery. This means that there must be two-way communication for the reverse channel that acknowledges packets. Missing packets are re-sent and duplicate packets are dropped, the result being perfect delivery of all data.

A broadcast datagram has its destination address set to all ones, i.e., 0xffffffff (more ffffs if it is IP-V6). Every receiver is required to "hear" these packets. If the packet is not for a particular receiver, it throws it away. Such broadcast is therefore not efficient because everyone on the network needs to receive that stuff. Broadcast packets are used for ICMP http://tools.ietf.org/html/rfc792 when a request for information is part of the packet such as "Who has 192.9.200.6." Once that question is answered, the sender uses the correct destination address and does not use broadcast packets anymore.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Dana Puopolo" <dpuopolo at usa.net>

It does. UDP is a BROADCAST protocol. It sends out and that's it. Imagine it
like a radio transmitter. The sound goes out. Period.