Multicast specifics ??

[AbsolutDealage]

Does anyone have a good general explanation of the theory behind a multicast driven download? We have a version of ghost at the office that runs multicast, and one of the firmware engineers wants to implement a multicast system for downloading to hardware. What i'm really looking for is a semi-detailed overview of the entire process. I couldn't really find a good explanation of it while scouring the net, and I was wondering if anyone here has had any experience implementing this...?

 

[reicherb]

Here is a dirty explanation as I understand it though I'm sure someone else can give you a much better and technical description.

Unicast streams go from the source to each destination. For example of you are imaging 10 PCs with the same image, the server will send the image 10 times.

Multicast streams are advertised to the entire network (actually requested by the destination in some cases I believe) and then the stream is only sent one time. The switches then only forward the actual steam to those hosts that have asked for it. This can reduce network traffic considerably if you are imaging multiple machines with the same image. Another good example would be streaming video. If multiple hosts are watching the same stream, multicasting it can save bandwidth. In order for multicast to work, you have to have switches that support it though. Otherwise the switches will just flood every port with the stream and you will be worse off then if you used unicast.

 

[AbsolutDealage]

Thanks. I am aware of the general functionality of a multicast system, but what I am more interested in is specifics about how lost packets are handled, how errors are dealt with, etc.

 

[spidey07]

multicast is a one-way medium. So there really is no TCP involved. No error checking or retransmissions.

As far as ghost is concerend you tell it a multicast address to use (224.0.1.1 - 239.255.255.254, class D address) and away you go.

-edit- ghost multicast is awesome, from a network and server resource side. We can ghost 100 machines in 20 minutes. no joke.

 

[cmetz]

AbsolutDealage, reliable multicast is a very difficult research problem and I am fully confident that the people who did Ghost didn't even remotely get it right. But it probably works well enough in a controlled environment.

What I would strongly suggest is that you make sure that the network that would be used is designed in such a way that you should have no packet loss. So, for example, if you have a server directly connected to an Ethernet switch directly connected to a bunch of PCs you're ghosting, you're in great shape. Hopping over routers, WAN links, etc., is a bad idea.

 

[spidey07]

Good point cmetz. Especially about "Get your network in shape first".

Actually ghost does have rudimentary retransmit algorithms. I don't remeber the specifics but the ghost server will slow down its multicast transmission to the slowest computer in the group.

We run worldwide multicast video on our network from speed of 100 Kbs - 9 Mbs (getting ready to test out HDTV via multicast, whoohoo!). No packet loss what soever. But then again I did do all my homework to make sure the net could accomodate it...making adjustments where necessary.