CrazyHelloDeli
Platinum Member
Anyone able to explain these two topics to me, or know of a site with an in-depth explanation? Thx.
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IP Multicasting And Demand Dial Routing...
- Thread starter CrazyHelloDeli
- Start date
I'll take a stab at both...
There are three categories of traffic on teh network:
Unicast - Specific source and destination address. Only the target receiver will pass the packet up the stack (all hosts on the segment "see" the packet, but only the addresee will act on it).
Broadcast - One source, with a destination address that will match all receiver addresses (conventionally all ones - ff.ff.ff.ff.ff.ff in the MAC/L2, 255.255.255.255 in an IP address (both are all ones, the notation is to help humans keep 'em seperate). Note: Early BSD Unix systems used an "All Zeros broadcast" - 0.0.0.0 instead of 255.255.255.255...convention has favored the "All Ones Broadcast." Every host receives the broadcast, and because it matches (all ones will match any address by convention), every machine will pass the packet up the stack for processing by a higher level protocol, even if the broadcast doesn't relate to any process in the machine. The logic and processing takes up CPU time; if the broadcast has no function for the receiving machine, then the processor time is wasted...that's why EXCESSIVE broadcasts are evil...some are necessary for some protocols....too many are a bad thing.
Multicast - One source, with a source/destination address (a Class D address) that is distinctive enough for a group of machines to listen for. If the host is not specifically listening for the multicast destination address, it will ignore it at a very low level, saving processing time. If the machine is listening for that destination address, then it's passed up the stack for processing by a higher level portocol.
Multicasts allow for traffic to a group of hosts, but only hosts "interested" in the multicast stream....saving CPU cycles for the machines that don't need to "hear" the transmission. If you had to UNICAST a stream to 100 hosts, and each stream was 10 megabits, you'd need no less than a Gigabit of bandwidth to accommodate the transmission. With a multicast, you send the same 10 Meg stream with a Class D source and destination (which the 100 machines are "listening"for, and you get the traffic to the 100 machines, but still only use 10megabits of bandwidth.
There are some "Well-Known" multicast addresses used, for example, by routing protocols (like BGP or OSPF) to communicate status information to a specific group of routers for routing table updates. Streaming video is another popular use for multicasts....think of it as a TV station, and the hosts are like your home TV. If you want to see a specific program, you tune to the channel. Multicasts are the same way.....if the hosts wants to see a specific program (data stream), it "tunes" to that stream's channel (Class D address).
Dial-on-demand routing is a little simpler. In a nutshell, you tell the router that "If you see traffic that's aimed at this/these address(es), dial this number, establish a connection, and send the traffic across the link." The addresses can be specified individually (like Corporate Headquaters) or as a group (like to the Internet). You tell the router what traffic is "Interesting" (a Cisco term), and when the router sees traffic destined for that/those addresses, it lights up, connects, sends the traffic...and...after a user-definable time-out period, hangs up and waits for more "interesting" traffic to come by.
While the router is dialing and connecting, it sends packets to the source host, basically saying "stand by, your request is being processed" (an ICMP message, or an outright spoof of a valid connection that buffers everything until the connection is established).
That's about it. There's a jillion details not covered...just because there are a mess of text books (whole books) just on multicasting, and many chapters on things like Cisco Access Lists (defining "Interesting Traffic").
This'll get ya started though, and I'm sure others will take a shot at filling in any gaps I've left.
Good Luck
Scott
There are three categories of traffic on teh network:
Unicast - Specific source and destination address. Only the target receiver will pass the packet up the stack (all hosts on the segment "see" the packet, but only the addresee will act on it).
Broadcast - One source, with a destination address that will match all receiver addresses (conventionally all ones - ff.ff.ff.ff.ff.ff in the MAC/L2, 255.255.255.255 in an IP address (both are all ones, the notation is to help humans keep 'em seperate). Note: Early BSD Unix systems used an "All Zeros broadcast" - 0.0.0.0 instead of 255.255.255.255...convention has favored the "All Ones Broadcast." Every host receives the broadcast, and because it matches (all ones will match any address by convention), every machine will pass the packet up the stack for processing by a higher level protocol, even if the broadcast doesn't relate to any process in the machine. The logic and processing takes up CPU time; if the broadcast has no function for the receiving machine, then the processor time is wasted...that's why EXCESSIVE broadcasts are evil...some are necessary for some protocols....too many are a bad thing.
Multicast - One source, with a source/destination address (a Class D address) that is distinctive enough for a group of machines to listen for. If the host is not specifically listening for the multicast destination address, it will ignore it at a very low level, saving processing time. If the machine is listening for that destination address, then it's passed up the stack for processing by a higher level portocol.
Multicasts allow for traffic to a group of hosts, but only hosts "interested" in the multicast stream....saving CPU cycles for the machines that don't need to "hear" the transmission. If you had to UNICAST a stream to 100 hosts, and each stream was 10 megabits, you'd need no less than a Gigabit of bandwidth to accommodate the transmission. With a multicast, you send the same 10 Meg stream with a Class D source and destination (which the 100 machines are "listening"for, and you get the traffic to the 100 machines, but still only use 10megabits of bandwidth.
There are some "Well-Known" multicast addresses used, for example, by routing protocols (like BGP or OSPF) to communicate status information to a specific group of routers for routing table updates. Streaming video is another popular use for multicasts....think of it as a TV station, and the hosts are like your home TV. If you want to see a specific program, you tune to the channel. Multicasts are the same way.....if the hosts wants to see a specific program (data stream), it "tunes" to that stream's channel (Class D address).
Dial-on-demand routing is a little simpler. In a nutshell, you tell the router that "If you see traffic that's aimed at this/these address(es), dial this number, establish a connection, and send the traffic across the link." The addresses can be specified individually (like Corporate Headquaters) or as a group (like to the Internet). You tell the router what traffic is "Interesting" (a Cisco term), and when the router sees traffic destined for that/those addresses, it lights up, connects, sends the traffic...and...after a user-definable time-out period, hangs up and waits for more "interesting" traffic to come by.
While the router is dialing and connecting, it sends packets to the source host, basically saying "stand by, your request is being processed" (an ICMP message, or an outright spoof of a valid connection that buffers everything until the connection is established).
That's about it. There's a jillion details not covered...just because there are a mess of text books (whole books) just on multicasting, and many chapters on things like Cisco Access Lists (defining "Interesting Traffic").
This'll get ya started though, and I'm sure others will take a shot at filling in any gaps I've left.
Good Luck
Scott
CrazyHelloDeli
Platinum Member
Hot damn. VERY Much appriciated Scott! Sometimes Books just dont do it for me and I need a human to spell it out...Once again Thx! I think ill go pick up a book on Multicasting tomarrow🙂
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