On Monday, July 14, 2003, at 2:55 PM, Daniel Frimerman wrote:
The cable modem is plugged into a 10mbit ethernet card.
I.e., the computer on which your WinPcap application is running has a 10Mb Ethernet interface, and that's the interface on which the application is sending packets?
If so, then...
Putting throttling will do, but isn't it strange that I'm capped at a certain speed, yet I can send out 10mbit?
No, it's not strange at all. Your application is sending packets on a 10Mb network interface; the fact that some network device upstream can't accept a full 10Mb worth of packets on its 10Mb interface isn't known to your machine - it's just a quirk of the fact that the network device in question happens to be forwarding those packets to a lower-speed network.
Use a protocol that does flow control, such as TCP, or put your own throttling into the sending loop.
Doesn't this mean that if I create a UDP socket and send it as fast as possible it would reach ~10mbit, since UDP has no flow control?
Yes.
I thought cable modem caps were physically on the cable modem itself
As far as I know, they are.
which I thought would block the packets until speed is synchronized :(
The cable modem link presumably will.
However, that blocks packets from being sent *over the cable modem link*. It *doesn't* block anybody from sending anything on the Ethernet segment going into the cable modem. The only way I know of to "block" somebody from sending something on a 10Mb Ethernet (and possibly on most if not all other forms of Ethernet) is to put carrier on the Ethernet; however
1) I don't know how long you can do that without actually transmitting a packet, if you can do so at all;
2) if you're not on a switch, you're not just preventing somebody from sending *you* something, you're preventing anybody from putting anything onto the Ethernet *at all*, which would, I think, be considered fairly impolite;
3) doing so might well be considered to be a violation of the Ethernet spec.
If I send too fast, above my cap limit, does it mean I lose packets?
Yes. The cable modem will probably discard packets sent to it if they arrive faster than it can forward them to the cable segment.
If yes, can't rely on their arrival can I?
No, you can't. But you couldn't do so anyway, at least not at the Ethernet level:
1) Ethernet doesn't guarantee packet delivery;
2) the cable service might not guarantee it, either, so even if the packet gets to the cable modem *and* gets put on the cable segment, it might not be guaranteed to arrive at the cable head end equipment, depending on what sort of protocols are used on the cable segment;
3) the cable company can't guarantee that the packet will arrive at an arbitrary destination, as it might get dropped by routers, switches, etc. along the path from the cable company's network to the destination, or it might get dropped by the destination machine.
That's why protocols such as TCP have acknowledgments and retransmissions.
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