Thanks for that, exactly what i was looking for, can i buy you a beer :-)
joolz
-----Original Message-----
From: [EMAIL PROTECTED] on behalf of David Empson
Sent: Wed 06/02/2008 22:35
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Not a bug report but a question regardingbroadcasting
RE: [lwip-users] Not a bug report but a question regardingbroadcastingThis
isn't really an LWIP problem, but hopefully the following should explain the
issue or at least point you in the right direction for further research.
If your LWIP host is trying to transmit to a specific IP address (192.168.1.49)
and there is no device on your network with that IP address, then it won't
actually send any of the UDP data. It will be sending ARP requests ("who has
192.168.1.49") and getting no reply, so it will throw away the UDP data. This
explains why you aren't seeing the data on the network.
Multicasts don't need a specific device to receive them, and therefore don't
need ARP to find the destination device's Ethernet MAC address. They will be
sent using Ethernet multicasts.
You won't be able to get a LAN multicast out to a specific device on the
Internet by doing something as simple as NAT translation in the router. It also
isn't possible for the router to "pick up" an existing LAN unicast and resend
it via NAT.
NAT operates by the external host sending a packet to your router's public IP
address, and the router maps that to a specific internal IP address and port
(given its configured rules), by replacing the destination address and port
fields in received packets and forwarding them to the LAN. The router also
remembers the mapping so it can be used for outgoing traffic.
For outgoing traffic, the LAN host must send the packets addressed to the
external host and port (matching the mapping that the router has remembered).
The router picks them up and does the NAT translation on the way through,
replacing the source address and port fields with the values from its rule.
You are trying to do NAT based on the destination address and port in packets
sent by your LWIP host. That isn't going to work. NAT operates from the source
address and port for outbound packets.
In order to support sending an MPEG stream over the Internet via NAT, the
router must have a NAT rule which maps UDP external port 7777 to your LWIP
host's IP address and port 1111. Your LWIP host is going to have to do
something like this:
1. Wait until it receives a UDP packet on port 1111 which requests the MPEG
stream.
2. Save the source IP address and port from that UDP packet.
3. Send the stream to the saved IP address and port.
This will produce a unicast UDP stream over the Internet to a specific IP
address.
If you also need to send the MPEG stream as a multicast over the LAN, you will
need to send it twice: the multicast and the directed external connection (once
you know where you are sending it). To support multiple external recipients,
you will need to support sending several copies of the MPEG stream at once. You
could offload this from the LWIP host by having another host on the LAN receive
the multicast and resend it to subscribers over the Internet. There may even be
an existing product which can achieve this: have a look at Apple's "QuickTime
Broadcaster".
Managing the Internet stream may be a little complicated. At a minimum, you
need a mechanism to turn it off again, otherwise your LAN host might keep
streaming data indefinitely. The simplest mechanism would be another UDP packet
from the external host which requests termination of the stream, but that seems
risky. At a minimum, it would have to be backed up with receiving ICMP
"destination unreachable" packets to cope with cases where the external host
drops off the network. Even that doesn't seem safe because of the possibility
of some routers on the Internet discarding ICMP traffic.
I'd say a safer approach would be to require the external host to regularly
send "I still want to receive the stream" packets, and your LAN host will stop
sending the stream if it times out on receiving these "keep alive" packets.
The other approach is to do everything with multicasts over the Internet. This
will require the use of a router which fully supports multicasts, your LWIP
host must use IGMP to report membership of a multicast group, and there may be
additional complexities at the other end. You should do more research on what
is involved with multicasting over the Internet before attempting this. (LAN
multicasts are easy. Internet multicasts are not.)
You might like to investigate RTP as a possible approach for sending your
stream over the Internet (whether you do it as a unicast or multicast). RTSP
may also be relevant.
----- Original Message -----
From: Julian Gardner [RSD]
To: Mailing list for lwIP users ; Mailing list for lwIP users
thats right, what i was wanting to do was put it on a unicast address as a
test!
did not know if it would work asking my router to NAT the multcast address.
joolz
-----Original Message-----
From: [EMAIL PROTECTED] on behalf of Kieran Mansley
Sent: Wed 06/02/2008 12:04
On Wed, 2008-02-06 at 12:00 +0000, Julian Gardner [RSD] wrote:
> Now i can see this if i vlc it on 224.10.0.7:1234
>
> but if i try and change the broadcast address to 192.168.1.49 (my dhcp
> starts at 50) i see nothing on the network.
As I understand it (and I'm no expert on multicast) the 224.10.0.7 is a
multicast address, but 192.168.1.49 is not.
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