The low down on echolink networking · Dec 17, 05:33 PM by Gregg Wonderly
Recently, I posted the following followup to a question about problems with echolink to a new echolink user. This type of question is a very common thread for people new to echolink.
Basic understanding of TCP/IP networking is something that a lot of people just don’t have. The issue is that IPv4 is an inhibiting factor. If we could get the internet switched over to IPv6, we’d have more addresses and more advanced routing control that would make it possible for everyone to enjoy direct connection to the internet.
oe6zh wrote:
> Hi, all friendly helpers!
>
> I am new to echolink, otherwise reasonably seasoned, but presently
> deeply frustrated. This is the situation:
>
> Windows 2000 Professional, SP4
> TCP/IP filters set to allow all udp ports
>
> DSL modem: Alcatel speed touch home (bridging, no firewall functionality)
> Zonealarm Pro: shut down and TrueVector service deactivated!
> Winroute Pro: shut down and winroute service deactivated!
>
> Echolink Troubleshooter results:
> IP: 10.0.0.140 (this is the network adapter to which the DSL modem is
> connected)
> TCP Test: Succeeded.
> UDP Test (5198): Receive failed (10060).
> Tests complete.
> Network adapters configured to allow all udp ports
>
> Echolink: can´t connect to test station
> Echolink with Echolink Proxy: ok
>
> All IPs on the system can be pinged ok.
>
> Local portscans on PC and network adapter suggest that ALL udp ports
> are closed! (Can´t believe it)
>
> The ISP says they allow udp to ports 5198-5200.
>
> The only other VoIP app tested is Skype and SkypeOut: ok
These systems use TCP connections which traverse firewalls just fine. They go through a central server for making “calls”. This means that the size of the central server needs to be sufficent to handle all of the traffic. Echolink uses station to station traffic over UDP. The station to station path avoids a central server. But it requires that the path between all stations be bi-directionally routed. When your router and/or DSL/Cable Modem provides Network Address Translation features, this means that it makes multiple devices on the local network appear on the internet side as a single machine. With TCP, this is done by automatic port translations. This is possible with TCP because it is a connection based protocol. The initial SYN packet tells the router what local address and port is associated with the local side. It translates the port, in the SYN packet on the way out to be a new port which is not already used on the internet side. When the remote machine sends back a SYN_ACK, it now knows from which machine and port the remote traffic will be. It can then institute a connection through the router and handle that without either end of the connection aware of what’s happening.
UDP is unconnected. The packets go out, but have no required ACK or other return packet that the router can associate with the outbound UDP packets. So, you must provide the translation manually, which the router does automatically for TCP. You must configure your modem (if needed) and your router (if needed) to use port forwarding. You must put in the local address and port that should receive the inbound UDP packets.
> Unless I am totally wrong that would suggest that there is some block
> on udp ports caused by my system that I cannot identify.
The web site, http://www.portforward.com, has a ton of information, for a vast array of devices. It describes how to configure port forwarding with words and in many cases screen snapshots. Your device(s) should have instructions in their manuals for how to configure port forwarding.
Unfortunately, there is a wide range of nomenclature that is used by the various manufactures. Some call it port forwarding, some call it something else.
As an asside, there is a feature in many routers call port triggering. Port triggering is a mechanism that is related to the TCP processing. It allows the router, to forward inbound traffic when a specific outbound connection is made. For echolink, the software always makes an initial TCP connection outbound to port 5200 to download the active station list. If you have port triggering, you should set that feature up to trigger on port 5200 and then forward UDP ports 5198 and 5199. The router will take the inside address that is the source of the port 5200 connection and make that the forward-to address of all traffic inbound for ports 5198 and 5199. The nice thing about this is that it will allow you to run echolink from any of the machines (one at a time) without having to manually reconfigure the forward to address.
