Can someone please tell me what is the purpose of daytime servers? I referred to some sites like wikipedia, but couldn't find an answer.
The Daytime Protocol is for computers to communicate a widely accepted known format RFC 867 allowing the 2 computer systems to agree on the exact time and date to the current second.
Its not a server its a protocol that might be supported by a server. Daytime Protocol works over TCP or UDP
A useful debugging and measurement tool is a daytime service. A daytime
service simply sends a the current date and time as a character string
without regard to the input.
Check this
http://tools.ietf.org/html/rfc867
Related
The RFC states that the browser has to assign a priority to ICE candidates. In my understanding, an ICE candidate may encompass a TURN server to use.
How the browser assigns this priority is left to the implementation. We would like to implement our own protocol.
If we can calculate such a priority, how can we influence the browser to use our priority when sending ICE candidates?
The priority of the TURN server is determined by the priority of the ICE candidate. That's right! And, as I googled, the text/document summarized for the priority of the ICE candidate was in Stack Overflow. Please read it once and refer to it:
How does WebRTC decide which TURN Servers to Use
Thank you.
I have a weird problem. I have a successfully working C++ (boost asio) P2P application which works on most of the NAT. The problem is when I give the initial start port number as 1000 it checks if 1000 is free else increment by one and chooses a port and starts handshaking. But when I have 10000, 20000, or any other huge port number the hole punching doesn't work on port restricted cone NAT.
How is that possible? I am pretty sure it nothing to do with the code. and recently it doesn't work on one of my friends' full cone NAT as well, but it has worked in many other full cone NATs. What could be the reason? Is there something I am missing about how a NAT behaves?
In many NAT implementations, there are protection rules in place which prevent one host from tying up a large percentage of ports on the WAN interface, e.g. like described here.
Depending on the router, the NAT table entries have different lifetimes, and there are always limits on how many ports can be allocated to a single client (I've seen numbers from 128 to 4096).
So I think when you get to the point where you need to use high ports, the NAT table for your source IP address is already full (or almost full) with entries from old connections, or connections from other apps, so the router either decides to decline or can't fit the new NAT entry for your port.
However, to be sure, I would try to repeat that on a controlled environment collecting Wireshark dumps on both sides of the NAT and analyze the packets. If possible, it would also be helpful to enable router logs and peek into them.
I understand this is not a "magic bullet", but hope it somehow helps you.
Don't try to choose the port number yourself. The operating system can do this faster and better than your code can.
Bind your socket to port 0 and let the OS choose an available port number for you. You didn't specify what programming language, but it usually involves a call to getsockname() after the bind() call is made to discover what local port is going to be used. Java and .NET have equivalent APIs for doing the same thing.
Then follow all the other steps here:
https://stackoverflow.com/a/8524609/104458
Not sure if this'll help but have you tried having one instance of the client application starting at 1001 and the other starting at 1000, then both increment by 1.
While the 1000 will fail on client B, client A has already tried 1001 and so punched that hole, so hopefully it'll work, right? In theory, it sounds OK in my head.
As you know we can use our ADSL modems to send/receive data with high bandwidth through our phone line to/from ISP servers. I'm interested to write a program to use this capability of ADSL modem to send/receive files directly between computers (like Hyper Terminal application which was used for 56k serial modems). Is it possible? Also I want to know does it need governmental support or not? :)
Best Regards
While this is definitly possible (at least with the Alcatel ADSL modems), it is not worth the hassle. Some random points - please mind that this is written from a european POV, things might be a little different in the USA.
Carriers are not amused, if you change the configuration of your modem. Some will call it "hacking" and disconnect you.
It is not possible, to use the modem for both (Internet access and file transfer) at the same time.
It won't work between modems on different carrier networks or even in different LM segments of the same carrier - the ADSL signal is terminated at the multiplexer in your carrier's LM switch. It does however work between modems attached to the same multiplexer.
For your problem, the solution ist to use a TCP connection instead of a fake serial - even HyperTerminal can do that AFAIR
We are running a course in robotics and Xbee is the most favorite communication protocol for the student. In last two years we helped them build around 62 various projects (40 more in pipeline).
All most all the projects involve sending different kind of data to the bot. Sometimes it is a 1 byte command where as sometimes it is a long string to be interpreted. Sometimes we face the issue of addressing a bot when one xbee is used in broadcast mode to send messages to a particular bot among several. Students use their creativity to address this issue each time.
I personally feel this is reinvesting the wheel. I wonder if any higher level protocol proposals exist for serial port communication and if there isn't any specific protocol design I wonder if if the worth designing one for the student needs.
Do you mean internal only protocol of your system? If yes, often embedded software engineers incline to roll their own protocols. Most of them talks that it lets them make most optimal system.
It is not ideal approach. I agree with you that it's good for students to learn good examples.
Unfortunately I don't know any protocol stack fitting well robotics application. But I advice you to try google's protocol buffer system, its able to simplify most efforts of building protocols engines, and it works with plain c too.
You can implement Modbus ASCII if you want to go with a standard protocol that's already open.
Comli is a master/slave protocol that is used in some older devices or when it is not possible to use ethernet. You can probably get the specification from ABB if you ask - it's no secret.
That said you can put an OPC server/client architecture on top of that to get a bit more powerful communication e.g.
+--------------+ +--------------+ +--------+
| OPC UA Client| -- | OPC UA Server| -comli- | Device |
+--------------+ +--------------+ +--------+
This would make your OPC UA client protocol indepedent which makes things a bit easier down the road.
Modbus is another serial protocol that is used a lot
I believe OPC will give you the highlevel operation that you want.
see
www.opcfoundation.org
www.abb.com
PS. OPC UA is not the same as the old OLE version and thus has nothing to do with COM/DCOM
Like mjh2007 said, Modbus is standard, open and easy. The only problem I can see is if you want the robot to respond "quickly" to a command, since serial Modbus uses timeouts to detect the end of a packet. You can get around this by ignoring the timeout requirements and calculating the expected size of a packet based on it's function code and parameters as you are receiving it, then you can start processing the command immediately upon receiving the last byte and verifying any checksums. This page has some more details on implementing such a scheme.
Be sure to make use of the XBee module's "Transmit Explicit" frame (type 0x11) running in API mode with ATAO set to 1. You can unicast to a particular bot on your network, instead of always broadcasting frames. On a mesh ZigBee network, you want to avoid broadcasts as much as possible.
I'm guessing you're either using "AT mode" for sending raw data, or using "API mode" with ATAO set to 0 (sometimes referred to as "transparent serial").
If you look at that frame type (0x11), you'll see that the recipient gets an 0x91 frame that contains multiple fields already (source/destination endpoint, cluster, profile ID). You can re-purpose those fields since you're not trying to do ZigBee networking.
I have a question about the skype protocol.
Supposedly, according to wiki, the supernodes in Skype are used in UDP hole punching. The supernodes are nodes without firewalls/NATs.
My question is, how is this reliable? Isn't the vast majority of internet users behind NAT?
And, if I was to create a P2P application using this technique, what happens if there are no peers without firewalls? I don't understand how you can launch an application that relies on that there will be some peers eventually without NAT
Thanks
I can't comment on Skype specifically, but I have some experience with this (http://wiki.squeak.org/squeak/5629). We called our supernodes "big friendly giants" or BFGs :).
The idea behind supernodes is that while you hope that they pop up in the network, giving new users more options for NAT hole punching, you provide, as p2p network operator, a minimal set yourselves (could be just one or two machines, they are just needed for initial hole punching, real traffic will get re-routed directly anyway). As far as I'm aware, Skype does that as well - they run a minimum set of supernodes themselves.
When Skype had issues earlier this year, a lot of people tried to reconnect and thus the supernodes got overloaded, resulting in a domino effect. Skype added supernodes, but the amount of people trying to reconnect at that time was so massive that it took quite a while before the network rebuilt itself. It's quite funny - we had that with the above project as well - that a P2P network can be extremely resilient until it gets pushed over some edge and the whole thing crumbles.
[disclaimer: I work for eBay, former owner of Skype, but this is all my personal opinion and based on public information]
Read the papers on libjingle with discussion about services like STUN. When both parties are behind NAT an external service is often required to relay through or assist in punching a hole open on one or other side.
http://code.google.com/apis/talk/libjingle/important_concepts.html