Develop Reference

UdpClient.BeginReceive() does not receive unless it haven't sent shortly before

I'm debugging a strange problem, happening on one of the machines in the live environment.
My app (slave) is supposed to be receiving UDP multicast messages at any time from another host (master) in the LAN, but apparently it does so only if the slave has previously sent a message.
What I expected is:
Slave asks for data
Master sends the data
Slave receives and consumes
Master waits 2-3 minutes
Master sends new data
Slave receives and consumes the new data
Steps from 4 to 6 are repeated
What I see is:
Slave doesn't receive anything
BUT if I make the slave asks for new data continuously (polling, i.e. repeat step 1) I finally get the message.
I see in Wireshark that the message from the master is indeed received by the slave host. Just my app is not receiving it. What is more surprising, is that another master-slave pair running on the same network, with the same apps, is working fine, as well as my pair in the test environment.
The slave app uses UdpClient in asynchronous mode. Here is how the listener is initialized:
private void ListenMain()
{
try
{
UdpClient udpClient = new UdpClient();
udpClient.Client.ExclusiveAddressUse = false;
udpClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
udpClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 1000);
IPv4InterfaceProperties p = adapter.GetIPProperties().GetIPv4Properties();
udpClient.Client.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastInterface, (int)IPAddress.HostToNetworkOrder(p.Index));
udpClient.Client.Bind(endPoint);
udpClient.JoinMulticastGroup(12345);
ListenState listenState = new ListenState();
listenState.udpClient = udpClient;
listenStates.Add(listenState);
logger.Debug("Waiting for messages");
udpClient.BeginReceive(new AsyncCallback(OnPacketReceived), listenState);
}
catch (Exception e)
{
logger.Error(e, "ListenMain() encountered an error");
}
}
And here is the handler of a received packet:
private void OnPacketReceived(IAsyncResult result)
{
logger.Trace("OnPacketReceived");
IPEndPoint recvAddress = new IPEndPoint(IPAddress.Any, MULTICAST_PORT);
ListenState state = result.AsyncState as ListenState;
byte[] receive_byte_array;
try
{
logger.Trace("before EndReceive");
receive_byte_array = state.udpClient.EndReceive(result, ref recvAddress);
logger.Trace("after EndReceive, got {0} bytes", receive_byte_array.Length);
// packet handling goes here...
// do the loop
logger.Trace("waiting for another packet");
state.udpClient.BeginReceive(new AsyncCallback(OnPacketReceived), state);
}
catch (ObjectDisposedException)
{
logger.Info("Socket is now closed");
return;
}
catch (Exception e)
{
logger.Warn(e, "exception in handling incoming message");
}
}
Of course, polling for new data is not an optimal solution and introduces unwanted delays. I'd like to know which phenomenon makes UdpClient lose incoming packets unless the same UdpClient has sent something before.

I think there is an error in the code: udpClient.JoinMulticastGroup(); takes the multicast IP address as argument, not the port. Does it work when you fix this? If so, this explains it:
Not joining a multicast group leads to the typical "multicast group not joined" erratic behavior, which includes the favorite "it works for two to five minutes and then suddenly stops" and "it works when I send something in the other direction and then suddenly stops" and "it works when using a different multicast address and then stops, leaving unusable multicast addresses behind".
The behaviour you see is typical for IPv4 multicast with more or less intelligent routers and switches. They all support some version of IGMP snooping (with timeouts, bugs and incompatible versions), and routers, switches and OSes cache network paths and MACs and registered and unregistered multicast IPs for an undefined amount of time. This makes it impossible to reason about the behaviour in a logical way.
Check whether you joined the expected multicast group on the receiver/listener. When this looks ok and you still have problems, trace IGMP messages and look for anything which does not make sense, like never seeing a join, or seeing erratic leaves.
(Note that IGMP messages are sent by the OS on a machine level, and not by your application. This means that not every JoinMulticastGroup() will generate an IGMP join message.)

Is there such a RTSP Ping?

I am currently working on a WinForm app to stream videos from IP camera using the RTSP protocol in C#. Everything worked fine. Part of the requirement for the app includes a function to check whether the IP camera is online or not.
So I did a ping function using the System.Net.NetworkInformation.Ping class to ping the IP camera. Say if the RTSP url of the camera is as follows rtsp://[CAMERA IP]:554/Master0-RTSP/1.0, I would only need to extract the [CAMERA IP] part and use the Ping class to see if the camera is online or not by using its IP.
Initially, it works until an issue came, say if one to enter an IP which may not be the intended IP Camera (say an IP of a computer) the ping function would still work if the entered IP of the entered device is online.
I tried to search for something like a RTSP ping but could not find one. Was hoping for any advices or opinions on this matter. Any example in C# are greatly appreciated. Thank you for your kind attention.

OPTIONS can possibly work but the standard specifies the correct way is through using theGET_PARAMETER.
RFC2326 outlines that clearly
http://www.ietf.org/rfc/rfc2326.txt
10.8 GET_PARAMETER
The GET_PARAMETER request retrieves the value of a parameter of a
presentation or stream specified in the URI. The content of the reply
and response is left to the implementation. GET_PARAMETER with no
entity body may be used to test client or server liveness ("ping").
While GET_PARAMETER may not be supported by the server there is no way to tell how that server will react to the OPTIONS request which does not even require a sessionID. Therefor it cannot be guaranteed it will keep your existing session alive.
This is clear from reading the same RFC about the OPTIONS request
10.1 OPTIONS
The behavior is equivalent to that described in [H9.2]. An OPTIONS
request may be issued at any time, e.g., if the client is about to
try a nonstandard request. It does not influence server state.
Example:
C->S: OPTIONS * RTSP/1.0
CSeq: 1
Require: implicit-play
Proxy-Require: gzipped-messages
S->C: RTSP/1.0 200 OK
CSeq: 1
Public: DESCRIBE, SETUP, TEARDOWN, PLAY, PAUSE
Note that these are necessarily fictional features (one would hope
that we would not purposefully overlook a truly useful feature just
so that we could have a strong example in this section).
If GET_PARAMETER is not supported then you would issue a PLAY request with the SessionId of the session you want to keep alive.
This should work even if OPTIONS doesn't as PLAY honors the Session ID and if you are already playing there is no adverse effect.
For the C# RtspClient see my project # https://net7mma.codeplex.com/
And the article on CodeProject # http://www.codeproject.com/Articles/507218/Managed-Media-Aggregation-using-Rtsp-and-Rtp

Regarding RTSP in C# see this thread Using RTMP or RTSP protocol in C#
Regarding Ping ... you can implement is as DESCRIBE operation ... but pay attention do not make it too frequently, the device should be affected.
http://www.ietf.org/rfc/rfc2326.txt

Instead of ICMP ping, you might want to keep a helper RTSP session without video/audio RTP streams, checking good standing of socket connection and sending OPTIONS or DESCRIBE command on a regular basis, e.g. once a minute, in order to see if the device is responsive.
Some suggest using GET_PARAMETER instead of options, however this is inferior method. OPTIONS is mandatory, GET_PARAMETER is not. Both serve different purpose. Both have small server side execution expense. OPTIONS is clearly the better of the two.
Some servers may not support setting stream parameters and thus not support GET_PARAMETER and SET_PARAMETER.

You can use RTSPClientSharp and do something like this:
public static async Task TestRTSPConnection(string rtspAddress, string user, string password)
{
var serverUri = new Uri(rtspAddress);
var credentials = new NetworkCredential(user, password);
var connectionParameters = new ConnectionParameters(serverUri, credentials);
var cancellationTokenSource = new CancellationTokenSource();
var connectTask = ConnectAsync(connectionParameters, cancellationTokenSource.Token);
if (await Task.WhenAny(connectTask, Task.Delay(15000 /*timeout*/)) == connectTask)
{
if (!connectTask.Result)
{
logger.Warn("Connection refused - check username and password");
}
logger.Info("Connection test completed");
}
else
{
logger.Warn("Connection timed out - check username and password");
}
}
private static async Task<bool> ConnectAsync(ConnectionParameters connectionParameters, CancellationToken token)
{
try
{
using (var rtspClient = new RtspClient(connectionParameters))
{
rtspClient.FrameReceived +=
(sender, frame) => logger.Info($"New frame {frame.Timestamp}: {frame.GetType().Name}");
while (true)
{
logger.Info("Connecting...");
try
{
await rtspClient.ConnectAsync(token);
}
catch (OperationCanceledException)
{
logger.Info("Finishing test before connection could be established. Check credentials");
return false;
}
catch (RtspClientException e)
{
logger.Error($"{e.Message}: {e.InnerException?.Message}");
return false;
}
logger.Info("Connected - camera is online");
return true;
}
}
}
catch (OperationCanceledException)
{
return false;
}
}
It works for me pretty well if you just care about pinging and if the camera is online or not. Also timeout happens when credentials are incorrect. You get direct failure if port is not exposed or connection is refused.

How can I read from a socket repeatedly?

To start I am coding in C#. I am writing data of varying sizes to a device through a socket. After writing the data I want to read from the socket because the device will write back an error code/completion message once it has finished processing all of the data. Currently I have something like this:
byte[] resultErrorCode = new byte[1];
resultErrorCode[0] = 255;
while (resultErrorCode[0] == 255)
{
try
{
ReadFromSocket(ref resultErrorCode);
}
catch (Exception)
{
}
}
Console.WriteLine(ErrorList[resultErrorCode[0] - 48]);
I use ReadFromSocket in other places, so I know that it is working correctly. What ends up happening is that the port I am connecting from (on my machine) changes to random ports. I think that this causes the firmware on the other side to have a bad connection. So when I write data on the other side, it tries to write data to the original port that I connected through, but after trying to read several times, the connection port changes on my side.
How can I read from the socket continuously until I receive a completion command? If I know that something is wrong with the loop because for my smallest test file it takes 1 min and 13 seconds pretty consistently. I have tested the code by removing the loop and putting the code to sleep for 1 min and 15 seconds. When it resumes, it successfully reads the completion command that I am expecting. Does anyone have any advice?

What you should have is a separate thread which will act like a driver of your external hardware. This thread will receive all data, parse it and transmit the appropriate messages to the rest of your application. This portion of code will give you an idea of how receive and parse data from your hardware.
public void ContinuousReceive(){
byte[] buffer = new byte[1024];
bool terminationCodeReceived = false;
while(!terminationCodeReceived){
try{
if(server.Receive(buffer)>0){
// We got something
// Parse the received data and check if the termination code
// is received or not
}
}catch (SocketException e){
Console.WriteLine("Oops! Something bad happened:" + e.Message);
}
}
}
Notes:
If you want to open a specific port on your machine (some external hardware are configured to talk to a predefined port) then you should specify that when you create your socket
Never close your socket until you want to stop your application or the external hardware API requires that. Keeping your socket open will resolve the random port change
using Thread.Sleep when dealing with external hardware is not a good idea. When possible, you should either use events (in case of RS232 connections) or blocking calls on separate threads as it is the case in the code above.

Find server listening on a specific port on local network

I have a server application. I also have a client application. I am able to establish a tcp connection between the applications when both applications happen to be on the same network. so let's say that the computer running the server application is listening from new connections on port 2121 and it has the LAN ip address 192.168.0.120. On a different computer running the client application I will be able to establish a connection by providing port number 2121 and ip address 192.168.0.120.
Is there a way to find all computers on a network that are listening on port 2121?
One algorithm that I am thinking now is like:
get ip address of current computer and lets say it comes out as 192.168.0.145.
now most likely the server will be listening on ip addresss 192.168.0.?
then ping 192.168.0.1 on port 2121 then 192.168.0.2 on port 2121 ... and then keep going.
I don't know if that method is efficient. moreover there might be a possibility that the server happens to be listening on ip address 192.168.1.x
So what changes will I have to make to my server and client application so that the client is able to find all the servers listening on port 2121?

The algorithm you proposed is the one you need. One problem is in the dynamic generation of the candidate IP addresses.
Normally, the possible IP address range is the one given by the subnet mask ( http://en.wikipedia.org/wiki/Subnetwork ). More exactly, the part of the IP that change is that part when in the subnet mask you have 0bits (always at the end of mask).
In your example:
if the mask is 255.255.255.0, then your possible ip address range is
192.168.0.*.
if the IP can also be 192.168.1.* then probably the mask should be 255.255.0.0
you can also have mask like 255.255.255.128 and the range would be 192.18.1.[1-126]. You can practically learn more using http://www.subnet-calculator.com/
The only other possibilities which cause your problem that I see to have these distinct ranges are:
you have more DHCP servers in your network, which is really bad as you will have "race conditions". The solution here is to fix your infrastructure by removing all but 1 DHCP server
you have manually set IP addresses (probably on laptops). The solution is to change to DHCP (if you need a specific IP that will always be assigned to a specific computer, use static DHCP)
Getting back to the problem of finding the problem of checking if "something" is listening on a specific port, the ICMP protocol is not the best here, as the majority of firewalls filter both the broadcast ICMP and single ICMP. If we are truly talking of a server, chances are you had to manually open the port you are looking for. Also, even if all computers respond, you still don't know if they host your wanted service.
The solution below involves computing the possible range of candidate IP addresses. After that you iterate through them to see if you can connect to your port.
In this implementation I test sequentially, which proves to be very slow as the timeout for connect is 30 seconds if the host is not on. For several hundred candidates, it doesn't sound too good. However, if the majority of host are available (even if they don't host your service), everything will go several times faster.
You can improve the program by either finding out how to decrease this timeout (I couldn't find out how in my allocated time) or to use a custom timeout as presented in How to configure socket connect timeout . You could also use multi-threading and adding the address that worked in a thread-safe collection and work with it from there.
Also, you could try pinging (ICMP) before, but you could miss valid servers.
static void Main(string[] args)
{
Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
int wantedPort = 21; //this is the port you want
byte[] msg = Encoding.ASCII.GetBytes("type msg here");
foreach (NetworkInterface netwIntrf in NetworkInterface.GetAllNetworkInterfaces())
{
Console.WriteLine("Interface name: " + netwIntrf.Name);
Console.WriteLine("Inteface working: {0}", netwIntrf.OperationalStatus == OperationalStatus.Up);
//if the current interface doesn't have an IP, skip it
if (! (netwIntrf.GetIPProperties().GatewayAddresses.Count > 0))
{
break;
}
//Console.WriteLine("IP Address(es):");
//get current IP Address(es)
foreach (UnicastIPAddressInformation uniIpInfo in netwIntrf.GetIPProperties().UnicastAddresses)
{
//get the subnet mask and the IP address as bytes
byte[] subnetMask = uniIpInfo.IPv4Mask.GetAddressBytes();
byte[] ipAddr = uniIpInfo.Address.GetAddressBytes();
// we reverse the byte-array if we are dealing with littl endian.
if (BitConverter.IsLittleEndian)
{
Array.Reverse(subnetMask);
Array.Reverse(ipAddr);
}
//we convert the subnet mask as uint (just for didactic purposes (to check everything is ok now and next - use thecalculator in programmer mode)
uint maskAsInt = BitConverter.ToUInt32(subnetMask, 0);
//Console.WriteLine("\t subnet={0}", Convert.ToString(maskAsInt, 2));
//we convert the ip addres as uint (just for didactic purposes (to check everything is ok now and next - use thecalculator in programmer mode)
uint ipAsInt = BitConverter.ToUInt32(ipAddr, 0);
//Console.WriteLine("\t ip={0}", Convert.ToString(ipAsInt, 2));
//we negate the subnet to determine the maximum number of host possible in this subnet
uint validHostsEndingMax = ~BitConverter.ToUInt32(subnetMask, 0);
//Console.WriteLine("\t !subnet={0}", Convert.ToString(validHostsEndingMax, 2));
//we convert the start of the ip addres as uint (the part that is fixed wrt the subnet mask - from here we calculate each new address by incrementing with 1 and converting to byte[] afterwards
uint validHostsStart = BitConverter.ToUInt32(ipAddr, 0) & BitConverter.ToUInt32(subnetMask, 0);
//Console.WriteLine("\t IP & subnet={0}", Convert.ToString(validHostsStart, 2));
//we increment the startIp to the number of maximum valid hosts in this subnet and for each we check the intended port (refactoring needed)
for (uint i = 1; i <= validHostsEndingMax; i++)
{
uint host = validHostsStart + i;
//byte[] hostAsBytes = BitConverter.GetBytes(host);
byte[] hostBytes = BitConverter.GetBytes(host);
if (BitConverter.IsLittleEndian)
{
Array.Reverse(hostBytes);
}
//this is the candidate IP address in "readable format"
String ipCandidate = Convert.ToString(hostBytes[0]) + "." + Convert.ToString(hostBytes[1]) + "." + Convert.ToString(hostBytes[2]) + "." + Convert.ToString(hostBytes[3]);
Console.WriteLine("Trying: " + ipCandidate);
try
{
//try to connect
sock.Connect(ipCandidate, wantedPort);
if (sock.Connected == true) // if succesful => something is listening on this port
{
Console.WriteLine("\tIt worked at " + ipCandidate);
sock.Close();
sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
}
//else -. goes to exception
}
catch (SocketException ex)
{
//TODO: if you want, do smth here
Console.WriteLine("\tDIDN'T work at " + ipCandidate);
}
}
}
Console.ReadLine();
}
sock.Close();
}

(sorry for my bad english) I am actually needing something similar to this and just found out about multicast. Here you can find an article and example. The sample app from the article worked fine on my lan. I do not know exactly how it works but maybe you can multicast something from the client and have the server(s) to respond with its IP? Or if that do not work, have the server multicasting his IP in a timed interval should do it. Sorry for the lack of informations, i just learned about this :)

An option i am not seeing beeing discussed here is to have a Master Server.
The idea is quite simple: A server where your application's server can register and where you application clients can get a list of active servers.
Server A is loaded and imediatly sends a hello message to the Master Server
Server B is loaded and sends an hello message to the Master Server
Both Server A and B keep sending hello's to Master Server every X Minutes so he knows they are still on
Client A is loaded - Needs to issue command - asks Master Server for List of Active Servers - Picks a server from the list - issues command
Things to keep in mind:
Master Server must be on a known address / port - either fixed ip or get ip throw well known ServerName
Purpose of Master Server is simply register servers and supply clients with their addresses - At first glance i see no other service it could provide your application
If any server is as good as any other for your application, i would advise the list to be ordered according to timestamp of last hello message received from that server - that way client will have at the top of that list the server most likelly to still be up (since it reported beeing up last) and can go down the list subsequentially.
More over, every time the Master Server receives an hello that list changes, so every so often client requests will get a different server list and use a different preferencial server, relieving load on servers accross the board.

can't you use the same method as when you get your ip.
let the client send a broadcast - if no response wait
server receive broadcast and send one back with its own ip.
now the client know that the server is out there and on what ip.

I assume you have a single server. If you can guarantee that the server location (ip address and port) is constant (or can be looked up) then each client application can 'register' with the server by connecting to it and informing the server about the ip address and local port to call back.

ICMP Ping does not determine if a computer is listening on a specific port, only if the computer is configured to response to a ping. ICMP is a protocol, different then TCP or UDP. It's only use for you would be to determine if an IP Address is use, and even then is becoming less viable.
You have two options.
Have the client constantly check every IP address on your local network and try to open port 2121. This is not a good option.
Have every server send out a ICMP ping to the broadcast address with specific data announcing it is on (and optionally not connected to a client) the never every so often (I would recommend a minute for testing, and 5 minutes minimum for production). All your software has to do is look for the broadcast ping and connect to the sending IP Address.
Update:
using System.Net.NetworkInformation;
private Ping _Ping = new Ping();
private PingOptions _PingOptions = new PingOptions(64, true);
private byte[] _PingID = Encoding.ASCII.GetBytes("MyPingID");
private _PingResponse = new AutoResetEvent(false);
public <classname> //Constructor
{
_Ping.PingCompleted += new PingCompletedEventHander(PingCompleted);
}
public void PingCompleted(object Sender, PingCompletedEventArgs e)
{
if (e.Cancelled)
{
//Status Unknown;
}
else if (e.Error != null)
{
//Status Error;
}
else if (e.Reply.Status == IPStatus.Success)
{
// Device Replying
}
else
{
// Status Unknown
}
}
public void StartPing(string AddressToPing)
{
IPAddress ipAddress = IPAddress.Parse(AddressToPing);
_Ping.SendAsync(ipAddress, 15000, _PingID, _PingOptions, _PingResponse);
}

you can make the server send his location to a specific port using UDP, and the client listen to it then the client establish a connection with the server based on the given ip and port.

Port Stuck in Time_Wait

I have a TCP Tunnel in C#. I need to open and close the tunnel which is my app between a server and a client. I'm using this to close the data connection to test out another app. I have to use particular ports.
On the second, third, nth connection depending on how long I wait to reconnect, I receive a 10048 error code - "Address already in use" when binding my socket. When closing the sockets, I do perform ShutDown.Both and Close in hopes of clearing out the ports, but when I do a netstat in a command prompt I still find the ports held in TIME_WAIT. I've also set the sockets to no linger. Lastly I tried to make a loop to check the status of the port, but it ends in a somewhat endless loop. I'm thinking it's that 4 minute TIME_WAIT rule.
I have a function to display a nestat query and I find that when I run that and check until the port goes from ESTABLISHED and into TIME_WAIT that I can bind, but when I use the same data from this query to bind on a loop when the status reaches TIME_WAIT, I get a 10048. Is there a brief moment in time allowed by my button click that allows me to bind? Is there a state between TIME_WAIT and ESTABLISHED I'm hitting in the loop and not when I do it with button clicks? I read TIME_WAIT should stop me from binding altogether, but this does not appear to be true. Can anybody explain this?
I apologize to you code lovers. Not thinking this will change anything though. I just need a better understanding of port states.
public bool CheckAvailablePorts()
{
int temp=0;
bool availPort= true;
m_config = new AppConfig();
if (!m_config.initialize())
{
System.Diagnostics.Debug.WriteLine("Error loading configuration file. Exiting...");
return false;
}
else
{
//checking through all the ports that have been set to connect on
foreach (ProxyConfig cfg in m_config.m_proxyConfigs)
{
availPort = true;
temp = cfg.localEP.Port;
DataView dv = FindEstablishedSockets();//returns netstat query
foreach (DataRowView rowView in dv)
{
DataRow row = rowView.Row;
if ((Convert.ToInt32(row["Local Port"].ToString()) == temp) && (row["Status"].ToString().Equals("Established")))
{
System.Diagnostics.Debug.WriteLine("Port: " + temp + " is still locked");
availPort = false;
break;
}
}
}
return availPort;
}
}
//snippet out of a bigger function which checks for availability and then sleeps if false and runs again
bool temp = false;
while (!temp)
{
temp = monitor.CheckAvailablePorts();
System.Threading.Thread.Sleep(2000);
}
System.Threading.Thread.Sleep(3000);
monitor.startApplication(); //starts all the binding

You need to be a bit more specific as it's hard to know what you are doing. Shorter text and a code sample would help.
I need to open and close connections and then reopen them again
That should not be a problem if it's in the client. If it's server-side, please explain why.
The config file on the server is looking for a particular port, so when I reconnect I need the same port open again
What do you mean? If you mean the listening port: You should never close the listener socket. If you do not want to accept more than one socket, simply do not call Accept again until the client socket have been disconnected.

I read TIME_WAIT should stop me from binding altogether, but this does not appear to be true.
There is an option you can use that will allow you to bind a local port that is in TIME_WAIT. This is very useful to ensure you don't have to wait 4 minutes after killing a server before restarting it.
int flag = 1;
sockfd = socket(...);
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &flag, sizeof(flag));
bind(...);

Before closing a socket, you must read all the data sent by its peer, otherwise it will stay in TIME_WAIT to ensure a new socket will not read data intended for the previous (closed one). You could also try the no lingering option of the socket.
Details: http://msdn.microsoft.com/en-us/library/windows/desktop/ms738547%28v=vs.85%29.aspx