For UDP (Multicasts), multiple applications can subscribe to the same port.
If one packet has arrived from client, which application receive it?
Nowadays, mDNS discovery is very popular in LAN D2D use cases. Different applications may bundle different mDNS discovery modules, which leads to the fact that multiple applications listen to the same port. When one packet comes, which application receive it? Or all get notified?
I can deliver normal UDP messages between two computers using a direct ethernet connection using my software. But I couldn't do the same for multicasting messages, I've tried other softwares that can send and receive multicast UDP messages and they didn't work as well. So I thought and wanted to ask if a direct Ethernet connection is a proper pyhsical setup or not. And if not what should I do?
Yes, a direct ethernet supports multicast. As opposed to an NBMA network like frame relay (rare these days) which does not support multicast for example.
Make sure you are not actually using a switch (which is a bridge) that has IGMP snooping enabled. IGMP snooping will not propagate multicast to nodes that have not sent an IGMP join (assuming your software does not do the join).
Short story: My DDS subscriber cannot see data from my DDS publisher. What am I missing?
Long story:
QNX 6.4.1 VM A -- Broken Publisher. IP ends with .113
QNX 6.4.1 VM B -- Working Publisher. IP ends with .114
Windows 7 -- Subscriber/Main Dev box. IP ends with .203
RTI DDS 5.0 -- Middleware version
I have a QNX VM (hosted on the network, not on my machine) that is publishing some data via RTI DDS. The data never shows up in my Windows 7 subscriber application.
Interestingly enough, I can put the same code on VM B, and the subscriber gets data. Thinking this must be a Windows 7 firewall issue, I swapped VM A's IP address with VM B, but this did not help.
Using Wireshark, I can see some heartbeat traffic from VM A, but no data. From VM B, I see the heartbeat traffic and the data. Below is a sanitized Wireshark snippet.
NDDS_DISCOVERY_PEERS is set to include both multicast and the explicit IP address of the other side of each conversation. The QOS profiles are the same, and the RTI Analyzer indicates the Match Analysis was successful (all green).
VM A:
NDDS_DISCOVERY_PEERS=udpv4://239.255.0.1,udpv4://127.0.0.1,udpv4://BLAH.203
VM B:
NDDS_DISCOVERY_PEERS=udpv4://239.255.0.1,udpv4://127.0.0.1,udpv4://BLAH.203
Windows 7 box:
NDDS_DISCOVERY_PEERS=udpv4://239.255.0.1,udpv4://127.0.0.1,udpv4://BLAH.113,udpv4://BLAH.114
When I include them in the NDDS_DISCOVERY_PEERS line, other folks on the network can see DDS traffic from VM A with DDS SPY on their Windows 7 box. My Windows 7 box can not.
Windows 7 event log does not appear to show any firewall or WFP stopping the data packets.
RTI DDS Spy run from my Windows 7 machine shows that VM A (0A061071) writers are visible on the network, but no data is being received. It also shows that the readers in my subscriber on my Windows 7 machine are visible, though it shows up at an odd IP address.
Bonus question (out of curiosity only, NOT the primary question): why does traffic on my local machine show up in DDS SPY as 192.168.11.1 instead of my machine's IP or even 127.0.0.1?
Main Question: What am I missing?
Update:
route print on my Windows 7 box appears to show that I have joined a multicast group with VM A.
netsh interface ip show joins seemed to concur.
Investigation Update:
I rebooted the VM to no effect.
I rebooted the Windows box to no effect.
I removed the multicast from the NDDS_DISCOVERY_PEERS environment
variables on both sides to no effect.
The Windows 7 box has three network interfaces (plus loopback): 1
LAN connection and 2 (unrelated) VM adapters. We are working with
the LAN connection. The QNX VM has one network interface (plus
loopback). Note that the working VM and the broken VM use a
different ethernet driver than each other, as they are slightly
different flavors of QNX 6.4.1. The broken one has wm0 as the
interface, and the working one has en0 as the interface. I don't believe this is the issue, but it is a difference.
I ran DDS SPY on the "broken" QNX VM while it was playing back, and
I got DDS data. I don't have a good method to sniff the network
between where the VM is hosted and my Windows 7 machine to see if it
makes it out of the interface, but looking at the transmitted packet
count out of the ethernet interface on the QNX VM indicates that it
is definitely transmitting something, and the Wireshark captures on the Windows 7 machine itself show that at least some traffic is making it through.
Other folks on the LAN here can see the DDS traffic from the
"broken" VM, which leads me to believe it is a Windows setup issue,
rather than a broken VM--I just can't see what it could be. I've
re-checked the firewall, to no avail. I would have thought that if it were a firewall issue, the problem would have gone away when I swapped IP addresses between VM A and VM B. In any case, the Windows 7 firewall is currently off, to no avail.
Below are several screens of Wireshark output. I skipped a bunch between the third and the fourth, as after the fourth, the traffic tended to look like the bottom of the fourth until the end.
(Skipped a bunch here)
(Pretty much continues on like the last 11 lines above)
What else should I try?
Update:
To answer Rose's question below, using rtiddsping -publisher on the bad VM and rtiddsping -subscriber works appropriately.
I wonder if this issue is caused by the weird IP address. The IP address it happens to publish and somehow latch on to is a local VM ethernet adapter (separate from VM A). See the screenshot below.
The address I would like it to attach to is 10.6.6.203. Any way I can specify that?
More than a year later this happened to me again with a different virtual machine. I had it working yesterday, so I was very suspicious. I scoured all my code changes for the past 24 hours for issues, but didn't find any. Then I decided to see if IT had pushed any patches to my computer.
Guess what? The Windows Firewall had been aggressively updated since the day before. Rules missing or changed, etc. The log said packets were being dropped. I opened the firewall filters up a bit, and suddenly, everything worked again. I hesitate to close this issue, as I am not 100% this was exactly the same as last year, but it feels very similar. I suspect that last year the settings in the firewall were not logging the packet drops.
Long and short of it: if DDS suddenly stops working, check your firewall settings.
A couple of things to try:
Try running rtiddsping -publisher on the broken VM and rtiddsping -subscriber on Windows. This has two advantages:
The data type is small and well-known, so if there's some problem with the data being fragmented due to the different Ethernet drivers, it will not happen with rtiddsping, and may help track down the problem.
Rtiddsping prints out when the publisher and subscriber discover each other, so you will be able to confirm that discovery is completing correctly on both sides. I am guessing discovery is working, because Analyzer is showing both applications, but it is good to confirm.
If you see the same problem with rtiddsping that you see with your application, increase the verbosity to rtiddsping -verbosity 3, and then 5. At the highest verbosity level, this will print (a lot of) additional output, which may give us a hint about what is happening.
To answer your bonus question about spy: The reason why spy is showing that IP address is because that is one of the addresses that is being announced as part of discovery. During discovery, a DomainParticipant can announce up to four IP addresses that can be used to reach it. Spy will choose one of those to display, but it may not be the actual address that is being used to communicate with the application. If your machine does not have any interface with the 192.168.11.1 IP address, this could indicate a larger problem. (This may be normal, though - as long as the correct IP is one of the four that are announced.)
Looking through the packet trace images, there is nothing that is obviously the problem. A few things I notice:
There seems to be a normal pattern of heartbeats/ACKNACKs in the final packet trace image. This indicates that there is some bidirectional communication between the two applications.
It is difficult to tell from these images whether the DATA being sent from .113 to .203 consists of participant-to-participant messages, or real discovery messages - except for two packets: packet #805, and packet #816 (fragments 811-815) look like discovery announcements that are being sent to .203. This indicates that you have at least four entities (DataWriters or DataReaders) in your application on .113.
So, discovery data is being sent by the application on .113. It is being received and reassembled by WireShark, but that doesn't always mean it was received correctly by the application.
Packet #816 has a heartbeat on the end of it. It is possible that packet #818 or #819 might be the ACKNACK that is responding to that heartbeat, but I can't be sure from the image. The next step is to look at those ACKNACKs from .203 to .113 to see if .203 thinks it has received all the discovery data. Here is an example of a HB/ACKNACK pair where a discovery DataReader has received all data:
Submessage: HEARTBEAT
...
firstSeqNumber: 1
lastSeqNumber: 1
The heartbeat sequence number is 1, which indicates it has only sent an announcement about a single DataReader.
Submessage: ACKNACK
...
readerSNState: 2/0:
bitmapBase: 2
numBits: 0
The readerSNState is 2/0, meaning it has received everything before sequence number two, and there is nothing missing. If there is something other than a 0 in the bitmap, it indicates the DataReader did not receive some data.
If you can confirm that the application is receiving all the discovery data correctly, it will be helpful if you can use a WireShark filter to show only user data, since the images aren't highlighting discovery vs. user data.
WireShark filter for just rtps2 user data:
rtps2 && (rtps2.traffic_nature == 3 || rtps2.traffic_nature == 1)
We had a similar issue with this. Here is the environment in a very summarized way:
A publisher
A working subscriber (laptop)
A non-working subscriber (desktop)
Both subscribers held exactly the same software (the desktop was a clone from the laptop, through Clonezilla), but rtiddsspy was blind from the desktop point of view; however, the opposite way worked well: the publisher machine's rtiddsspy saw the desktop. Laptop and publisher machines' always worked well. Laptop and desktop too (they saw each other's subscriptions)
The workaround for this (based on https://community.rti.com/content/forum-topic/discovery-issues) was to increase the MTU on the desktop NIC. Don't ask me why, but it worked.
EDIT: At the beginning, the MTU in the publisher was set to a higher value than the subscriber. So, we changed the MTU in the subscriber to match the publisher's.
Can anyone tell be where to use the UDP protocol except live streaming of music/video? What are default usecases for UDP?
UDP is also good for broadcast, such as service discovery - finding that newly plugged in printer.
Also of note is that broadcast is anonymous, you don't need to specify target hosts, as such it can form the foundation of a convenient plug-and-play or high-availability network.
UDP is stateless and is good for applications that have large numbers of clients connecting to a server such as time servers or DNS. The fact that no connection has to established and maintained reduces the memory required by the server. There is no handshaking involved and so this reduces the traffic on the network. On the downside, if the information transferred requires multiple packets there is no transmission control to ensure that all packets arrive and in the correct order - but in games packets lost are probably better than late or disordered.
Anything else where you need performance but can survive if a packet gets lost along the way. Multiplayer games come to mind, for example.
A very common use case is DNS, since the overhead of creating a TCP connection would by far outweight the actual payload.
Additional use cases are NTP (network time service) and most video games.
I use UDP to add chat capabilities to our applications. No need to create a server. It is also useful to dispatch events to all users of our applications.
I have an application where I have potentially many processes must send a message to one process. I figured the most expedient way to do this would be to just send a UDP datagram.
When opening a socket, I need to specify which port to listen to. As I just want to fire and forget the UDP datagram, I'd like to just have the OS pick an available port. I'd rather not have to hunt for one, especially as many instances of my process may be running. Is there a way I can tell the OS to just pick an available port (or better yet, not even open a listening socket) to fire and forget my UDP datagram?
Yes. Specify 0 as the port. The OS will pick an available port for you.
Answering the "Problem" rather than the "Question"
If all the processes are operating on the same PC, you might want to look into shared files or some other means of communications other than a networking stack.
I'd suggest you explore the options in Boost's Interprocess library
http://www.boost.org/doc/libs/1_37_0/doc/html/interprocess.html
Quick guide here:
http://www.boost.org/doc/libs/1_37_0/doc/html/interprocess/quick_guide.html
Specifically, I suggest you use a named pipe or shared memory between your processes.