Results 1 to 9 of 9

Thread: QUdpSocket error

  1. 24th October 2007, 14:34 #1
    mdecandia
    mdecandia is offline Novice
    Join Date
    Mar 2007
    Posts
    29
    Thanks
    2

    Default QUdpSocket error

    Hi all,
    I've done a client-server application to send data between two nodes. If I use both applications on linux machines, everything works fine.
    When I recompile the client app on Windows platform, UDP datagram is sent correctly by windows client (I monitor network traffic by Ethereal but I don't know if it is really what network adapter sends) but very often server application doesn't receive entire datagram: It looses some IP packet (UDP datagram is fragmented in more than one IP packet because data is bigger than 1,5Kb).
    I think there is some Windows network problem because server runs correctly with linux client.

    Somebody may help me?

    Thanks

    Michele
    Last edited by mdecandia; 24th October 2007 at 14:39.
    Reply With Quote Reply With Quote
  2. 24th October 2007, 15:58 #2
    DeepDiver
    DeepDiver is offline Intermediate user
    Join Date
    Oct 2007
    Location
    Munich, Bavaria
    Posts
    144
    Thanks
    1
    Thanked 19 Times in 19 Posts
    Qt products
    Qt3 Qt4
    Platforms
    Unix/X11 Windows

    Default Re: QUdpSocket error

    http://en.wikipedia.org/wiki/User_Datagram_Protocol
    ...
    UDP does not guarantee reliability or ordering in the way that TCP does. Datagrams may arrive out of order, appear duplicated, or go missing without notice.
    ...
    As you see: the behavior you describe is according to the definition of UDP.

    In case you need reliability: you TCP.

    Good luck,

    Tom
    Reply With Quote Reply With Quote
  3. 24th October 2007, 16:32 #3
    mdecandia
    mdecandia is offline Novice
    Join Date
    Mar 2007
    Posts
    29
    Thanks
    2

    Default Re: QUdpSocket error

    Thank you but I know what is UDP.
    My client/server application was developed to run in LAN context. I haven't any problem on Linux client so I don't understand why Windows must be a problem.
    Reply With Quote Reply With Quote
  4. 24th October 2007, 16:53 #4
    jpn
    jpn is offline Guru
    Nokia Certified Qt Developer
    Join Date
    Feb 2006
    Location
    Oslo, Norway
    Posts
    6,264
    Thanks
    36
    Thanked 1,519 Times in 1,389 Posts
    Qt products
    Qt4
    Platforms
    MacOS X Unix/X11 Windows Symbian S60 Maemo/MeeGo

    Default Re: QUdpSocket error

    Just curious, may we see the code which reads data on the server side?
    J-P Nurmi
    Reply With Quote Reply With Quote
  5. 24th October 2007, 16:56 #5
    DeepDiver
    DeepDiver is offline Intermediate user
    Join Date
    Oct 2007
    Location
    Munich, Bavaria
    Posts
    144
    Thanks
    1
    Thanked 19 Times in 19 Posts
    Qt products
    Qt3 Qt4
    Platforms
    Unix/X11 Windows

    Default Re: QUdpSocket error

    Can you past some code to see what you do?

    In this case I can have a closer look.

    Nevertheless from my point of view: due to UDP nature there is nothing you can do against losing packets.
    In a linux-linux environment you are simply lucky due to implementation and setup of drivers, module and network subsystem implementation.
    I assume in a windows-windows environment you will be as lucky as in the linux-linux environment.

    Tom
    Reply With Quote Reply With Quote
  6. 25th October 2007, 09:14 #6
    mdecandia
    mdecandia is offline Novice
    Join Date
    Mar 2007
    Posts
    29
    Thanks
    2

    Default Re: QUdpSocket error

    The client code follows:
    Qt Code: 
    1. QByteArray message;
    2.  
    3. ..
    4. // filling up message
    5. ..
    6.  
    7. QUdpSocket udpSocket;
    8. udpSocket.writeDatagram(message.data(), message.size(), address,port);
    To copy to clipboard, switch view to plain text mode 

    The server code is

    Qt Code: 
    1. ..
    2. QUdpSocket udpSocket;
    3. connect(&udpSocket, SIGNAL(readyRead()),
    4.         this, SLOT(processPendingDatagrams()),Qt::DirectConnection);
    5.  
    6. ...
    7.  
    8. void Receiver::processPendingDatagrams()
    9. { 	
    10. 	while (udpSocket.hasPendingDatagrams()) {
    11. 			QByteArray datagram;
    12. 			datagram.resize(udpSocket.pendingDatagramSize());
    13. 			udpSocket.readDatagram(datagram.data(), datagram.size());
    14.  
    15. 		}
    16. }
    To copy to clipboard, switch view to plain text mode 

    I paste you the different output of ethereal on server with linux client:

    Qt Code: 
    1. No.     Time        Source                Destination           Protocol Info
    2. ..
    3.       3 0.004583    192.168.32.2          192.168.32.11         UDP      Source port: 50259  Destination port: wizard
    4.       4 0.074725    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=0) [Reassembled in #17]
    5.       5 0.074845    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=1480) [Reassembled in #17]
    6.       6 0.074969    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=2960) [Reassembled in #17]
    7.       7 0.075092    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=4440) [Reassembled in #17]
    8.       8 0.075215    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=5920) [Reassembled in #17]
    9.       9 0.075338    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=7400) [Reassembled in #17]
    10.      10 0.075461    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=8880) [Reassembled in #17]
    11.      11 0.075583    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=10360) [Reassembled in #17]
    12.      12 0.075707    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=11840) [Reassembled in #17]
    13.      13 0.075830    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=13320) [Reassembled in #17]
    14.      14 0.075953    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=14800) [Reassembled in #17]
    15.      15 0.076076    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=16280) [Reassembled in #17]
    16.      16 0.076199    192.168.32.11         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=17760) [Reassembled in #17]
    17.      17 0.076258    192.168.32.11         192.168.32.2          UDP      Source port: 33747  Destination port: 35147
    18.  ..
    To copy to clipboard, switch view to plain text mode 

    and windows client:

    Qt Code: 
    1. No.     Time        Source                Destination           Protocol Info
    2. ..
    3.       7 17.149508   192.168.32.2          192.168.32.12         UDP      Source port: 50146  Destination port: wizard
    4.       8 17.196854   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=0)
    5.       9 17.196978   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=1480)
    6.      10 17.197100   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=2960)
    7.      11 17.197223   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=4440)
    8.      12 17.197346   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=5920)
    9.      13 17.197469   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=7400)
    10.      14 17.197592   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=13320)
    11.      15 17.197715   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=23680)
    12.      16 17.197845   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=25160)
    13.      17 17.197969   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=26640)
    14.      18 17.198091   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=28120)
    15.      19 17.198214   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=29600)
    16.      20 17.198288   192.168.32.12         192.168.32.2          IP       Fragmented IP protocol (proto=UDP 0x11, off=31080)
    17. ..
    To copy to clipboard, switch view to plain text mode 

    As you can see in windows case, many IP fragments were loosed.
    Reply With Quote Reply With Quote
  7. 25th October 2007, 09:34 #7
    DeepDiver
    DeepDiver is offline Intermediate user
    Join Date
    Oct 2007
    Location
    Munich, Bavaria
    Posts
    144
    Thanks
    1
    Thanked 19 Times in 19 Posts
    Qt products
    Qt3 Qt4
    Platforms
    Unix/X11 Windows

    Default Re: QUdpSocket error

    What is the return value of writeDatagram and readDatagram?
    In general: you shall take care about return values - they have a meaning.

    Here is the Qt documentation on writeDatagram:
    Datagrams are always written as one block. The maximum size of a datagram is highly platform-dependent, but can be as low as 8192 bytes. If the datagram is too large, this function will return -1 and error() will return DatagramTooLargeError.

    Sending datagrams larger than 512 bytes is in general disadvised, as even if they are sent successfully, they are likely to be fragmented by the IP layer before arriving at their final destination.

    Believe it or not: ip packet loss is part of udp - if you need reliability: use tcp.
    Reply With Quote Reply With Quote
  8. 25th October 2007, 10:18 #8
    mdecandia
    mdecandia is offline Novice
    Join Date
    Mar 2007
    Posts
    29
    Thanks
    2

    Default Re: QUdpSocket error

    I take care of return values and they are about 20000 (bytes) on writeDatagram on linux and windows client.
    Server receives that data size only when linux client is running. processPendingDatagram slot is not called when windows client is running becauese readyRead() signal is not emitted (Udp datagram isn't received entirely).
    Reply With Quote Reply With Quote
  9. 25th October 2007, 10:47 #9
    DeepDiver
    DeepDiver is offline Intermediate user
    Join Date
    Oct 2007
    Location
    Munich, Bavaria
    Posts
    144
    Thanks
    1
    Thanked 19 Times in 19 Posts
    Qt products
    Qt3 Qt4
    Platforms
    Unix/X11 Windows

    Default Re: QUdpSocket error

    MTU on Ethernet is ~1500 bytes.
    Meaning the IP packets have a max size of 1500 bytes.
    In case you send data bigger than the MTU - you will get fragmentation.

    This is what we see from your ethereal trace.

    UDP doesn't take care about ordering of packet delivery and/or guarantee of delivery.

    This is what we see in your ethereal trace as well.

    As soon as one ip fragment is lost - the whole datagram is gone.

    My advice: try TCP or send the 20000 bytes in chunks of less than 1480 bytes and re-assemble the whole data pack on server side on your own.

    And believe me: not loosing data in the linux-linux setup is pure luck.
    Reply With Quote Reply With Quote
« Previous Thread | Next Thread »

Similar Threads

  1. Error compiling psql plugin
    By vieraci in forum Installation and Deployment
    Replies: 4
    Last Post: 7th October 2007, 02:49
  2. qt 4.2.2 install on aix
    By try to remember in forum Installation and Deployment
    Replies: 2
    Last Post: 28th March 2007, 12:19
  3. Qt-x11-commercial-src-4.2.0-snapshot-20060824 error
    By DevObject in forum Installation and Deployment
    Replies: 4
    Last Post: 24th August 2006, 23:31
  4. use qpsql
    By raphaelf in forum Installation and Deployment
    Replies: 34
    Last Post: 22nd August 2006, 12:52
  5. Am I the only one with "make" error ?
    By probine in forum Installation and Deployment
    Replies: 1
    Last Post: 13th February 2006, 12:54

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Forum Rules

Digia, Qt and their respective logos are trademarks of Digia Plc in Finland and/or other countries worldwide.